Chap. 4—Genetics and Cellular Function

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Ch. 4 Study Guide 1. Critically read Chapter 4 up to page 129 right before

4.3 “DNA Replication and the Cell Cycle” section2. Comprehend Terminology (those in bold in the

textbook)3. Study-- Figure questions, Think About It

questions, and Before You Go On (section-ending) questions

4. Do end-of-chapter questions:– Testing Your Recall— 2, 4, 5, 6, 7, 18– True or False– 1, 2, 4-7

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§ 4.1 DNA and RNA– The Nucleic Acids

§ DNA structure (1)• General

– DNA– deoxyribonucleic acid– Most human cells have 46 molecules of DNA– A uniform diameter of 2 nm and the average length @ 2-in.

• Molecular level—– Nucleic acids (DNA + RNA) are polymers of

__________________________– A nucleotide consists of (1) ________ + (2) ________ +

(3) ___________________ – DNA is a double helix (@ spiral staircase)

Fig. 4.1 a +b and 4.2 4

HC

N C

N

NH2

NH

C

C

CH

N

H

CH2OHO

O

OH

P

H

HOH

HH

O

Adenine

Phosphate Deoxyribose(a)

See next slideA nucleotide consists of three components

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C

NH2 N

NH

CCH

CH

N

N C

Adenine (A)

C

O N

NH

CCH

C N

HN C

Guanine (G)

HC C

NH

C

HC N

O

Cytosine (C)

Uracil (U)

C

C

O

C

O

CH

HN CH

NH

NH

C

C

HC

CH3

NH

O

O

Thymine (T)

Pyrimidines

(b)

NH2

NH2

Purines

Five nitrogenous bases:

T: Only in DNA

U: Only in RNA6

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§ DNA Structure (2)

“Twisted ladder”Space-filling model

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§ DNA Structure (3)• DNA = a double helix molecule; a spiral

staircase; a soft rubber ladder that you can twist

• Details:– Each sidepiece is a backbone-- composed of

phosphate groups alternating with the sugar deoxyribose.

– Step-like connections-- between the backbones are pairs of nitrogenous bases.

– The arrangement of these nitrogenous bases– How? (Next slide)

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§ DNA Structure (4)

Law of complementary base pairing:

•Base pairs (2 kinds):

– A-T and C-G

•Nitrogenous bases form hydrogen bonds

Su

gar-p

ho

sph

ate backb

on

e

Su

gar-p

ho

sph

ate backb

on

e

Segment of DNA

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§ DNA Function

• Carry instructions of genes for protein synthesis

• A gene – a segment of DNA that codes for one polypeptide (or closely related proteins)– Genes determine the characteristics of a species and

each individual

• Genome - all the genes of one person– humans have estimated 25,000-35,000 genes (2% of

DNA)– The other 98% of DNA is noncoding – either “junk”

or organizational DNA

Think About It1. What would be the base sequence

of the DNA strand across from ATTGACTCG?

2. If a DNA molecule were known to be 20% adenine, predict its percentage of cytosine.

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§ Chromatin and Chromosomes1. Chromatin—filamentous material making

up 46 chromosomes (DNA and proteins) in the interphase nucleus

– Chromatin appears like “beads on a string” packed together (Fig. 4.2 a-f)

– The beaded string is divided into segments called nucleosomes (consist of histones and linker DNA)

2. In dividing cells, DNA coils and supercoils itself to form chromosomes (can be seen with light microscope) . Fig. 4.5

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2 nm DNA doublehelix

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Nucleosome

Linker DNA

Core particle

2 DNA windsaround coreparticles

11 nm

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30 nm 3 NucleosomesFold into zigzagfiber

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300 nm fiber isthrown intoirregular loops

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700 nm

In dividing cells only

5 loopedchromatin coilsfurther into achromatid

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700 nm

Chromatids Centromere

6 Chromosomeat the midpoint(metaphase) ofcell division

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(a)

Centromere

Kinetochore

Sister chromatids

Chromosome structure at metaphase

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§ RNA (ribonucleic acids): Structure1. RNA-- much smaller than DNA (fewer bases)

A. messenger RNA (mRNA) has over 10,000 basesB. ribosomal RNA (rRNA)C. transfer RNA (tRNA), smallest, has 70 - 90 bases

(Fig. 4.8)– Are these bases (of RNAs) paired or unpaired?

2. Only one nucleotide chain (not a double helix)– ribose replaces deoxyribose as the sugar– uracil replaces thymine as a nitrogenous base

Figure 4.8Transfer RNA (tRNA)

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§ RNA: Function• DNA directs the synthesis of proteins by means

of its smaller cousins, the RNAs• Essential function of RNA--

– interpret DNA code– direct protein synthesis in the cytoplasm

• (Location) RNA works mainly in the cytoplasm while DNA remains safely behind in the nucleus

• Table 4.1 is an excellent summary (Comparison of DNA/RNA)

Check Point QuestionsWhat are four nitrogenous bases found in RNA?

a) U, G, C, T; b) A, G, C, T

c) A, U, G, C; d) A, T, G, C

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In RNA, when does the secondary structure called a hairpin form?a)When hydrophilic residues act with waterb)When complementary base pairing between ribonucleotides on the same strand creates a stem-and-loop structurec)When complementary base pairing forms a double helix

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§ 4.2 Genes and Their Action

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§ Protein Synthesis: Genetic Control of Cell Action

• DNA codes for the synthesis of all cell proteins– including enzymes that direct the synthesis of

nonproteins– For example, Testosterone production

• Different cells synthesize different proteins– Why?– Due to differing gene activation

• See Fig. 4.13 (next slide)

= LH

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§ Summary of Protein Synthesis• DNA contains a genetic code that specifies which

proteins a cell can make; protein synthesis as: DNA mRNA protein

1. Transcription (DNA mRNA); What? Details?– messenger RNA (mRNA) is formed next to an

activated gene– mRNA migrates to cytoplasm

2. Translation (mRNA protein) (Fig. 4.7) What? How?– mRNA code is “read” by ribosomes– transfer RNA (tRNA) delivers the amino acids to the

ribosome– Ribosomes assemble amino acids in the order . . .

§ Genetic Code• Def. -- System that enables the 4 nucleotides (A,T,G,C) to

code for the 20 amino acids• Base triplet: (of DNA) Fig. 4.10

– Def.– A sequence of 3 nucleotides that stand for 1 amino acid

– found on DNA molecule (ex. TAC codes for AUG in mRNA)

• Codon: (genetic code is expressed in terms of codons)– Def.--“mirror-image” sequence of nucleotides found in

mRNA (ex. AUG is the codon of mRNA, code for methionine, an amino acid) (Table 4.2)

– 64 possible codons (43)

• often 2-3 codons represent the same amino acid

• start codon = AUG

• 3 stop codons = UAG, UGA, UAA 28

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Seven base triplets

A

B

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§ Protein Synthesis (details)

• Three sites of the large subunit of ribosome:

i. P (peptidyl) site—

ii. A (acceptor) site—

iii. E (exit) site--

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• Watch a video-- An animation: protein synthesis, when available

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