Principles of Biology contents 12 Nucleic Acids12) 12...Nucleic acids are polymers of nucleotides....

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12 Nucleic Acids

The structure of nucleic acid molecules allows for the coding and transmission ofinformation.

The best-known nucleic acid, DNA, occurs in the form of a double helix.The DNA double helix is made up of two intertwined strands of nucleotides. The nucleotide strands have a sugar-phosphate backbone, and each nucleotide contains one of four bases, adenine, cytosine, guanine, or thymine.© 2014 Nature Education All rights reserved.

Topics Covered in this Module Structure of Nucleic AcidsFunctions of Nucleic Acids

Major Objectives of this Module Describe the structure of nucleotides.Explain how nucleotides assemble into polymers.Distinguish between DNA and RNA.Explain the roles of nucleic acids in transmission and expression of hereditary information.


12 Nucleic Acids

Structure of Nucleic AcidsThe human body contains an amazing variety of different types of cells thatoriginate from just one fertilized egg. But how does that single cell manage togrow and differentiate into the diverse cells that make up a human being? Allthe information needed for this amazing transformation from a single cell to acomplex organism comes from one type of biological molecule calleddeoxyribonucleic acid (DNA). DNA encodes all the information needed tocreate life's diversity. It is responsible for the large variety of cells within anorganism as well as the diversity among organisms of all species. How is itthat a simple DNA molecule encodes the complexity of life? What are thebuilding blocks of this amazing molecule?

Nucleic acids are polymers of nucleotides.DNA is a nucleic acid. Along with lipids, carbohydrates, and proteins, nucleicacids are one of the four classes of large biological molecules that areessential to cellular structure and function. Nucleic acids include both DNA,which encodes genetic information, and ribonucleic acid (RNA), amacromolecule that has multiple roles, the most well studied of which is todeliver information from DNA to sites of protein synthesis. Likecarbohydrates and proteins, nucleic acids are polymers, meaning theyconsist of multiple repeating monomers. For both DNA and RNA, therepeating monomers are nucleotides that are arranged in a wide variety ofsequences to make up polynucleotides.

The structure of the nucleotide.Both DNA and RNA are made up of nucleotide monomers, each of whichcontains three parts: a phosphate functional group (–PO4

2-), a five-carbonsugar, or pentose, and a nitrogenous base (often referred to simply as the"base") (Figure 1a). When a sugar is linked to a nitrogenous base but thephosphate group is absent, this is called a nucleoside. Five different typesof nitrogenous base occur in nucleic acids: guanine, thymine, adenine,cytosine, and uracil.

Because the nucleotide is made up of two different carbon-based units (thenitrogenous base and the pentose), a special notation has been developedto distinguish the carbons in the two structures. The carbons in thenitrogenous base are numbered 1, 2, 3, etc., while the carbons in thepentose are numbered 1′, 2′, 3′, etc. The symbol that looks similar to anapostrophe is referred to as a prime, so the pentose number 1′ is written"one-prime." Notice that in a nucleic acid, the phosphate group is attached tothe 5′ (five-prime) carbon. This numbering becomes important when we learnin the next section about the linkages that connect nucleotides into nucleicacids.

DNA and RNA differ in two different ways. First, the chemical structure of thenucleotides found in these two classes of nucleic acids differs. DNA containsguanine, thymine, adenine, and cytosine. RNA contains uracil in place ofthymine. Second, a different type of sugar is present in DNA than in RNA.The sugar found in RNA is ribose, and the sugar found in DNA isdeoxyribose. Ribose and deoxyribose are nearly identical: both arepentoses that are closed into a ring in the nucleotide. The only difference isthat ribose has a hydroxyl group attached to the 2′ carbon and deoxyribosedoes not. As the oxygen atom is missing from the second carbon indeoxyribose, the sugar is more accurately called 2-deoxyribose.

Figure 1: The structure of thenucleotide.

© 2014 Nature Education All rightsreserved.

a) The three components of anucleotide include a phosphatefunctional group, a pentose, and anitrogenous base. b) The pentosesugar is a key feature that distinguishesRNA from DNA. The sole differencebetween ribose (found in RNA) anddeoxyribose (used in DNA) is the 2′carbon. In ribose, the 2′ carbon isbound to a hydroxyl group. Indeoxyribose, the –OH is absent, and ahydrogen is in its place. This smalldifference makes RNA much morechemically reactive than DNA.

Nitrogenous bases include purines and pyrimidines.The five nucleotides found in DNA and RNA are divided into two separatefamilies — pyrimidine or purine — based on their structure (Figure 2). Apyrimidine has a six-member ring containing four carbons and twonitrogens. The pyrimidines include cytosine (C), thymine (T), and uracil (U).Cytosine and thymine are found in DNA, and cytosine and uracil (U) arefound in RNA. A handy mnemonic to remember the pyrimidine bases is"CUT the py" — the letters in CUT represent the three pyrimidines: cytosine,uracil, and thymine. A purine molecule consists of a five-member ringattached to a six-member ring. Each ring has two nitrogens. The purinesinclude adenine (A) and guanine (G), which are found in both DNA and RNA(Figure 2).

Figure 2: Two types of nitrogenous bases are found in DNA and RNA.

© 2012 Nature Education All rights reserved.

a) Pyrimidines consist of a single ring of four carbon atoms and twonitrogen atoms. The three pyrimidines, cytosine, thymine, and uracil, differin the presence of amine (–NH2) and methyl (–CH3) functional groups. b)Purines consist of a five-member ring connected to a six-member ring.The rings contain four nitrogens. The two purines, adenine and guanine,differ in the number and position of amine and carbonyl functional groups.

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Compare the main features of a pyrimidine molecule with a purine molecule.

Nucleic acids form when phosphodiester linkages connect nucleotidemonomers.A phosphodiester linkage bonds two nucleotides together. Aphosphodiester linkage (also called a phosphodiester bond) forms when thephosphate group of one nucleotide becomes covalently bound to thehydroxyl group at the 3′ carbon of another nucleotide (Figure 3). Note inFigure 3 that the entire polymer molecule has a 5′ end and a 3′ end. The 3′end typically is a free –OH, and the 5′ end typically is a phosphate group(although it could be an –OH). Thus, a nucleic acid molecule has polarity.This concept of polarity is important and will appear again in this modulewhen we talk about the sequence of DNA and about DNA replication andtranscription. Together, the repeating phosphate and pentose units form thebackbone of the nucleic acid. Because the phosphate groups carry anegative charge, they face outward, toward water. The hydrophobicnitrogenous bases face inward, where they are sequestered from water.

Figure 3: Section of a strand of DNA.


DNA is a repeating polymer of nucleotides held together byphosphodiester linkages (also called phosphodiester bonds). In thisillustration, one of each of the four types of nucleotides found in DNA isincluded.

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Test Yourself

Describe a phosphodiester linkage. In what molecules is this linkage found?

Controversy.

Figure 4: British scientist Rosalind Franklin provided data that werekey to discovering the structure of DNA.

Who discovered the structure of DNA? Most people would answer, "Watsonand Crick." But the populist version of the discovery of DNA omits some keyplayers. One of these players was Maurice Wilkins who, along with Crick andWatson, was awarded the Nobel Prize for the 1953 discovery of the DNAdouble helix structure. Rosalind Franklin, however, was not so honored.During the early 1950s, several research groups were racing to solve thepuzzle of DNA's structure. Francis Crick and James Watson proposed oneidea, based on a triple helix. At a scientific meeting, Rosalind Franklin(Figure 4), working in another group, pointed out that such a model did not fitwith the data she had seen using X-ray crystallography. She insteadproposed that DNA forms a double helix with the phosphate backbone on theoutside, but she wanted to get better data before publishing. While Franklinwas toiling away, Crick and Watson obtained Franklin's unpublished X-raydiffraction patterns of DNA from Maurice Wilkins, without Franklin'spermission or knowledge. They then used this information and Franklin'sinterpretation of the data to propose their new model, the subject of theirlandmark paper published in 1953. Franklin died in 1958 at the age of 38 anddid not share in the Nobel Prize, which is not awarded posthumously. Thereis no doubt, however, that Franklin's work was a key to unlocking the puzzleof DNA structure.

Science Source.

Rosalind Franklin provided exceptional quality data from X-ray diffractionpatterns of purified DNA. Her data allowed Crick and Watson to derive themolecule's double helix structure.

DNA and RNA form complex secondary structures.As often happens in science, Watson and Crick built their model on afoundation laid by earlier scientists. By the 1950s, it was already knownthrough the work of Oswald Avery and his colleagues that DNA is themolecule of heredity that makes up genes and chromosomes. Around thesame time, Austrian chemist Erwin Chargaff established what is now knownas Chargaff's Rule: the amount of cytosine in cellular DNA always equals theamount of guanine, and the amount of thymine always equals the amount ofadenine.

Based on this information and Rosalind Franklin's X-ray crystal structure,Crick and Watson created a three-dimensional model and proposed thatDNA is a double helix consisting of two intertwined strands of DNA, with aphosphate-sugar backbone on the exterior. On the interior, hydrogen bondsform between adenine and thymine and between guanine and cytosine(Figure 5). This hydrogen bonding is referred to as base pairing. Thespecificity of base pairing accounts for Chargaff's rule. Furthermore, thehydrogen bonds that form between the base pair provide the "glue" thatholds two strands of DNA together. The beauty of this model is that itsuggested, in the words of Crick and Watson, "a possible copyingmechanism for genetic material." The Watson-Crick model confirmedFranklin's inference from her X-ray diffraction data that the phosphate groupswere on the outside of the helix. It also corresponded with the measurementsFranklin provided on the dimensions of the DNA molecule.

Crick and Watson's model also showed that each strand in the double helixhad directionality. That is, the 5′ to 3′ direction of one strand was opposite tothe complementary strand, oriented in a 3′ to 5′ direction. This distinction iscrucial because a DNA sequence of bases running in one direction is not thesame as the identical sequence running in the other direction. For example,the sequence of bases 5′-ATCGTG-3′ is not the same as 3′-ATCGTG-5′. Twocomplementary strands of DNA running in opposite directions are said to beantiparallel. Again, by convention, the sequence of DNA is always written inthe 5′-to-3′ direction. So the second sequence above would properly bewritten as follows: 5′-GTGCTA-3′.

Figure 5: DNA is antiparallel and forms hydrogen bonds betweenspecific pairs of bases.


In a double-stranded molecule of DNA, the two strands are antiparallel —they run in opposite directions. The arrows indicate the "forward" directionof DNA from the 5′ carbons of the deoxyribose sugars in the backbone tothe 3′ carbons. In double-stranded DNA, guanine (G) forms threehydrogen bonds with cytosine (C), and adenine (A) forms two hydrogenbonds with thymine (T). The specific base pairing between G–C and A–Texplains Chargaff's observations and suggests that if the sequence of onestrand is known, the sequence of the other strand can be inferred.

BIOSKILL

Determine Whether a DNA Sample Is from a Bacterium or VirusExamine the data in Table 1, which shows the proportions of each of the fourbases present in DNA samples from various organisms. Notice that in mostcases, there is an approximately equal ratio of A:T and G:C. What does thissignify? It means that the DNA is likely double-stranded. This is because anytime there is an A on one strand, there will be a T on the complementarystrand, and vice versa. The same is true for a C or G; every time there is a Con one strand, there is a G on the complementary strand, and vice versa.However, a virus can be a single strand of DNA, so these ratios would notapply.

Relative proportions (%) of bases inDNA

Sample A T G C

A 30.9 29.4 19.9 19.8

B 28.8 29.2 20.5 21.5

C 29.3 29.3 20.5 20.7

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BIOSKILL

Figure 6: Molecular representations of DNA.

© 2012 Nature Education http://www.ks.uiuc.edu/Research/vmd/Used with permission. All rights reserved.

Figure Detail

(a) The DNA double helix contains two polynucleotide chains arrangedantiparallel to one another. The sugar-phosphate backbones are on theoutside. On the inside, A-T and G-C base pairs hold the two strands of thenucleic acid together. (b) A molecular model of DNA showing much of thesame information as the ribbon model in panel (a). nm = nanometers.

D 19.1 30.2 18.9 31.7

E 32.8 32.1 17.7 17.3

F 27.3 27.1 22.7 22.8

G 31.3 32.9 18.7 17.1

H 24.7 23.6 26.0 25.7

Table 1: Base ratios in DNAsamples.Based on the relative proportions of thedifferent bases in DNA, it is possible todetermine whether a sample of DNAcomes from a single-stranded DNAvirus or an organism whose DNA isdouble-stranded.

Test Yourself

Which of the samples listed in Table 1 is likely a single-stranded DNA virus? Explain youranswer.

Other structural characteristics of DNA.Examine Figure 6 to see the dimensions of a molecule of DNA.

DNA in the helix can be twisted in one of several forms, or conformations.There are three DNA helix conformations thought to be biologically active.The most common shape in living cells is called B-DNA form, which closely

aligns with the Watson and Crick model of DNA structure. A-DNA is similar toB-DNA but is shorter and more compact in nature. Both A-DNA and B-DNAtake the shape of a right-handed double helix, which twists to the right-handside. A third type is Z-DNA, which resembles B-DNA except the helix isleft-handed in directionality.

RNA structure differs dramatically from DNA structure.Like the bases in DNA, the bases in RNA can form base pairs. However,while base pairing in DNA usually occurs between two different nucleic acidmolecules, base pairing in RNA typically occurs within a single RNAmolecule. The RNA molecule folds over, and bases on one part of the strandform base pairs with complementary bases on another part. This feature ofRNA results in a wide variety of so-called secondary structures. Onecommon structure is the stem-loop, which forms a secondary structure calleda hairpin (Figure 7).

Most often, DNA is double-stranded and serves as the genetic material,while RNA is single-stranded and serves many functions, including shuttlinggenetic instructions to the protein-making machinery of the cell. However,there are exceptions, particularly in viruses. In viruses, RNA can serve as thegenetic material, and the RNA can be double-stranded. For example, therotavirus, which causes diarrhea in children, is a double-stranded RNA virus.In other viruses, DNA serves as the genetic material, but the DNA may bedouble-stranded or single-stranded.

Figure 7: A stem-loop hairpin occurs in RNA when a sequence foldsback on itself.

© 2014 Nature Education All rights reserved. Figure Detail

When a strand of RNA folds over itself, the two folded strands maybecome joined due to hydrogen bonding between complementary basepairs. An unpaired loop may arise in an area where base pairing betweenthe folded strands cannot occur.

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What is the main difference between the double helix formed in a DNA molecule comparedwith that formed in the RNA molecule?

Structure of Nucleic Acids

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Functions of Nucleic Acids

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Chargaff In His Own Words

Who Discovered the Shape of DNA?

DNA Structure and Function

How Fast Can You Build DNA?

PRIMARY LITERATURE

Classic paper: The discovery ofthe neutron (1932)Possible existence of a neutron.

Classic paper: The idea of the DNAdouble helix (1953)Molecular structure of nucleic acids.

Classic paper: X-ray diffractionreveals helical structure of DNA(1953)Molecular configuration in sodiumthymonucleate.

Classic paper: The firstsequencing of a complete DNAgenome (1977)Nucleotide sequence of bacteriophageΦX174 DNA.

Classic paper: X-rays reveal thestructure of myoglobin (1958)A three-dimensional model of the myoglobinmolecule obtained by X-ray analysis.

SCIENCE ON THE WEB

Listen to an interview with Erwin Chargaffand how he discovered base pairing

Read a short bio on a woman who made alarge contribution to discovering helicalstructure of DNA

Read about the discovery of DNA

Match the base pairs to build a DNA doublehelix.


12 Nucleic Acids

Functions of Nucleic AcidsWhy were Crick, Watson, Franklin, and other teams so motivated to discoverthe structure of DNA? From cellular studies, scientists knew that hereditarymaterial was located in the nucleus of the cell. They also believed that DNAwas the molecule most likely to convey hereditary information. They believedthat revealing the structure of DNA was perhaps the most significantcontribution in biology since Charles Darwin wrote On the Origin of Species. WhenCrick and Watson wrote that the structure "suggests a possible copyingmechanism for the genetic material," they ushered in a new era of biology.Their publication was the beginning of a transformation in genetics, cellbiology, and biochemistry and was arguably the birth of molecular biology.

Replication of DNA copies hereditary information.So why did Crick and Watson think that DNA could copy genetic information?The key, they realized, was the base-pairing rules. When the two DNAstrands were separated, each would be able to form another helix identical tothe original. While the details took years of subsequent research to work out,the basic principle is beautifully simple. The double helix unravels,separating the two strands. Enzymes then use each parent strand as atemplate to make a new, complementary strand by attaching thecomplementary nucleotides. One of the enzymes aids in the formation of thephosphodiester bonds that make up the backbones of the two new strands.The result is two DNA double helices identical to the original (Figure 8).

Figure 8: Two identical DNA molecules can be produced from asingle original molecule.


During DNA synthesis, the DNA double helix partly unravels, and eachparent strand is used as a template to form identical daughter strands.

The ordering of nucleotide bases encodes hereditary information.Although Crick and Watson deduced the structure of DNA and speculated asto how it could replicate, they did not know how the bases stored hereditaryinformation. How did the sequence of bases on the DNA molecule storeinformation that could be passed on from one generation to the next? Thekey is in the order of the bases. The order is preserved from the DNAtemplate strand, or parent molecule, to the new daughter molecules. In thisway, the daughter molecules preserve the hereditary information encoded inthe parent DNA.

Scientists conducting the earliest genetic studies were aware that units ofheredity are discrete. That is, traits are passed on as units, individual parts ofa whole. Gregor Mendel showed that traits of pea plants were inherited asunits. In the late 19th century, geneticist Hugo de Vries coined the termgene: the smallest unit of heredity in organisms. But what were these units?How did the structure of DNA relate to genes? That was the challenge forscientists after Crick and Watson published the structure of DNA.

Although the details that related DNA to genes are complex, the principle is

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straightforward. A sequence of nucleotide bases in DNA makes up a gene.With its ordered sequence of bases, a gene contains the instructions tomake proteins. The type of proteins present in a cell determines theparticular properties or functions of that cell. Thus, despite being a relativelysimple molecule, DNA directs the synthesis of the proteins that createcomplex structures and perform diverse cellular functions. We will learnmuch more about these processes in later modules.

RNA has many roles in a cell.The roles of DNA in a cell are few. DNA is the keeper of the genes thatencode hereditary information. DNA provides the blueprint for what cellsform and how they function. In contrast, RNA plays multiple roles in a celland an organism. RNA molecules come in many different forms, and theirfunctions vary depending on the type of RNA. The most famous RNA ismessenger RNA (mRNA), which encodes the information from a gene andcarries it to the machinery that decodes the mRNA and makes proteins.Associated with the process of making proteins are two other well-studiedtypes of RNA: ribosomal RNA (rRNA) and transfer RNA (tRNA). Morerecently, many other types of RNA molecules have been identified thatregulate key cellular processes, such as the making of proteins, celldifferentiation, and chromosome stability. For now, let's focus only on theRNA molecules involved in protein synthesis. We will look at all of these RNAmolecules in more detail in later modules.

How does DNA communicate the information it contains in its genes tosynthesize proteins? Again, the key is in the base pair rules that govern howcomplementary polynucleotide strands form from a template. The process ofRNA synthesis is known as transcription. During transcription, the DNAdouble helix partly unwinds so that RNA can be synthesized from thecomplementary strand. In eukaryotes, a messenger RNA molecule typicallyencodes a single gene, while in prokaryotes, the mRNA can contain multiplegenes in a row. In eukaryotes, the mRNA migrates out of the nucleus toribosomes in the cytoplasm. Ribosomes are large complex structures madefrom a combination of rRNA and over 50 proteins. The ribosomes decodethe RNA sequence and synthesize the appropriate protein, linking aminoacids together. The process of converting the "message," that is, thesequence of nucleotides in mRNA, into a sequence of amino acids requirestRNA as the adaptor molecule.

The structural diversity of RNA molecules, which is exemplified by mRNA,rRNA and tRNA, is the basis for RNA's many functional roles in cells. RNAeven functions as an enzyme in certain cellular processes. The discoverythat RNA could function as an enzyme was a surprise because people longthought enzymatic activity was the sole province of proteins.

The functions of both DNA and RNA are directly related to their structures.The double-stranded helix of DNA provides a way for the genome to bereplicated and passed on to progeny. RNA, in contrast, plays a multitude ofroles and comes in various forms, including mRNA, tRNA, and rRNA. Eachform of RNA serves a specialized function. Without RNA, the cell would haveno way to make proteins, which serve as enzymes, structural components,and membrane components. Without DNA, RNA would have no informationwith which to produce these important cellular components.

Test Yourself

How does DNA communicate the hereditary information it contains to synthesize proteins?

© 2012 Nature Education All rights reserved. Transcript

CAREERS

CAREERS

Figure 9 reviews the correct structure of a nucleotide.

Figure 9: Creating a nucleotide.A nucleotide contains a nitrogenous base, a five-carbon sugar, and aphosphate group.

Knowledge of Nucleic Acids Applied to BiotechnologyOne of the top challenges humankind faces is how to feed Earth'sburgeoning population. Many scientists believe that biotechnology holds theanswers. Private companies, governments, and research foundations arepumping funds into biotechnology to develop high-yielding, disease resistantand nutritionally enhanced crop varieties. These organizations need expertswho understand the biochemistry of nucleic acids and the techniques tostudy their structure and function. Other growing fields includebiotechnological approaches to treating human disease and greenapproaches to remediation of waste and pollution, such as the use ofgenetically altered bacteria to consume oil spilled into the environment.There is no doubt that demand for expertise in biotechnology will outstripsupply. There are many opportunities in these fields that apply knowledge ofnucleic acids, their structure, and their composition. By all accounts,biotechnology is a growing industry that will have an impact on our Earth'sfuture.



Summary

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12 Nucleic Acids

OBJECTIVE Describe the structure of nucleotides.A nucleotide is made up of three components: a phosphate group, a ribosesugar with five carbon atoms in a ring shape, and a nitrogenous base.

OBJECTIVE Explain how nucleotides assemble into polymers.Nucleotides assemble into polymers via the formation of phosphodiesterbonds between the 5′ carbon of one nucleotide's sugar and the 3′ carbon ofanother nucleotide's sugar. The phosphate functional group is part of thislinkage. These covalent bonds hold together the sugar-phosphate backboneof the polynucleotide.

OBJECTIVE Distinguish between DNA and RNA.DNA is a double helix made from two strands of deoxyribonucleic acids. Inthe double helix, A base pairs with T, and G base pairs with C. RNA istypically single-stranded and is made from ribonucleic acids. RNA can foldinto hairpins in which A base pairs with U and G base pairs with C. Thisstem-loop configuration is a common secondary structure of RNA.

OBJECTIVE Explain the roles of nucleic acids in transmission andexpression of hereditary information.

The sequence of bases in the DNA molecule encodes hereditary information.The RNA molecule is a copy of information encoded in the DNA sequence.RNA can function as a template that transfers information out of the nucleusto the cytoplasm (in eukaryotes), where ribosomes read the RNA andsynthesize proteins based on the sequence of bases in the RNA molecule.

antiparallelDescribing the opposite orientation of two complementary strands: 5' to 3' and 3'to 5'.

base pairingHydrogen bonding between nitrogenous bases within polynucleotides; specificallyhydrogen bonding between guanine and cytosine and between adenine andthymine in DNA; in RNA the base pairs are guanine and cytosine, and adenineand uracil.

conformationThe spatial arrangement or shape of a macromolecule, such as a protein ornucleic acid.

deoxyribonucleic acid (DNA)The primary molecule of inheritance in all organisms (with the exception of someviruses); a double-stranded nucleic acid containing nucleotides that containdeoxyribose.

deoxyriboseFive-carbon sugar found in DNA nucleotides; differs from ribose in that it lacks ahydroxyl (OH) group at the 2′-carbon.

DNASee deoxyribonucleic acid.

double helixTwo antiparallel strands held together by hydrogen bonds coiled into a helicalshape. Sometimes also referred to as a "twisted ladder."

Summary

Key Terms

geneSegment of DNA that encodes a particular protein.

hairpinImportant secondary structure formation in RNA.

nitrogenous baseComponent of nucleotides that makes each unique: adenine, guanine, cytosine,thymine, and uracil; grouped into two families: pyrimidines and purines. Keyelement of complementary base-pairing of nucleic acids.

nucleic acidOne of the four classes of large biological molecules; DNA and RNA are nucleicacids.

nucleosideComponent of a nucleotide: a five-carbon sugar linked to a nitrogenous basewithout the phosphate group present.

nucleotideAn organic molecule containing a nitrogenous base, one or more phosphates, anda five-carbon sugar molecule; the monomer of nucleic acids.

phosphodiester bondCovalent bond in nucleic acids that links nucleotides together; connects the5′-carbon on one sugar with the 3′-carbon on the next sugar via the phosphategroup.

polynucleotideRepeating monomers of nucleotides linked together.

purineA nitrogenous base containing a five-member ring connected to a six-memberring. The rings contain four nitrogens; one of the two types of nitrogenous bases innucleotides; includes adenine and guanine.

pyrimidineComposed of a six-member ring containing four carbons and two nitrogens; one ofthe two types of nitrogenous bases in nucleotides; includes cytosine, thymine, anduracil.

ribonucleic acid (RNA)Nucleic acid containing ribose as the sugar. Used to transmit genetic informationto ribosomes in the cell.

riboseFive-carbon sugar found in RNA nucleotides.



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12 Nucleic Acids

1.

Bases in RNA include adenine, cytosine, guanine and thymine. DNA bases includeadenine, cytosine, guanine and uracil.Bases in DNA include adenine, cytosine, guanine and thymine. RNA bases includeadenine, cytosine, guanine, and uracil.Only purines are found in DNA and only pyrimidines are found in RNA.The nucleotide compositions of DNA and RNA do not differ except that RNA has aribose sugar and DNA has a deoxyribose sugar.Only purines are found in RNA, whereas only pyrimidines are found in DNA.

How does the base composition of DNA and RNA differ?

2.

It is a disaccharide.It is a hexose.It includes an amine functional group.It is connected by the 1-position carbon to a phosphate functional group.None of the answers are correct.

Which of the following correctly describes the sugar component of a nucleotide?

3.

The orientation of the phosphodiester bond on the 5′ and 3′ carbons of linkedsugar molecules gives the polynucleotide directionality.The orientation of the phosphodiester bond on the 5′ and 3′ carbons of linkedphosphate functional groups gives the polynucleotide directionality.The orientation of the phosphodiester bond on the 5′ and 3′ carbons of linkedamine functional groups gives the polynucleotide directionality.There is no significance of the directionality because 3′ to 5′ directionality is fornotational convenience.None of the answers are correct.

Why is the directionality of a nucleic acid specified as running from 5′ to 3′ ?

4.

cytosinepyrimidineThere is not enough information to determine the answer.purineeither guanine or thymine

What kind of nucleotide base has a two-ring structure with an amine functionalgroup?

5.

5′-AACTTGA-3′3′-GGTCCAG-5′5′-AACGGTA-3′3′-TTGCCAT-5′5′-AACGGTA-3′5′-TTGCCAT-3′5′-AACGGTA-3′3′-AACGGTA-5′5′-AACGGUA-3′3′-UUGCCAU-5′

Which of the following sequences could theoretically base pair in a short segmentof DNA?

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6.

migrates to the nucleus, it is copied by the DNA strand, which is then "read" byribosomes to synthesize proteins.is synthesized in the nucleus using DNA as the template, the mRNA migrates tothe cytoplasm, where ribosomes use mRNA as a template to direct the synthesisof proteins.copies a sequence of bases from DNA in the nucleus, it rearranges the basesequence to form genes that code for specific proteins.replicates from a parent strand, two daughter strands form that are different fromthe parent strand.is transcribed from a sequence of bases in DNA in the cytoplasm, it migrates to thenucleus, where ribosomes use the RNA as a template to direct the synthesis ofproteins.

Complete the following sentence: After a eukaryotic mRNA strand...

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