Chapter 11: DNA and Genes - Polson...

286 DNA AND GENES

DNA and Genes

What You’ll Learn■ You will relate the structure

of DNA to its function. ■ You will explain the role of

DNA in protein production.■ You will distinguish among

different types of mutations.

Why It’s ImportantAn understanding of birthdefects, viral diseases, cancer,aging, genetic engineering,and even criminal investiga-tions depends upon knowingabout DNA, how it holds infor-mation, and how it plays a rolein protein production.

Scan the vocabulary words list-ed in the Section Preview atthe start of each section. Notefamiliar words. Make a list ofcurrent events issues you mayhave heard about that involvegenetics, DNA, or cloning. Asyou read the chapter, referback to your list to add notesfrom the material.

To find out more about DNAand genes, visit the GlencoeScience Web site.science.glencoe.com

READING BIOLOGYREADING BIOLOGY

11ChapterChapter

The appearance of thesetwo flies depends on thetype of genes they contain.Chromosomes, made ofgenes, which are made ofDNA, determine how anorganism looks and how itfunctions.

BIOLOGY

http://science.glencoe.com

Section

What is DNA? Although the environment influ-

ences how an organism develops, thegenetic information that is held inthe molecules of DNA ultimatelydetermines an organism’s traits.DNA achieves its control by produc-ing proteins. Living things containproteins. Your skin contains protein,your muscles contain protein, andyour bones contain protein mixedwith minerals. All actions, such aseating, running, and even thinking,depend on proteins called enzymes.Enzymes are critical for an organ-

ism’s function because they controlthe chemical reactions needed forlife. Within the structure of DNA isthe information for life—the com-plete instructions for manufacturingall the proteins for an organism.

The structure of DNADNA is capable of holding all this

information because it is a very longmolecule. Recall that DNA is a poly-mer made of repeating subunitscalled nucleotides. Nucleotides havethree parts: a simple sugar, a phos-phate group, and a nitrogen base.The simple sugar in DNA, called

11.1 DNA: THE MOLECULE OF HEREDITY 287

Can you imagine all of the information that could be con-tained in 1000 textbooks?

Remarkably, that much information—and more—is carried by the genes of asingle organism. Scientists have foundthat the substance DNA, contained ingenes, holds this information. Because ofthe unique structure of DNA, newcopies of the information can be easilyreproduced.

SECTION PREVIEW

ObjectivesAnalyze the structureof DNA.Determine how thestructure of DNAenables it to reproduceitself accurately.

Vocabularynitrogen basedouble helixDNA replication

11.1 DNA: The Molecule of Heredity

Model of a DNA molecule

deoxyribose (dee ahk sih RI bos),gives DNA its name—deoxyribonu-cleic acid. The phosphate group iscomposed of one atom of phosphorussurrounded by four oxygen atoms. Anitrogen base is a carbon ring struc-ture that contains one or more atomsof nitrogen. In DNA, there are fourpossible nitrogen bases: adenine (A),guanine (G), cytosine (C), andthymine (T). Thus, in DNA thereare four possible nucleotides, eachcontaining one of these four bases, asshown in Figure 11.1.

Nucleotides join together to formlong chains, with the phosphategroup of one nucleotide bonding tothe deoxyribose sugar of an adjacentnucleotide. The phosphate groupsand deoxyribose molecules form thebackbone of the chain, and the nitro-gen bases stick out like teeth on azipper. In DNA, the amount of ade-nine is always equal to the amount ofthymine, and the amount of guanineis always equal to the amount of

cytosine. You can see this in theProblem-Solving Lab on the next page.

In 1953, James Watson andFrancis Crick published a journalarticle that was only one page inlength, yet monumental in impor-tance. Watson and Crick proposedthat DNA is made of two chains ofnucleotides joined together by thenitrogen bases. Just as the teeth of azipper hold the two sides of the zip-per together, the nitrogen bases ofthe nucleotides hold the two strandsof DNA together with weak hydro-gen bonds. The two strands can beheld together in this way becausethey are complementary to eachother; that is, the bases on one stranddetermine the bases on the otherstrand. Specifically, adenine on onestrand bonds with a thymine on theother strand, and guanine on onestrand bonds with a cytosine on theother strand. These two bondedbases, called a complementary basepair, explain why adenine and

288 DNA AND GENES

Figure 11.1Adenine and guanine aredouble-ring bases calledpurines (A). Thymine andcytosine are smaller, single-ring bases called pyrim-idines (B). Each of the fournucleotides that make upDNA contains a phosphategroup, the sugar deoxyri-bose, and one of four dif-ferent nitrogen bases (C).

Purines Pyrimidines

Nucleotide

AA BB

CC

H HH

OO

N

N

N

Adenine (A)

C

C

C

O

O

O = P – O–

C C

C

OH

CH

NHHC

HSugar

(deoxyribose)

Nitrogen base

Phosphate groupCH2

NH2

NH2

NH2

HN

N

N

Guanine (G)

C

C

C

CH

NHC

O

N

Thymine (T)

C

CH

C

C

H

CH3

HN

Cytosine (C)

O

O NH

CH

CHC

C

N

O

thymine are always present in equalamounts. Likewise, the guanine andcytosine base pairs result in equalamounts of these nucleotides inDNA. Watson and Crick also pro-posed that DNA is shaped like a longzipper that is twisted. When some-thing is twisted like a coiled spring,the shape is called a helix. BecauseDNA is composed of two strandstwisted together, its shape is called adouble helix. This shape is shown inFigure 11.2.


Figure 11.2DNA normally exists in the shape of adouble helix. This shape is similar to thatof a twisted zipper.

What does chemical analysis reveal about DNA? Muchof the early research on the structure and composition ofDNA was done by carrying out chemical analyses. The datafrom these experiments provide evidence of a relationshipamong the nitrogen bases of DNA.

AnalysisExamine Table 11.1. Compare the amounts of adenine,

guanine, cytosine, and thymine found in the DNA of each ofthe cells studied.

Thinking Critically1. Compare the amounts of A, T, G, and C in each kind

of DNA. Why do you think the relative amounts are sosimilar in human liver and thymus cells?

2. How do the relative amounts of each base in herring spermcompare with the relative amounts of each base in yeast?

3. What fact can you state about the overall composition ofDNA, regardless of its source?

Problem-Solving Lab 11-1Problem-Solving Lab 11-1 Interpreting the Data

OriginWORDWORD

helixFrom the Latinword helix, meaning“spiral.” A doublehelix has twotwisted strands thatform a spiral.

The importance of nucleotide sequences

An elm, an elk, and an eel are alldifferent organisms composed of dif-ferent proteins. If you compare thechromosomes of these organisms, youwill find that they all contain DNAmade up of nucleotides with adenine,thymine, guanine, and cytosine bases.How can organisms be so differentfrom each other if their geneticmaterial is made of the same fournucleotides? Their differences result

Source of sample TCGA

Human liver

Human thymus

Herring sperm

Yeast

30.3

29.4

27.5

32.6

19.9

19.8

22.6

17.4

19.5

19.9

22.2

18.2

30.3

30.9

27.8

31.7

Table 11.1 Percent of each base in DNA samples

from the sequence of the four differ-ent nucleotides along the DNAstrands, as you can see in Figure 11.3.

The sequence of nucleotides formsthe unique genetic information of anorganism. For example, a nucleotidesequence of A-T-T-G-A-C carriesdifferent information from asequence of T-C-C-A-A-A. In a simi-lar way, two six-letter words made ofthe same letters but arranged in dif-ferent order have different meanings.The closer the relationship betweentwo organisms, the greater the simi-larity in their order of DNAnucleotides. The DNA sequences of

a chimpanzee are similar to those of agorilla, but different from those of arose bush. Scientists use nucleotidesequences to determine evolutionaryrelationships among organisms.Nucleotide sequences can also beused to determine whether two peo-ple are related, or whether the DNAfrom a crime scene matches theDNA of a suspected criminal.

Replication of DNAA sperm cell and an egg cell of a

fruit fly, both produced throughmeiosis, unite to form a fertilized egg.From one fertilized egg, a fruit flywith billions of cells is produced bythe process of mitosis. Each cell has acopy of the DNA that was in theoriginal fertilized egg. As you havelearned, before a cell can divide bymitosis or meiosis, it must first makea copy of its chromosomes. TheDNA in the chromosomes is copiedin a process called DNA replication.Without DNA replication, new cells

290 DNA AND GENES

Figure 11.3The structure of DNAis shown here.

In each chain of nucleotides,the sugar of one nucleotideis joined to the phosphategroup of the next nucleotideby a covalent bond.

AA The two chains ofnucleotides in a DNA mole-cule are held together byhydrogen bonds betweenthe bases. In DNA, cytosine

forms hydrogen bondswith guanine, and

thymine bonds withadenine.

BB

This pairing produces a long, two-stranded molecule that is often com-pared to a zipper. As you can see, thesides of the zipper are formed by thesugar and phosphate units, while theteeth of the zipper are the pairs of bases.

CC

Sugar-phosphate backbone

Hydrogen bonds betweennitrogen bases

Chromosome

A

A

A

P

P

P

P

D

D

D

D

D

D

D

D

P

T

T

T

G

GC

C

would have only half the DNA oftheir parents. Species could not sur-vive, and individuals could not growor reproduce successfully. All organ-isms undergo DNA replication.Figure 11.4B shows bacterial DNAreplicating.

How DNA replicatesYou have learned that a DNA mol-

ecule is composed of two strands,each containing a sequence ofnucleotides. As you know, an adenineon one strand pairs with a thymineon the other strand. Similarly, gua-nine pairs with cytosine. Therefore,if you knew the order of bases on onestrand, you could predict thesequence of bases on the other, com-plementary strand. In fact, part of theprocess of DNA replication is donein just the same way. During replica-tion, each strand serves as a patternto make a new DNA molecule. Howcan a molecule serve as a pattern?Read the Inside Story on the next pageto find out.

DNA replication begins as anenzyme breaks the hydrogen bondsbetween nitrogen bases that hold thetwo strands together, thus unzippingthe DNA molecule. As the DNAcontinues to unzip, nucleotides thatare floating free in the surroundingmedium bond to the single strands bybase pairing. Another enzyme bondsthese new nucleotides into a chain.

This process continues until theentire molecule has been unzippedand replicated. Each new strandformed is a complement of one of theoriginal, or parent, strands. Theresult is the formation of two DNAmolecules, each of which is identicalto the original DNA molecule.

When all the DNA in all the chro-mosomes of the cell has been copiedby replication, there are two copiesof the organism’s genetic informa-tion. In this way, the genetic makeupof an organism can be passed on tonew cells during mitosis or to newgenerations through meiosis followedby sexual reproduction.


Replication

DNA

Replication

Figure 11.4DNA replication producestwo molecules from one.

When a DNA mole-cule replicates, two molecules areformed. Each mole-cule has one origi-nal strand and one new strand.Newly-synthesizedstrands are shownin red.

AA

This circular bacterial DNA is replicating. Thephoto shows two loops. The bottom loop istwisted into a figure-8 shape. Replication istaking place at the intersections of the twoloops, as indicated by the arrows.

BB

Magnification: 188 000�

D

P

D

P

D

P

P

D

D

P

D

P

D

P

Original DNA strand

Original DNA

A

A

A

T

T

T

T

G

G

C

C

292 DNA AND GENES

Copying DNA

DNA is copied during interphase prior tomitosis and meiosis. It is important that

the new copies are exactly like the original molecules. The structure of DNA provides amechanism for accurate copying of the molecule.The process of making copies of DNA is calledDNA replication.

Critical Thinking What would be the outcome if mitosis occurred before replicationtook place?

Two molecules of DNA from one

INSIDESSTORTORYY

INSIDE

Separation of strands When a cell begins tocopy its DNA, the two nucleotide strands of aDNA molecule first separate at their base pairswhen the hydrogen bonds connecting the basepairs are broken. As the DNA molecule unzips,the nucleotides are exposed.

11

Base pairing Free nucleotides base pair withexposed nucleotides. If one nucleotide on astrand has thymine as a base, the free nucleotidethat pairs with it would be adenine. If the strandcontains cytosine, a free guanine nucleotide willpair with it. Thus, each strand builds its comple-ment by base pairing with free nucleotides.

22

Magnification:280 000�

Free nucleotides

New DNA strand

New DNA strand

New DNA molecule

New DNA molecule

AA

TG

G

C

C


Bonding of bases Thesugar and phosphate partsof adjacent nucleotidesbond together to form thebackbone of the new strand.Each original strand is nowbonded to a new strand.

33

Results of replication The process of repli-cation produces two molecules of DNA. Eachnew molecule has one strand from the originalmolecule and one strand that has been newlysynthesized from free nucleotides in the cell.

44

Section AssessmentSection AssessmentUnderstanding Main Ideas1. Describe the structure of a nucleotide.2. How do the nucleotides in DNA pair?3. Explain why the structure of a DNA molecule is

often described as a zipper.4. How does DNA hold information?

Thinking Critically5. The sequence of nitrogen bases on one strand

of a DNA molecule is GGCAGTTCATGC. What would be the sequence of bases on the comple-mentary strand?

6. Sequencing Sequence the steps that occur during DNA replication. For more help, refer to Organizing Information in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

Section

294 DNA AND GENES

Genes and ProteinsThe sequences of nucleotides in

DNA contain information. Thisinformation is put to work throughthe production of proteins. Proteinsform into complex three-dimensionalshapes to become key cell structuresand regulators of cell functions.Some proteins become importantstructures, such as the filaments inmuscle tissue, walls of blood vessels,and transport proteins in membranes.Other proteins, such as enzymes,control chemical reactions that per-form key life functions—breakingdown glucose molecules in cellularrespiration, digesting food, or mak-ing spindle fibers during mitosis. Infact, enzymes control all the chemicalreactions of an organism. Thus, by

encoding the instructions for makingproteins, DNA controls cells.

You learned earlier that proteinsare polymers of amino acids. Thesequence of nucleotides in each genecontains information for assemblingthe string of amino acids that makeup a single protein. It is estimatedthat each human cell contains about80 000 genes.

RNARNA, like DNA, is a nucleic acid.

However, RNA structure differs fromDNA structure in three ways, shownin Figure 11.5. First, RNA is singlestranded—it looks like only one-halfa zipper—whereas DNA is doublestranded. The sugar in RNA isribose; DNA has deoxyribose.

Morse code was a method ofcommunicating that wasdeveloped in the nineteenth

century. This code used a pattern ofdots and dashes to represent letters of the alphabet. In this way, long se-quences of dots and dashes could pro-duce an infinite number of differentmessages. Living organisms havetheir own code, called the geneticcode, in which the sequence ofnucleotides in DNA can beconverted to the sequence ofamino acids in proteins.

SECTION PREVIEW

ObjectivesRelate the concept ofthe gene to thesequences ofnucleotides in DNA.Sequence the stepsinvolved in protein synthesis.

Vocabularymessenger RNAribosomal RNAtransfer RNAtranscriptioncodontranslation

11.2 From DNA to ProteinSign

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

Signal Sign

S

T

U

V

W

X

Y

Z

1

2

3

4

5

6

7

8

9

0

Signal

This sample ofMorse code (above)is being sent (inset).

Finally, both DNA and RNA containfour nitrogen bases, but rather thanthymine, RNA contains a similarbase called uracil (U). The uracilforms a base pair with adenine, just asthymine does in DNA.

What is the role of RNA in thecell? Let’s look at an analogy. Perhapsyou have seen a car being built on anautomobile assembly line. Complexautomobiles are built in many simplesteps. Engineers tell workers how tomake the cars, and the workers fol-low directions to build the cars onthe assembly line. Suppliers bringparts to the assembly line so they canbe installed in the car. Protein pro-duction is similar to car production.DNA provides workers with the

instructions for making the proteins,and the workers build the proteins.Other workers bring parts, the aminoacids, over to the assembly line. Theworkers for protein synthesis areRNA molecules. They take fromDNA the instructions on how theprotein should be assembled, then—amino acid by amino acid—theyassemble the protein.

There are three types of RNA thathelp to build proteins. Extending thecar-making analogy, you can considerthese RNA molecules to be theworkers in the protein assembly line.One type of RNA, messenger RNA(mRNA) brings information from theDNA in the nucleus to the cell’s factory floor, the cytoplasm. On the

11.2 FROM DNA TO PROTEIN 295

Figure 11.5The three chemicaldifferences betweenDNA and RNA areshown here.

RNA contains the nitrogen base uracil (U)in place of thymine (T). Uracil base pairswith adenine just as thymine does in DNA.

CC

The sugar in RNA isribose, rather thanthe deoxyribosesugar of DNA.

BB

An RNA molecule usually consists of asingle strand of nucleotides, not a doublestrand. This single-stranded structure isclosely related to its function.

AA

CH

C

C

N

N

O

O

H

C

HO CH2

O = P – O

C

Hydrogenbonds

Adenine

Uracil

Ribose

A

Phosphate

H

OH OH

H

O

H

O

N

N

N

N

N

H

H

H

C

C C

C

CH

C C

C

P

R

C

P

RU

RG

factory floor, the mRNA becomespart of the assembly line. Ribosomes,made of ribosomal RNA (rRNA),clamp onto the mRNA and use itsinformation to assemble the aminoacids in the correct order. The thirdtype of RNA, transfer RNA (tRNA)

is the supplier. Transfer RNA trans-ports amino acids to the ribosome tobe assembled into a protein.

TranscriptionHow does the information in DNA,

which is found in the nucleus, moveto the ribosomes in the cytoplasm?Messenger RNA carries this informa-tion from the DNA to the cell’s ribo-somes for manufacturing proteins,just as a worker brings informationfrom the engineers to the factoryfloor for manufacturing a car. In thenucleus, enzymes make an RNA copyof a portion of a DNA strand in aprocess called transcription (transKRIHP shun). Follow the steps inFigure 11.6 as you read about tran-scription.

The process of transcription issimilar to that of DNA replication.

296 DNA AND GENES

Figure 11.6Messenger RNA is madeduring the process oftranscription.

When the process ofbase pairing is com-pleted, the mRNAmolecule breaksaway as the DNAstrands rejoin. ThemRNA leaves thenucleus and entersthe cytoplasm.

CC

As the DNA molecule unzips, free RNA nucleotides pair with complemen-tary DNA nucleotideson one of the DNAstrands. Thus, if asequence of bases on theDNA strand were AGCTAA CCG, the sequence ofbases on the RNA strandwould be UCG AUU GGC.

BB

DNAstrand

DNAstrand

RNAstrand

RNAstrand

A

A

A

T

T

U U

G

G G

G

G

C

C

C

C

The process of tran-scription begins asenzymes unzip themolecule of DNA,just as they do dur-ing DNA replication.

AA

The main difference is that transcrip-tion results in the formation of onesingle-stranded RNA molecule ratherthan a double-stranded DNA mole-cule. You can find out how scientistsuse new microscopes to “watch”transcription take place by readingthe BioTechnology at the end of thechapter. Modeling the process oftranscription in the BioLab will helpyou to understand this process.

The Genetic CodeThe nucleotide sequence tran-

scribed from DNA to a strand ofmessenger RNA acts as a geneticmessage, the complete informationfor the building of a protein. Thinkof this message as being written in alanguage that uses nitrogen bases asits alphabet. As you know, proteinsare built from chains of smaller mol-ecules called amino acids. You couldsay that the language of proteins usesan alphabet of amino acids. A code isneeded to convert the language ofmRNA into the language of proteins.There are 20 different amino acids,but mRNA contains only four typesof bases. How can these bases form acode for proteins? Biochemists beganto crack the code when they discov-ered that a group of three nucleotidescodes for one amino acid. For exam-ple, a sequence of three uracilnucleotides in mRNA (U-U-U)results in the amino acid phenylala-nine being placed in a protein. Eachset of three nitrogen bases in mRNAcoding for an amino acid is known asa codon. You can follow the bio-chemists’ reasoning for why threebases are needed by doing theProblem-Solving Lab on this page.

The order of nitrogen bases in themRNA will determine the type andorder of amino acids in a protein.Sixty-four combinations are possible


How many nitrogen bases determine an amino acid?After the structure of DNA had been discovered, scientists triedto predict the number of nucleotides that code for a singleamino acid. It was already known that there were 20 aminoacids, so at least 20 codons were needed. If one nucleotidecoded for an amino acid, then only four amino acids could berepresented. How many nucleotides are needed?

AnalysisExamine the

three safes. Letters representing nitrogen bases have replaced numbers on the dials. Copy the data table. Calculate the possible number of combinations that will open the safe in each dia-gram using the formula provided in the table. The 4 corre-sponds to the number of letters on each dial; the superscriptrefers to the number of available dials.

Thinking Critically 1. Using safe 1, write down several examples of dial settings

that might open the safe. Do the total possible combina-tions seen in safe 1 equal or surpass the total number ofamino acids known?

2. Could a nitrogen base (A, T, C, or G) taken one at a timecode for 20 different amino acids? Explain.

3. Using safe 2, write down several examples of dial combi-nations that might open the safe. Do the total possiblecombinations seen in safe 2 equal or surpass the totalnumber of amino acids known?

4. Could nitrogen bases taken two at a time code for 20 different amino acids? Explain.

5. Using the same procedure for safe 3, see whether thetotal possible combinations equal or surpass the totalnumber of amino acids known.

6. Could nitrogen bases taken three at a time code for 20different amino acids? Explain.

7. Does the analogy prove that three bases code for anamino acid? Explain.

Problem-Solving Lab 11-2Problem-Solving Lab 11-2 Formulating Models

Formula

Totalpossible

combinations

Number of lettersper dial

Numberof dials

Safe 1

Safe 2

Safe 3

41

42

43

Data Table

T

G

C

A

T

G

C

A

T

G

C

A T

G

C

A

T

G

C

A

T

G

C

A

Safe 1 Safe 2 Safe 3

when a sequence of three bases isused; thus, 64 different mRNAcodons are in the genetic code,shown in Table 11.2. Some codonsdo not code for amino acids; theyprovide instructions for assemblingthe protein. For example, UAA is astop codon indicating that proteinproduction ends at that point. AUGis a start codon as well as the codonfor the amino acid methionine. Asyou can see, more than one codoncan code for the same amino acid.However, for any one codon, therecan be only one amino acid.

All organisms use the same geneticcode for amino acids and assemblingproteins; UAC codes for tyrosine inthe messenger RNA of bacteria, birchtrees, and bison. For this reason, thegenetic code is said to be universal,and this provides evidence that all lifeon Earth evolved from a commonorigin. From the chlorophyll of a

birch tree to the digestive enzymes ofa bison, a large number of proteinsare produced from DNA. It may behard to imagine that only fournucleotides can produce so manydiverse proteins; yet, think aboutcomputer programming. You mayhave seen computer code, such as00010101110000110. Through abinary language with only twooptions—zeros and ones—manytypes of software are created. Fromcomputer games to World WideWeb browsers, complex software isbuilt by stringing together the zerosand ones of computer code into longchains. Likewise, complex proteinsare built from the long chains ofDNA carrying the genetic code. Ifthe DNA in all the human cells of anadult were lined up end-to-end, itwould stretch to about 60 billionmiles—about 16 times the distancefrom the sun to Pluto, the outermost

298 DNA AND GENES

U

C

A

G

Second letter

U

Phenylalanine (UUU)

Phenylalanine (UUC)

Leucine (UUA)

Leucine (UUG)

Leucine (CUU)

Leucine (CUC)

Leucine (CUA)

Leucine (CUG)

Isoleucine (AUU)

Isoleucine (AUC)

Isoleucine (AUA)

Methionine;Start (AUG)

Valine (GUU)

Valine (GUC)

Valine (GUA)

Valine (GUG)

C

Serine (UCU)

Serine (UCC)

Serine (UCA)

Serine (UCG)

Proline (CCU)

Proline (CCC)

Proline (CCA)

Proline (CCG)

Threonine (ACU)

Threonine (ACC)

Threonine (ACA)

Threonine (ACG)

Alanine (GCU)

Alanine (GCC)

Alanine (GCA)

Alanine (GCG)

A

Tyrosine (UAU)

Tyrosine (UAC)

Stop (UAA)

Stop (UAG)

Histadine (CAU)

Histadine (CAC)

Glutamine (CAA)

Glutamine (CAG)

Asparagine (AAU)

Asparagine (AAC)

Lysine (AAA)

Lysine (AAG)

Aspartate (GAU)

Aspartate (GAC)

Glutamate (GAA)

Glutamate (GAG)

G

Cysteine (UGU)

Cysteine (UGC)

Stop (UGA)

Tryptophan (UGG)

Arginine (CGU)

Arginine (CGC)

Arginine (CGA)

Arginine (CGG)

Serine (AGU)

Serine (AGC)

Arginine (AGA)

Arginine (AGG)

Glycine (GGU)

Glycine (GGC)

Glycine (GGA)

Glycine (GGG)

Firstletter

Third letter

U

C

A

G

U

C

A

G

U

C

A

G

U

C

A

G

Table 11.2 The messenger RNA genetic code

OriginWORDWORD

codon From the Latinword codex, meaning“a tablet for writ-ing.” A codon is thethree-nucleotidesequence that codesfor an amino acid.

Transcribe and Translate Molecules of DNA carry thegenetic instructions for protein formation. Converting theseDNA instructions into proteins requires a series of coordi-nated steps in transcription and translation.

Procedure! Copy the data table.@ Complete column B by writing the correct mRNA codon

for each sequence of DNA bases listed in the columnmarked DNA Base Sequence. Use the letters A, U, C, or G.

# Identify the process responsible by writing its name on thearrow in column A.

$ Complete column D by writing the correct anticodon thatbonds to each codon from column B.

% Identify the process responsible by writing its name on the arrow in column C.

^ Complete column E by writing the name of the correctamino acid that is coded by each base sequence. Use Table 11.2 on page 298 to translate the mRNA basesequences to amino acids.

Analysis1. Where within the cell:

a. are the DNA instructions located?b. does transcription occur?c. does translation occur?

2. Describe the structure of a tRNA molecule. 3. Explain why specific base pairing is essential to the

processes of transcription and translation.

MiniLab 11-1MiniLab 11-1 Predicting planet in our solar system. With pro-teins, as in software, elaborate thingsare constructed from a simple code.

Translation: FrommRNA to Protein

How is the language of mRNAtranslated into the language of pro-teins? The process of converting theinformation in a sequence of nitro-gen bases in mRNA into a sequenceof amino acids that make up a proteinis known as translation. You cansummarize transcription and transla-tion by completing the MiniLab.

Translation takes place at the ribo-somes in the cytoplasm. In prokary-otic cells that have no nucleus, themRNA is made in the cytoplasm. Ineukaryotic cells, mRNA leaves thenucleus through an opening in thenuclear membrane and travels to thecytoplasm. When the strands ofmRNA arrive in the cytoplasm, ribo-somes attach to them like clothespinsclamped onto a clothesline.

The role of transfer RNAFor proteins to be built, the 20 dif-

ferent amino acids dissolved in thecytoplasm must be brought to theribosomes. This is the role of transferRNA (tRNA), Figure 11.7. Each


T

Anticodon

Aminoacid

Chain of RNAnucleotides

ransfer RNAmolecule

AA

U A C

Figure 11.7A tRNA molecule is composed of about 80 nucleotides.Each tRNA recognizes only one amino acid. The aminoacid becomes bonded to one side of the tRNA molecule.Located on the other side of the tRNA molecule arethree nitrogen bases, called an anticodon, that pair upwith an mRNA codon during translation.

DNA basesequence

D

tRNA anticodon

C

Process

B

mRNAcodon

A

Process

AAT

GGG

ATA

AAA

GTT

E

Amino acid

→

→

→

→

→

→

→

→

→

→

Data Table

tRNA molecule attaches to only onetype of amino acid.

Correct translation of the mRNAmessage depends upon the joining ofeach mRNA codon with the correcttRNA molecule. How does a tRNAmolecule carrying its amino acid rec-ognize which codon to attach to?The answer again involves base pair-ing. On the opposite side of thetransfer-RNA molecule from theamino-acid attachment site, there is asequence of three nucleotides thatare the complement of the nucleotidesin the codon. These three nucleotidesare called an anticodon because theybond to the codon of the messengerRNA. The tRNA carries only theamino acid that the anticodon speci-fies. For example, one tRNA mole-cule for the amino acid cysteine hasan anticodon of A-C-A. This anti-codon bonds with the mRNA codonU-G-U. Now, use Table 11.2 to findthe mRNA codon for tryptophan,then determine its tRNA anticodon.

Translating the mRNA codeFollow the steps in Figure 11.8 as

you read how translation occurs. Astranslation begins, a tRNA moleculebrings the first amino acid to themRNA strand that is attached to theribosome, Figure 11.8A. The anti-codon forms a temporary bond with

the codon of the mRNA strand,Figure 11.8B. This places the aminoacid in the correct position for form-ing a bond with the next amino acid.The ribosome slides down themRNA chain to the next codon, anda new tRNA molecule brings anotheramino acid, Figure 11.8C . Theamino acids bond, the first tRNAreleases its amino acid and detachesfrom the mRNA, Figure 11.8D. ThetRNA molecule is now free to pickup and deliver another molecule ofits specific amino acid to a ribosome.Again, the ribosome slides down tothe next codon; a new tRNA mole-cule arrives, and its amino acid bondsto the previous one. A chain of aminoacids begins to form. When a stopcodon is reached, translation ends,and the amino acid strand is releasedfrom the ribosome, Figure 11.8E.

Like Silly String sprayed into afriend’s hair, amino acid chainsbecome proteins when they are freedfrom the ribosome and twist and curlinto complex three-dimensionalshapes. Unlike Silly String, however,each protein chain forms the sameshape every time it is produced.These proteins become enzymes andcell and tissue structures. The forma-tion of protein, originating from theDNA code, produces the diverse andmagnificent living world.

300 DNA AND GENES

Figure 11.8A protein is formedby the process oftranslation.

As translation begins, thestarting end of the mRNAstrand attaches to a ribo-some. Then, tRNA mole-cules, each carrying a spe-cific amino acid, approachthe ribosome. When atRNA anticodon pairs withthe first mRNA codon, thetwo molecules temporarilyjoin together.

AA

AUGA U G AAG U GCUC C A A

mRNA codon

Ribosome


Section AssessmentSection AssessmentUnderstanding Main Ideas1. In what ways do the chemical structures of DNA

and RNA differ?2. What is a codon, and what does it represent?3. What is the role of tRNA in protein synthesis?4. Compare and contrast the final products of DNA

replication and transcription.

Thinking Critically5. You have learned that there is a stop codon that

signals the end of an amino acid chain. Why is itimportant that a signal to stop translation bepart of protein synthesis?

6. Sequencing Sequence the steps involved inprotein synthesis from the production of mRNAto the final translation of the DNA code. Formore help, refer to Organizing Information inthe Skill Handbook.


Usually, the first codon on mRNA is AUG, whichcodes for the amino acid methionine. AUG signalsthe start of protein synthesis. When this signal isgiven, the ribosome slides along the mRNA to thenext codon.

BB A new tRNA molecule carrying an amino acidpairs with the second mRNA codon.

CC

When the first and second amino acids are inplace, an enzyme joins them by forming a peptidebond between them.

DD As the process continues, a chain of amino acids isformed until the ribosome reaches a stop codonon the mRNA strand.

EE

Methionine

AUGA

A

U

U

G AAG U GCUC

C

C A A

tRNA anticodon

AUGA U G AAG

G

U

U

GCUC

C

C A A

Alanine

U AC

AUGA U G AAG

G

U

U

GCUC

C

C A A

Alanine

Peptidebond

Methionine

AA

A A

U UGA U G AAG UU

C

C

C A A

Stopcodon

Section

302 DNA AND GENES

Mutation: A Change in DNA

Radiation may be given off in thereactor areas of nuclear power plants.If a person is exposed to this radiation,serious problems may result. Thegamma radiation found in nuclearreactors can break apart a moleculeof DNA, causing the nucleotidesequence to be changed. For this rea-son, nuclear power plant workers wearradiation-detecting devices such as theones shown in Figure 11.9. As youknow, the sequences of nucleotides inDNA molecules control the structureand function of cells. Any change inthe DNA sequence that also changes

the protein it codes for is called amutation.

Mutations in reproductive cellsMutations can affect the repro-

ductive cells of an organism bychanging the sequence of nucle-otides within a gene in a sperm or an egg cell. If these cells take part in fertilization, the altered genewould become part of the geneticmakeup of the offspring. The muta-tion may produce a new trait or itmay result in a protein that does notwork correctly, resulting in structuralor functional problems in cells andin the organism. Sometimes, themutation is so severe that the

DNA controls the structures and functions of a cell. Whathappens when the sequence of

DNA nucleotides is changed in a gene?Sometimes it may have little or noharmful effect, as in this tailless Manxcat, and the DNA changes are passedon to offspring of the organism. Atother times, however, the changecan cause the cell to behavedifferently. For example, inthe type of skin cancershown here, UV rays fromthe sun change the DNAand cause the cells to growand divide rapidly.

SECTION PREVIEW

ObjectivesCategorize the differ-ent kinds of mutationsthat can occur in DNA.Compare the effects of different kinds ofmutations on cells andorganisms.

Vocabularymutationpoint mutationframeshift mutationchromosomal mutationmutagen

11.3 Genetic Changes

Manx cat (above)and melanoma(inset)

OriginWORDWORD

mutation From the Latinword mutare, mean-ing “to change.”Mutations arechanges in DNA.

CAREERS IN BIOLOGY

Genetic Counselor

A re you fascinated by howyou inherit traits from your

parents? If so, you couldbecome a genetic counselorand help people assess theirrisk of inheriting or passing ongenetic disorders.

Skills for the JobGenetic counselors are medical

professionals who work on a healthcare team. They analyze families’ medicalhistories to determine their risk of having children withgenetic disorders, such as hemophilia or cystic fibrosis.Counselors also educate the public and help families withgenetic disorders find support and treatment. These coun-selors may work in a medical center, a private practice,research, or a commercial laboratory. To become a counselor,you must earn a two-year master’s degree in medical geneticsand pass a test to become certified. The most importantrequirement is the ability to listen and to help families makedifficult decisions.

For more careers in related fields, be sure to check the Glencoe Science Web site.science.glencoe.com

resulting protein is nonfunctional,and the embryo does not survive.

In some rare cases, a gene muta-tion may have positive effects. Anorganism may receive a mutation thatmakes it faster or stronger; such amutation may help an organism—and its offspring—better survive inits environment. You will learn laterthat mutations that benefit a speciesplay an important role in the evolu-tion of that species.

Mutations in body cellsWhat happens if powerful radia-

tion, such as gamma radiation, hitsthe DNA of a nonreproductive cell, acell of the body such as in skin, mus-cle, or bone? If the cell’s DNA ischanged, this mutation would not bepassed on to offspring. However, themutation may cause problems for theindividual. Damage to a gene mayimpair the function of the cell; forexample, it may make a muscle celllose its ability to make a protein thatcontracts, or a skin cell may lose itselasticity. When that cell divides, the

11.3 GENETIC CHANGES 303

Figure 11.9Nuclear power plant workerswear radiation badges (a) andpocket dosimeters (b) to monitortheir exposure to radiation.

a

b

BIOLOGY


new cells also will have the samemutation. Many scientists suggestthat the buildup of cells with lessthan optimal functioning is animportant cause of aging.

Some mutations of DNA in bodycells affect genes that control celldivision. This can result in the cellsgrowing and dividing rapidly, pro-ducing the disease called cancer. Asyou learned earlier, cancer is theuncontrolled dividing of cells.Cancer may result from gene muta-tions. For example, ultraviolet radia-tion in sunlight can change the DNAin skin cells, altering their behavior.The cells reproduce rapidly, causingskin cancer.

The effects of point mutationsConsider what might happen if an

incorrect amino acid were insertedinto a growing protein chain during

the process of translation. The mis-take might affect the structure of theentire molecule. Such a problem canoccur if a point mutation arises. Apoint mutation is a change in a sin-gle base pair in DNA.

A simple analogy can illustratepoint mutations. Read the two sen-tences below to see what happenswhen a single letter in the first sen-tence is changed.

THE DOG BIT THE CAT.THE DOG BIT THE CAR.

As you can see, changing a singleletter changes the meaning of theabove sentence. Similarly, a change ina single nitrogen base can change theentire structure of a protein becausea change in a single amino acid canaffect the shape of the protein.Figure 11.10A shows what can hap-pen with a point mutation.

304 DNA AND GENES

Figure 11.10The results of a pointmutation and aframeshift mutationare different. The dia-grams show themRNA that would beformed from eachcorresponding DNA.

Proteins that are produced as a result of frameshift mutations seldom functionproperly because such mutations usually change many amino acids. Adding ordeleting one base of a DNA molecule will change nearly every amino acid in theprotein after the addition or deletion.

BB

In this point mutation, the mRNA produced by the mutated DNA had the base gua-nine changed to adenine. This change in the codon caused the insertion of serinerather than glycine into the growing amino acid chain. The error may or may notinterfere with protein function.

AA

Normal

Pointmutation

Frameshiftmutation

. . .

. . .

AC AUGUGA U G A A G U U U G G C U AmRNA

StopMet Lys Phe Gly Ala LeuProtein

mRNA

StopMet Lys Phe Ser Ala LeuProtein

A AC UGUGA U G A A G U U U U AG C

Replace G with A

A

mRNA

Met Lys Leu Ala His CysProtein

G AC AAUGUGA U G A G U U G C U

UDeletion of U

A

What type of mutation results in sickle-cell anemia?A condition called sickle-cell anemia results from a geneticchange in the base sequence of DNA. Red blood cells inpatients with sickle-cell anemia have molecules of hemoglo-bin that are misshapen. As a result of this change in proteinshape, sickled blood cells clog capillaries and prevent normalflow of blood to body tissues, causing severe pain.

AnalysisTable 11.3 shows the sequence of bases in a short seg-

ment of the DNA that controls the order of amino acids inthe protein, hemoglobin.

Thinking Critically1. Use Table 11.2 on page 298 to transcribe and translate the

DNA base sequence for normal hemoglobin and for sick-led hemoglobin into amino acids. Remember that thetable lists mRNA codons, not DNA base sequences.

2. Does this genetic change illustrate a point mutation orframeshift mutation? Explain your answer.

3. Explain why the correct sequence of DNA bases is impor-tant to normal development of proteins.

4. Assume that the base sequence reads GGG CTT CTT AAAinstead of the normal sequence for hemoglobin. Wouldthis result in sickled hemoglobin? Explain your answer.

Problem-Solving Lab 11-3Problem-Solving Lab 11-3 Making and Using Tables

Frameshift mutationsWhen the mRNA strand moves

across the ribosome, a new amino acidis added to the protein for every codonon the mRNA strand. What wouldhappen if a single base were lost from aDNA strand? This new sequence withthe deleted base would be transcribedinto mRNA. But then, the mRNAwould be out of position by one base.As a result, every codon after thedeleted base would be different, asshown in Figure 11.10B. This muta-tion would cause nearly every aminoacid in the protein after the deletion tobe changed. In the sentence THEDOG BIT THE CAT, deleting a Gwould produce the sentence THEDOB ITT HEC AT. The same effectwould also result from the addition ofa single base. A mutation in which asingle base is added or deleted fromDNA is called a frameshift mutationbecause it shifts the reading of codonsby one base. In general, point muta-tions are less harmful to an organismbecause they disrupt only a singlecodon. The MiniLab on the next pagewill help you distinguish point muta-tions from frameshift mutations, andthe Problem-Solving Lab on this pagewill show you an example of an actualhuman mutation.

ChromosomalMutations

Changes may occur at the level ofchromosomes as well as in genes.Mutations to chromosomes may occurin a variety of ways. For example,sometimes parts of chromosomes arebroken off and lost during mitosis ormeiosis. Often, chromosomes breakand then rejoin incorrectly.Sometimes, the parts join backwardsor even join to the wrong chromo-some. These changes in chromosomesare called chromosomal mutations.

Effects of chromosomal mutations

Chromosomal mutations occur inall living organisms, but they areespecially common in plants. Suchmutations affect the distribution ofgenes to gametes during meiosisbecause they cause nondisjunction,the failure of chromosomes to sepa-rate. Nondisjunction occurs becausehomologous chromosomes cannotpair correctly when one has extra ormissing parts. Gametes that shouldhave a complete set of genes may endup with extra copies or a completelack of some genes.


Normal hemoglobin

Sickled hemoglobin

GGG CTT CTT TTT

GGG CAT CTT TTT

Table 11.3 DNA base sequences

A B C D E F G H A B C E F G H

A B C D E F G H A B C B C D E F G H

A B C D E F G H A D C B E F G H

A B C D E F G H W X A B C D E F G H

TranslocationW X Y Z Y Z

Deletion

Insertion

Inversion

Gene Mutations and Proteins Genemutations often have serious effects onproteins. In this activity, you will demon-strate how such mutations affect proteinsynthesis.

Procedure! Copy the following base sequence of one strand of an

imaginary DNA molecule: AATGCCAGTGGTTCGCAC.@ Then, write the base sequence for an mRNA strand that

would be transcribed from the given DNA sequence.# Use Table 11.2 to determine the sequence of amino acids

in the resulting protein fragment.$ If the fourth base in the original DNA strand were

changed from G to C, how would this affect the resultingprotein fragment?

% If a G were added to the original DNA strand after thethird base, what would the resulting mRNA look like?How would this addition affect the protein?

Analysis1. Which change in DNA was a point mutation? Which was a

frameshift mutation?2. In what way did the point mutation affect the protein?3. How did the frameshift mutation affect the protein?

MiniLab 11-2MiniLab 11-2Few chromosome mutations are

passed on to the next generationbecause the zygote usually dies. Incases where the zygote lives anddevelops, the mature organism isoften sterile and thus incapable ofproducing offspring. The mostimportant of these mutations—dele-tions, insertions, inversions, andtranslocations—are illustrated inFigure 11.11.

Causes of MutationsSome mutations seem to just hap-

pen, perhaps as a mistake in basepairing during DNA replication.These mutations are said to be spon-taneous. However, many mutationsare caused by factors in the environ-ment. As you learned earlier, gammaradiation is capable of causing muta-tions. Any agent that can cause achange in DNA is called a mutagen(MYEWT uh jun). Mutagens includehigh energy radiation, chemicals, andeven high temperatures.

306 DNA AND GENES

DNA segment

Figure 11.11Study the four kinds of chromosomal mutations.

Deletions occur when part of achromosome is left out.

AA

Insertions occur when a part of achromatid breaks off andattaches to its sister chromatid.The result is a duplication ofgenes on the same chromosome.

BB

Inversions occur when part of achromosome breaks off and isreinserted backwards.

CC

Translocations occur when part ofone chromosome breaks off and isadded to a different chromosome.

DD

Making andUsing Tables

Forms of radiation, such as X rays,cosmic rays, ultraviolet light, andnuclear radiation, are dangerousmutagens because they contain alarge amount of energy that can blastDNA apart. The breaking andreforming of a double-stranded DNAmolecule can result in deletions.Radiation can also cause substitutionsof incorrect nucleotides in the DNA.

Chemical mutagens include diox-ins, asbestos, benzene, cyanide, andformaldehyde, compounds that arecommonly found in buildings and inthe environment, Figure 11.12.These mutagens are highly reactivecompounds that interact with theDNA molecule and cause changes.Chemical mutagens usually result ina substitution mutation.

Repairing DNAThe cell processes that copy

genetic material and pass it from onegeneration to the next are usuallyaccurate. This accuracy is importantto ensure the genetic continuity ofboth new cells and offspring. Yet,mistakes sometimes do occur. Thereare many sources of mutagens in anorganism’s environment. Althoughmany of these are due to humanactivities, others—such as cosmic

rays from outer space—have affectedliving things since the beginning oflife. Repair mechanisms that fixmutations in cells have evolved.Much like a book editor, enzymesproofread the DNA and replaceincorrect nucleotides with correctnucleotides. These repair mecha-nisms work extremely well, but theyare not perfect. The greater theexposure to a mutagen such as UVlight, the more likely is the chancethat a mistake will not be corrected.Thus, it is wise for people to limittheir exposure to mutagens.


Figure 11.12Asbestos was for-merly used to insu-late buildings. It isnow known to causelung cancer andother lung diseasesand must beremoved from thesebuildings, as shownhere.

Section AssessmentSection AssessmentUnderstanding Main Ideas1. What is a mutation?2. Describe how point mutations and frameshift

mutations affect the synthesis of proteins.3. Describe why a mutation of a sperm or egg cell

has different consequences than a mutation of aheart cell.

4. What is the relationship between mutations andcancer?

Thinking Critically5. Why do you think low levels of mutation might

be an adaptive advantage to a species, whereashigh levels of mutation might be a disadvantage?

6. Recognizing Cause and Effect In an experi-ment with rats, the treatment group receivesradiation while the control group does not.Months later, the treatment group has a greaterpercentage of rats with cancer and newborn ratswith birth defects than the control group.Explain these observations. For more help, referto Thinking Critically in the Skill Handbook.


308 DNA AND GENES

RNA Transcription

A lthough DNA remains in the nucleus of a cell, it passes its infor-mation into the cytoplasm by way of another nucleic acid, messen-

ger RNA. The base sequence of this mRNA is complementary to thesequence in the strand of DNA, and is produced by base pairing duringtranscription. In this activity, you will demonstrate the process of tran-scription through the use of paper DNA and mRNA models.

INVESTIGATEINVESTIGATE

ProblemHow does the order of bases in

DNA determine the order of bases inmRNA?

ObjectivesIn this BioLab, you will:■ Formulate a model to show how the

order of bases in DNA determinesthe order of bases in mRNA.

■ Infer why the structure of DNAenables it to be easily copied.

Materialsconstruction paper, 5 colorsscissorsclear tape

Safety PrecautionsBe careful when using scissors.

Always use goggles in the lab.

Skill HandbookUse the Skill Handbook if you need

additional help with this lab.

PREPARATIONPREPARATION

Adenine

Deoxyribose

Parts for DNA Nucleotides Extra Parts for RNANucleotides

Phosphate

Ribose

Phosphate

Guanine

Uracil

Thymine

Cytosine


1. Observing and Inferring Doesthe mRNA model more closelyresemble the DNA strand fromwhich it was transcribed or thecomplementary strand that wasn’tused? Explain your answer.

2. Recognizing Cause and EffectExplain how the structure ofDNA enables the molecule to beeasily transcribed. Why is thisimportant for genetic informa-tion?

3. Relating Concepts Why is RNAimportant to the cell? How does

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

1. Copy the illustrations of the fourdifferent DNA nucleotides ontoyour construction paper, makingsure that each differentnucleotide is on a different colorpaper. You should make tencopies of each nucleotide.

2. Using scissors, carefully cut outthe shapes of each nucleotide.

3. Using any order of nucleotidesthat you wish, construct a double-stranded DNA molecule. If youneed more nucleotides, copythem as before.

4. Fasten your molecule togetherusing clear tape. Do not tapeacross base pairs.

5. As in step 1, copy the illustrationsof A, G, and C nucleotides. Usethe same colors of constructionpaper as in step 1. Use the fifthcolor of construction paper tomake copies of uracil nucleotides.

6. With scissors, carefully cut outthe nucleotide shapes.

7. With your DNA molecule in

PROCEDUREPROCEDURE

an mRNA molecule carry infor-mation from DNA?

Going FurtherGoing Further

Biology Journal Do library research tofind out more about how the bases in DNAwere identified and how the base pairingpattern was determined.

To find out more about DNA, visit the Glencoe

Science Web site.science.glencoe.com

front of you, demonstrate theprocess of transcription by firstpulling the DNA molecule apartbetween the base pairs.

8. Using only one of the strands ofDNA, begin matching comple-mentary RNA nucleotides with the exposed bases on the DNA model to make mRNA.

9. When you are finished, tape your new mRNA molecule together.

BIOLOGY


Scanning probe microscopes can show thearrangement of atoms on the surface of a

molecule. They make it possible for scientists topick up molecules, and even atoms, and movethem around. They can also be used to observehow biological molecules interact. There aremany types of scanning probe microscopes. All ofthem use a very sharp probe that may be only asingle atom wide at its tip. The probe sits veryclose to the specimen, but does not actually touchit. As the probe moves across, or scans, the speci-men, it measures some property of the specimen.

The scanning tunneling microscope (STM)The STM uses a probe through which a tinyamount of electric current flows. As the probescans a molecule, it encounters ridges and valleysformed by the different kinds of atoms on themolecule’s surface. The probe moves up anddown as needed to keep the current constant. Themovements of the probe are recorded by a com-puter, which produces an image of the molecule.

The atomic force microscope (AFM) TheAFM can measure many different properties,including electricity, magnetism, and heat. As theprobe moves across the specimen, changes in theproperty being measured move the probe. Thesechanges are used to create the image.

What can they do? One of the primary advan-tages of scanning probe microscopy, besides itsatomic-level resolution, is the ability to observemolecules in air or liquid. This means that biolo-gists can “watch” molecules interact as theywould inside a cell. In 1998, for example, biolo-gists used the AFM to observe the behavior of anenzyme called RNA polymerase. This enzyme isinvolved in transcription. The AFM images showhow the polymerase molecule binds to a strand

of DNA and creates a strand of mRNA by gath-ering nucleotides from the surrounding liquid.

Applications for the FutureBiologists have used a combination of lasers

and an AFM to study the physical properties ofDNA molecules. Laser “tweezers” hold downone end of a coiled DNA helix and pull on theother end. The AFM measures the forces thathold the strand together and the forces thatcause it to coil and uncoil as it performs its func-tions in the cell.

310 DNA AND GENES

Scanning ProbeMicroscopes

Have you ever heard of someone dissecting a chromosome to get a closer look at DNA?Imagine an instrument small enough to allow you to grab hold of one of the nucleotides

in a strand of DNA, yet powerful enough to provide a detailed image.

TechnologyTechnologyBIOBIO

Thinking Critically What advantages do scan-ning probe microscopes have over other types ofmicroscopes?

To find out more about microscopes, visit the

Glencoe Science Web site.science.glencoe.com

INVESTIGATING THE TECHNOLOGYINVESTIGATING THE TECHNOLOGY

STM image of DNA

Magnification: 3 200 000�

BIOLOGY


Chapter 11 AssessmentChapter 11 Assessment

SUMMARYSUMMARY

Section 11.1

Section 11.2

Section 11.3

Main Ideas■ DNA, the genetic material of organisms, is

composed of four kinds of nucleotides. A DNAmolecule consists of two strands of nucleotideswith sugars and phosphates on the outside andbases paired by hydrogen bonding on the inside.The paired strands form a twisted-zipper shapecalled a double helix.

■ Because adenine can pair only with thymine,and guanine can pair only with cytosine, DNAcan replicate itself with great accuracy. Thisprocess keeps the genetic information constantthrough cell division and during reproduction.

VocabularyDNA replication (p. 290)double helix (p. 289)nitrogen base (p. 288)

Main Ideas■ Genes are small sections of DNA. Most

sequences of three bases in the DNA of a genecode for a single amino acid in a protein.

■ The order of nucleotides in DNA determinesthe order of nucleotides in messenger RNA in aprocess called transcription.

■ Translation is a process through which theorder of bases in messenger RNA codes for theorder of amino acids in a protein.

Vocabularycodon (p. 297)messenger RNA (p. 295)ribosomal RNA (p. 296)transcription (p. 296)transfer RNA (p. 296)translation (p. 299)

Main Ideas■ A mutation is a change in the base sequence of

DNA. Mutations may affect only one gene, orthey may affect whole chromosomes.

■ Mutations in eggs or sperm affect future gener-ations by producing offspring with new charac-teristics. Mutations in body cells affect only theindividual and may result in cancer.

Vocabularychromosomal mutation

(p. 305)frameshift mutation

(p. 305)mutagen (p. 306)mutation (p. 302)point mutation (p. 304)

CHAPTER 11 ASSESSMENT 311

1. Which of the following processes requiresDNA replication?a. transcription c. mitosisb. translation d. protein synthesis

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS 2. In which of the following processes does theDNA unzip?a. transcription and translationb. transcription and replicationc. replication and translationd. all of these

DNA: TheMolecule ofHeredity

From DNA toProtein

GeneticChanges

D


3. Which DNA strand can base pair to the fol-lowing DNA strand?

a. T-A-C-G-A-T c. U-A-C-G-A-Ub. A-T-G-C-T-A d. A-U-G-C-U-A

4. Which of the following nucleotide chainscould be part of a molecule of RNA?a. A-T-G-C-C-A c. G-C-C-T-T-Gb. A-A-T-A-A-A d. A-U-G-C-C-A

5. Which of the following mRNA codonswould cause synthesis of a protein to termi-nate? Refer to Table 11.2.a. G-G-G c. U-A-Gb. U-A-C d. A-A-G

6. A DNA sequence of A-C-C would create anmRNA codon for which amino acid? Refer toTable 11.2.a. tryptophan c. leucineb. serine d. phenylalanine

7. The genetic code for an oak tree is ________.a. more similar to an ash tree than to a squirrelb. more similar to a chipmunk than to

a maple treec. more similar to a mosquito than to

an elm treed. exactly the same as for an octopus

8. Which of the following base pairs would notbe found in a cell?a. adenine—thymine c. thymine—uracilb. cytosine—guanine d. adenine—uracil

9. A protein is assembled amino acid-by-aminoacid during the process of ________.a. replication c. transcriptionb. translation d. mutation

10. A deer is born normal, but UV rays cause amutation in its retina. Which of the follow-ing statement is least likely to be true?a. The mutation may be passed on to the

offspring of the deer.b. The mutation may cause retinal cancer.c. The mutation may interfere with the

function of the retinal cell.d. The mutation may interfere with the

structure of the retinal cell.11. In the process of ________, enzymes make an

RNA copy of a DNA strand.12. The RNA copy that carries information from

DNA in the nucleus into the cytoplasm is________ RNA.

13. DNA is copied before a cell divides in theprocess called ________.

14. Molecules of ________ bring amino acids tothe ________ for assembly into proteins.

15. The shape of a molecule of DNA is called a________.

16. RNA has a ribose sugar, whereas DNA has a________ sugar.

17. Nucleotides form base pairs through a weakbond called a ________ bond.

18. A female lab rat is exposed to X rays. Itsfuture offspring will be affected only if amutation occurs in the rat’s ________ cells.

19. Chemical Q causes the following change inthe sequences of nucleotides. This change isan example of a ________. Chemical Q is a________.

20. DNA ________ is necessary before a celldivides so each cell has a complete copy ofthe chromosomes.

21. Explain why a mutation in a lung cell wouldnot be passed on to offspring.

APPLYING MAIN IDEASAPPLYING MAIN IDEAS

312 CHAPTER 11 ASSESSMENT

TEST–TAKING TIPTEST–TAKING TIP

Use as Much Time as You Can You will not get extra points for finishing early.Work slowly and carefully on any test and makesure you don’t make careless errors because youare hurrying to finish.

AA TT G C

TAA T G AA T A AA A


CHAPTER 11 ASSESSMENT 313

22. Explain why codons can’t consist of two basesinstead of three for each amino acid.

23. A bricklayer has an assistant who bringsbricks to the bricklayer so she can build awall. What part of translation most closelyresembles the assistant’s job? What do thebricks represent?

24. Making Inferences Explain how the univer-sality of the genetic code is evidence that allorganisms alive today evolved from a com-mon ancestor in the past.

25. Analyzing Identify the type of chromosomalmutation illustrated in each diagram below.

26. Concept Mapping Complete the conceptmap by using the following vocabulary terms:transfer RNA, codons, messenger RNA,transcription, translation, ribosomal RNA.

THINKING CRITICALLYTHINKING CRITICALLY

ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS

The following graph records the amount ofDNA in liver cells that have been grown ina culture so that all the cells are at the samephase in the cell cycle.

Interpreting Data Use the data in thegraph to answer the following questions.1. During the course of the experiment,

these cells went through cell division.What is this type of division called?a. transcription c. mitosisb. translation d. meiosis

2. During which hours were the cells carrying out cell division?a. 0-8 hours c. 12-14 hoursb. 8-10 hours d. 14-20 hours

3. Which phase of the cell cycle were thecells in during hours 2-6?a. interphase c. prophaseb. telophase d. anaphase

4. If you added radioactive thymine to theculture at 0 hour, what would happen tothe amount incorporated into the DNAbetween hours 20 and 28 relative to theamount at 0 hour?a. stay the same c. doubleb. divide in half d. triple

5. Predicting Predict what will be theDNA content of the cells, in picograms,at 33 hours.

For additional review, use the assessmentoptions for this chapter found on the Biology: TheDynamics of Life Interactive CD-ROM and on theGlencoe Science Web site.science.glencoe.com

CD-ROM

5. 6.

3.

which is madeup of

2.

4.1.

combines amino acids in process of

Protein synthesis

firstrequires

to makethat occurs with

the help of

and

for use in

a

b

c

K L M N O P R Q S

K L M N O P Q X Y R

F G H I J K M ND

NA

(pic

ogra

ms/c

ell)

Time (hours)2

0.1

0

0.2

0.3

0.4

0.5

0.6

6 10 14 18 22 26

Change in DNA content per cell


Chapter 11: DNA and Genes - Polson...

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Transcript of Chapter 11: DNA and Genes - Polson...