Nucleic acids (DNA and RNA)

BiologyMolecular Basis of InheritanceFor NEET aspirants

Nucleic acids (DNA and RNA) are the molecules that store and express genetic information. This page covers the polynucleotide chain, the Watson-Crick double helix and the central dogma. It then explains the experiments of Griffith, Avery and Hershey-Chase that proved DNA is the genetic material, the RNA world, and semiconservative replication with the Meselson-Stahl experiment and the replication fork, as in the NCERT Class 12 chapter Molecular Basis of Inheritance. NEET regularly asks base pairing, Chargaff's rule and replication from nucleic acids (DNA and RNA).

On this page1Genetic material2Polynucleotide chain3Double helix4Central dogma5Search for genetic material6DNA versus RNA7RNA world8Replication9Exam essentials10Quick revision11Solved examples12Practice

Key Points at a Glance

★ Must learn DNA is the genetic material in most organisms; RNA is the genetic material in some viruses.
Nucleotide = nitrogenous base + pentose sugar + phosphate group.
★ Must learn Nucleotides join by 3′-5′ phosphodiester linkages; a chain has a 5′ phosphate end and a 3′ OH end.
★ Must learn Chargaff's rule: in double-stranded DNA, A : T = G : C = 1. A pairs with T by two H-bonds, G with C by three.
Double helix: anti-parallel, right-handed, pitch 3.4 nm, about 10 bp per turn, 0.34 nm between base pairs.
Central dogma: DNA $\to$ RNA $\to$ protein; some viruses show RNA $\to$ DNA (reverse transcription).
★ Must learn Griffith (1928): transforming principle. Avery, MacLeod and McCarty: it is DNA. Hershey and Chase (1952): unequivocal proof.
DNA is chemically less reactive and structurally more stable, so it stores genetic information; RNA is better for transmitting it.
RNA was the first genetic material (RNA world).
★ Must learn Replication is semiconservative: proved by Meselson and Stahl (1958) using $^{15} N$ and a CsCl density gradient.
DNA polymerase works only $5^{'} \to 3^{'}$ : synthesis is continuous on one template and discontinuous on the other; DNA ligase joins the pieces.

1. DNA and RNA: the Genetic Material

In Mendel's time, the nature of the factors that control inheritance was not clear.
Over the next hundred years, DNA (deoxyribonucleic acid) was shown to be the genetic material, at least in most organisms.
Nucleic acids: polymers of nucleotides. Living systems have two types, DNA and RNA (ribonucleic acid).
★ Exam imp DNA is the genetic material in most organisms. RNA is the genetic material in some viruses.
RNA mostly works as a messenger. It also works as an adapter, a structural molecule and, in some cases, a catalytic molecule.

1.1 Length of DNA

DNA is a long polymer of deoxyribonucleotides.
Its length is given as the number of nucleotides, or of nucleotide pairs, called base pairs (bp).
This length is a characteristic of each organism.

Organism	Length of DNA
Bacteriophage $ϕ$ X174	5386 nucleotides
Bacteriophage lambda	48502 bp
Escherichia coli	$4.6 \times 1 0^{6}$ bp
Human (haploid content)	$3.3 \times 1 0^{9}$ bp

Memory Trick Read the table from the smallest genome to the largest: phage $ϕ$ X174 (thousands), lambda (tens of thousands), E. coli (millions), human (billions).

★ Exam imp Watch the unit: $ϕ$ X174 is given in nucleotides, while lambda, E. coli and human DNA are given in base pairs.

Key idea

DNA is the genetic material of most organisms, and the length of its DNA is a fixed feature of each organism.

2. Structure of a Polynucleotide Chain

A nucleotide has three parts: a nitrogenous base, a pentose sugar and a phosphate group.
The sugar is ribose in RNA and deoxyribose in DNA.
★ Exam imp Purines: adenine (A) and guanine (G). Pyrimidines: cytosine (C), uracil (U) and thymine (T).
Cytosine is common to both DNA and RNA. Thymine is present in DNA; uracil takes its place in RNA.

Memory Trick Pure As Gold: the purines are Adenine and Guanine. CUT the pie: the pyrimidines are Cytosine, Uracil and Thymine.

2.1 From base to polynucleotide

Nucleoside: a nitrogenous base linked to the OH of the 1′ C of the pentose sugar by an N-glycosidic linkage.
Nucleotide: a phosphate group linked to the OH of the 5′ C of a nucleoside by a phosphoester linkage. With deoxyribose it is a deoxynucleotide.
Dinucleotide: two nucleotides joined by a 3′-5′ phosphodiester linkage.
Polynucleotide chain: many nucleotides joined in the same way.

Nitrogenous base	Nucleoside (with ribose)	Deoxynucleoside (with deoxyribose)
Adenine	Adenosine	Deoxyadenosine
Guanine	Guanosine	Deoxyguanosine
Cytosine	Cytidine	Deoxycytidine
Uracil (RNA) / thymine (DNA)	Uridine	Deoxythymidine

★ Exam imp 5′ end of the chain: the sugar carries a free phosphate on its 5′ C. 3′ end: the sugar carries a free OH on its 3′ C.
The backbone is made of sugar and phosphate. The bases are linked to the sugars and project from the backbone.
In RNA, every ribose has an extra -OH group at the 2′ position.
In RNA, uracil is found in place of thymine. Thymine is 5-methyl uracil.

Test yourself: hide labels

Figure 1: A polynucleotide chain. Sugar and phosphate form the backbone and the bases project from the sugars; the chain has a 5′ phosphate end and a 3′ hydroxyl end.

★ Very important Three different links build a nucleic acid: base to sugar by an N-glycosidic linkage, phosphate to sugar by a phosphoester linkage, and nucleotide to nucleotide by a 3′-5′ phosphodiester linkage.

NucleosideBase + pentose sugar.
Example: adenosine, deoxythymidine.
No phosphate.

NucleotideBase + pentose sugar + phosphate.
The monomer of DNA and RNA.
Phosphate on the 5′ C.

Key idea

Base + sugar = nucleoside; + phosphate = nucleotide; nucleotides joined by 3′-5′ phosphodiester links = polynucleotide.

3. The DNA Double Helix

3.1 Discovery

★ Exam imp Friedrich Miescher (also spelt Meischer) first identified DNA in 1869, as an acidic substance in the nucleus. He named it nuclein.
Such a long polymer was hard to isolate intact, so its structure stayed unknown for a long time.
In 1953, James Watson and Francis Crick proposed the double helix model of DNA.
Their model used the X-ray diffraction data of Maurice Wilkins and Rosalind Franklin.
A hallmark of the model was base pairing between the two polynucleotide chains.
It also used Erwin Chargaff's observation: in double-stranded DNA, the ratios A : T and G : C are constant and equal to one.

3.2 Complementary strands

Base pairing makes the two chains complementary to each other.
So if the base sequence of one strand is known, the sequence of the other can be predicted.
If each strand of a parental DNA acts as a template for a new strand, the two daughter DNA molecules will be identical to the parent.
This is how the structure at once explained the copying of genetic material.

3.3 Salient features of the double helix

It has two polynucleotide chains. The sugar-phosphate backbone is on the outside and the bases project inside.
The two chains have anti-parallel polarity: if one runs $5^{'} \to 3^{'}$ , the other runs $3^{'} \to 5^{'}$ .
Bases of the two strands pair through hydrogen bonds. Adenine forms two H-bonds with thymine; guanine forms three with cytosine.
A purine always faces a pyrimidine, so the distance between the two strands stays almost uniform.
The chains coil in a right-handed fashion. The pitch is 3.4 nm (1 nm = $1 0^{- 9}$ m), with about 10 bp per turn.
So the distance between two adjacent base pairs is about 0.34 nm.
The plane of one base pair stacks over the next. Stacking, along with H-bonds, makes the helix stable.

Test yourself: hide labels

Figure 2: A double-stranded polynucleotide chain. The chains run in opposite directions; A pairs with T by two hydrogen bonds and G pairs with C by three.

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Figure 3: The DNA double helix. Two sugar-phosphate backbones twist round each other in a right-handed helix, with base pairs stacked inside like the rungs of a ladder.

★ Very important Chargaff's rule: in double-stranded DNA, A = T and G = C. Since every purine faces a pyrimidine, purines = pyrimidines (A + G = T + C).

Memory Trick T for Two: A pairs with T by two H-bonds, so the other pair, G with C, has three. For the helix, remember 3.4, 10, 0.34: pitch 3.4 nm, 10 base pairs per turn, 0.34 nm per base pair.

Tips and Tricks For percentage problems, pair the bases first. If C = 20%, then G = 20%; A + T = 100 - 40 = 60%, so A = T = 30%. One known base gives all four.

Quick Recall: tap to check

In Figure 1, which group lies free at the left end of the chain?

A phosphate on the 5′ C of the sugar: the 5′ phosphate end.

In Figure 2, how many hydrogen bonds join G and C?

Three. A and T are joined by two.

Why does the distance between the two strands stay almost constant?

A purine (larger) always pairs with a pyrimidine (smaller), so every base pair has about the same width.

Key idea

Two anti-parallel, complementary chains coil right-handedly; H-bonds and base stacking hold the helix together.

4. The Central Dogma

Soon after the double helix, Francis Crick proposed the central dogma of molecular biology.
★ Exam imp It states that genetic information flows from DNA $\to$ RNA $\to$ protein.

Process	Information flows	Result
Replication	DNA $\to$ DNA	A copy of DNA
Transcription	DNA $\to$ mRNA	RNA copied from one DNA strand
Translation	mRNA $\to$ protein	A polypeptide

In some viruses, information flows in the reverse direction, from RNA to DNA.
This reverse flow is called reverse transcription.

Memory Trick Copy, rewrite, translate: replication copies DNA, transcription rewrites it in the RNA alphabet, and translation changes it into the language of amino acids.

Key idea

The central dogma: DNA makes RNA, and RNA makes protein; reverse transcription is the viral exception.

5. The Search for Genetic Material

Miescher's discovery of nuclein and Mendel's principles of inheritance came at almost the same time.
Yet proving that DNA is the genetic material took a long time.
By 1926, the search had reached the molecular level.
Work by Gregor Mendel, Walter Sutton, Thomas Hunt Morgan and others had narrowed the search to the chromosomes in the nucleus.
Which molecule was the genetic material was still unanswered.

5.1 The transforming principle (Griffith, 1928)

★ Exam imp Frederick Griffith (1928) worked with Streptococcus pneumoniae (pneumococcus), the bacterium that causes pneumonia.
He saw a living organism change its physical form, a process called transformation.
On a culture plate, the S strain forms smooth, shiny colonies and the R strain forms rough colonies.
The S strain has a mucous (polysaccharide) coat; the R strain does not.
The S strain is virulent: infected mice die of pneumonia. Mice infected with the R strain do not develop pneumonia.
Griffith killed bacteria by heating them, and then tested four injections.

Injected into mice	Result
S strain (live)	Mice die
R strain (live)	Mice live
S strain (heat-killed)	Mice live
S strain (heat-killed) + R strain (live)	Mice die; living S bacteria recovered from the dead mice

Conclusion: the R strain bacteria had been transformed by the heat-killed S strain bacteria.
Some transforming principle from the heat-killed S strain let the R strain make a smooth polysaccharide coat and become virulent.
This must be due to the transfer of the genetic material.
Griffith's experiments did not reveal the biochemical nature of the genetic material.

5.2 Biochemical nature of the transforming principle

Before the work of Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44), the genetic material was thought to be a protein.
They purified biochemicals (proteins, DNA, RNA, etc.) from heat-killed S cells.
They tested which of these could transform live R cells into S cells.
★ Exam imp DNA alone from S bacteria transformed R bacteria.
Proteases (protein-digesting enzymes) and RNases (RNA-digesting enzymes) did not affect transformation.
DNase (DNA-digesting enzyme) inhibited transformation, so DNA caused it.
They concluded that DNA is the hereditary material, but not all biologists were convinced.

5.3 The genetic material is DNA (Hershey and Chase, 1952)

★ Exam imp The unequivocal proof came from Alfred Hershey and Martha Chase (1952).
They worked with bacteriophages, viruses that infect bacteria.
A phage attaches to a bacterium and its genetic material enters the cell. The cell treats this material as its own and makes more virus particles.
They asked: is it the protein or the DNA of the virus that enters the bacterium?

Labelling: some phages were grown on a medium with radioactive phosphorus ( $^{32} P$ ), others with radioactive sulfur ( $^{35} S$ ).
Phages grown with phosphorus had radioactive DNA but not radioactive protein, because DNA contains phosphorus and protein does not.
Phages grown with sulfur had radioactive protein but not radioactive DNA, because DNA does not contain sulfur.
Infection: the radioactive phages were allowed to attach to E. coli bacteria.
Blending: as infection went on, the viral coats were removed from the bacteria by agitating them in a blender.
Centrifugation: the virus particles were separated from the bacteria by spinning them in a centrifuge.

Bacteria infected with phages carrying radioactive DNA were radioactive: DNA had passed from the virus into the bacteria.
Bacteria infected with phages carrying radioactive protein were not radioactive: protein had not entered.
So DNA is the genetic material that passes from virus to bacteria.

Test yourself: hide labels

Figure 4: The Hershey-Chase experiment. Radioactive phosphorus (in DNA) entered the bacteria, but radioactive sulfur (in protein) stayed in the supernatant, so DNA is the genetic material.

^{32} P

phagesLabel DNA (DNA has phosphorus).
After centrifugation: radioactivity in the cells.
Shows DNA enters.

^{35} S

phagesLabel protein (DNA has no sulfur).
After centrifugation: radioactivity in the supernatant.
Shows protein stays out.

Memory Trick P for the phosphate backbone of DNA; S for the sulfur of protein. The label that ends up inside the cell marks the genetic material.

NEET Focus Know which experiment used which organism and tool: Griffith used Streptococcus pneumoniae and mice; Avery, MacLeod and McCarty used purified biochemicals and digesting enzymes; Hershey and Chase used bacteriophages, E. coli,

^{32} P

and

^{35} S

, a blender and a centrifuge. A common trap pairs

^{32} P

with protein or names pneumococcus as the Hershey-Chase host.

Key idea

Griffith found the transforming principle, Avery's group showed it is DNA, and Hershey and Chase proved it beyond doubt.

6. Properties of Genetic Material: DNA versus RNA

The Hershey-Chase experiment settled the debate between protein and DNA as the genetic material.
★ Exam imp Later it became clear that in some viruses RNA is the genetic material, for example Tobacco Mosaic Virus (TMV) and Q $β$ bacteriophage.
Why DNA is the main genetic material, while RNA does dynamic jobs as messenger and adapter, lies in their chemical differences.
The two chemical differences: DNA has deoxyribose and RNA has ribose (with a 2′-OH); DNA has thymine and RNA has uracil.

6.1 Criteria for a genetic material

It should be able to generate its replica (replication).
It should be stable chemically and structurally.
It should allow slow changes (mutation) that are needed for evolution.
It should be able to express itself as Mendelian characters.

Memory Trick Really Stable Molecules Express: Replication, Stability, Mutation, Expression.

Criterion	How DNA and RNA compare
Replication	Both DNA and RNA can direct their own duplication, because of base pairing and complementarity. Proteins fail this first criterion.
Stability	The genetic material should not change with the stage of the life cycle, age or physiology. In Griffith's work, heat killed the bacteria but did not destroy the genetic material. DNA strands separated by heating come together again under suitable conditions. In RNA, the 2′-OH on every nucleotide is reactive, so RNA is labile and easily degraded; RNA is also catalytic, hence reactive. DNA is chemically less reactive and structurally more stable, so it is the better genetic material.
Thymine in place of uracil	Gives DNA additional stability (the reason involves DNA repair).
Mutation	Both can mutate. RNA, being unstable, mutates at a faster rate, so viruses with an RNA genome and a short life span mutate and evolve faster.
Expression	RNA can directly code for the synthesis of proteins, so it expresses characters easily. DNA depends on RNA for protein synthesis. The protein-synthesising machinery evolved around RNA.

★ Very important Both DNA and RNA can work as genetic material. DNA, being more stable, is preferred for storage of genetic information; RNA is better for its transmission.

Key idea

DNA wins as the store of genetic information because it is stable; RNA wins at expressing and transmitting it.

7. The RNA World

★ Exam imp RNA was the first genetic material.
There is enough evidence that essential life processes, such as metabolism, translation and splicing, evolved around RNA.
RNA acted as a genetic material as well as a catalyst.
Some important biochemical reactions in living systems are still catalysed by RNA catalysts, not by protein enzymes.
Being a catalyst, RNA was reactive and hence unstable.
So DNA evolved from RNA, with chemical modifications that make it more stable.
DNA is double stranded with a complementary strand, and it further resists change by evolving a process of repair.

Key idea

Life began in an RNA world; DNA evolved later from RNA as a more stable store of information.

8. DNA Replication

8.1 The semiconservative scheme

While proposing the double helix, Watson and Crick also proposed a scheme for copying DNA.
Their 1953 statement reads: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."
In this scheme, the two strands separate and each acts as a template for a new complementary strand.
★ Exam imp Each new DNA molecule then has one parental strand and one newly synthesised strand. This is semiconservative DNA replication.

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Figure 5: Watson-Crick model for semiconservative replication. Each daughter helix keeps one parental strand (orange) and gains one new strand (brown), and both daughters carry the same base pairs.

8.2 The experimental proof (Meselson and Stahl, 1958)

DNA replication is now proven to be semiconservative, first in Escherichia coli and later in higher organisms such as plants and human cells.
★ Exam imp Matthew Meselson and Franklin Stahl did the experiment in 1958.

They grew E. coli for many generations in a medium with $^{15} N H_{4} Cl$ as the only nitrogen source. $^{15} N$ , the heavy isotope of nitrogen, entered the new DNA and other nitrogen compounds.
This heavy DNA could be told apart from normal DNA by centrifugation in a caesium chloride (CsCl) density gradient.
$^{15} N$ is not a radioactive isotope; it is separated from $^{14} N$ only by its density.
They then moved the cells to a medium with normal $^{14} N H_{4} Cl$ , and took samples at definite time intervals as the cells multiplied.
They extracted the DNA, which stayed as double-stranded helices, and ran each sample on a CsCl gradient to measure its density.

Under centrifugal force, a molecule with higher mass or density sediments faster, so denser DNA forms its band lower in the gradient.
★ Exam imp After one generation in $^{14} N$ (20 minutes, since E. coli divides every 20 minutes), the DNA had a hybrid (intermediate) density.
After two generations (40 minutes), the DNA was made of equal amounts of hybrid and light DNA.

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Figure 6: Meselson and Stahl's experiment. One generation in

^{14} N

gives only hybrid DNA, and two generations give equal amounts of hybrid and light DNA, as semiconservative replication predicts.

Time in $^{14} N$ medium	Generation	Hybrid DNA	Light DNA
0 min	Parent	None (all heavy)	None
20 min	I	100%	0%
40 min	II	50%	50%
60 min	III	25%	75%
80 min	IV	12.5%	87.5%

Tips and Tricks The two original

^{15} N

strands never break up, so after the first generation there are always exactly two hybrid molecules. After

n

generations there are

2^{n}

molecules, so the hybrid fraction is

\frac{2}{2 ^{n}}

. At 80 minutes (

n

= 4): 2 hybrid : 14 light, that is 1 : 7.

Taylor and colleagues (1958) did similar experiments on Vicia faba (faba beans), using radioactive thymidine to trace newly made DNA.
They proved that the DNA in chromosomes also replicates semiconservatively.

8.3 The machinery and the enzymes

In living cells such as E. coli, replication needs a set of catalysts (enzymes).
★ Exam imp The main enzyme is DNA-dependent DNA polymerase, which uses a DNA template to polymerise deoxynucleotides.
It is highly efficient. E. coli, with only $4.6 \times 1 0^{6}$ bp, completes replication in about 18 minutes, an average rate of about 2000 bp per second.
Compare this with the human diploid content of $6.6 \times 1 0^{9}$ bp.
The polymerase must also be highly accurate, because any mistake during replication causes mutations.
Replication is energetically very expensive.
Deoxyribonucleoside triphosphates serve two purposes: they are the substrates, and they provide energy for polymerisation.
Their two terminal phosphates are high-energy phosphates, as in ATP.
Many additional enzymes are needed to complete replication with a high degree of accuracy.

Extra Depth: 4.6 million bp in 18 minutes works out to about 4,300 bp per second in all. E. coli copies its DNA from the origin in two directions at once, so the 2000 bp per second is the rate at each of the two replication forks.

8.4 The replication fork

The two strands of a long DNA cannot be separated over the entire length, because that needs very high energy.
★ Exam imp So replication occurs within a small opening of the DNA helix, called the replication fork.
DNA-dependent DNA polymerases catalyse polymerisation in only one direction, $5^{'} \to 3^{'}$ .
On the template with polarity $3^{'} \to 5^{'}$ , replication is continuous.
On the template with polarity $5^{'} \to 3^{'}$ , replication is discontinuous.
The discontinuously synthesised fragments are later joined by the enzyme DNA ligase.

Test yourself: hide labels

Figure 7: The replicating fork. The new strand grows continuously along one template and in short pieces along the other, because the polymerase works only in the 5′ to 3′ direction.

Extra Depth: The short pieces made discontinuously are called Okazaki fragments. The strand made continuously is the leading strand, and the one made in pieces is the lagging strand.

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Figure 8: Leading and lagging strands at a replication fork. The leading strand grows continuously towards the fork; the lagging strand is made away from the fork as Okazaki fragments, which DNA ligase later joins.

DNA polymerases cannot initiate replication on their own.
Replication does not start at random. It starts at a definite region in E. coli DNA, the origin of replication.
Because an origin is needed, a piece of DNA to be propagated in recombinant DNA procedures requires a vector; the vector provides the origin of replication.
In eukaryotes, DNA replication takes place in the S phase of the cell cycle.
Replication and cell division must be highly coordinated. If the cell fails to divide after replication, polyploidy (a chromosomal anomaly) results.
Many details of replication, such as the events at the origin, are still being studied.

Memory Trick Polymerase drives one way: 5′ to 3′. The template that runs the opposite way (3′ to 5′) lets it drive straight on, so that strand is continuous.

NEET Focus Statement questions often swap the template polarity. Correct: synthesis is continuous on the template with polarity 3′ to 5′ and discontinuous on the template with polarity 5′ to 3′. Also remember: the polymerase cannot start replication by itself, and DNA ligase joins the pieces.

Quick Recall: tap to check

If E. coli grows for 80 minutes in

^{14} N

medium, what is the ratio of hybrid to light DNA?

Four generations: 16 molecules, 2 hybrid and 14 light, so 1 : 7 (12.5% hybrid, 87.5% light).

Why does heavier DNA settle lower in the CsCl gradient?

A molecule of higher mass or density feels a larger centrifugal force and sediments faster.

Which enzyme joins the discontinuously made fragments?

DNA ligase.

Key idea

Replication is semiconservative, runs only 5′ to 3′, starts at an origin, and is continuous on one template and discontinuous on the other.

9. Exam Essentials

Pairs to Match

List I	List II
Friedrich Miescher	Identified DNA as nuclein (1869)
Watson and Crick	Double helix model (1953)
Wilkins and Franklin	X-ray diffraction data of DNA
Erwin Chargaff	A : T and G : C ratios equal one
Francis Crick	Central dogma
Frederick Griffith	Transforming principle (1928)
Avery, MacLeod and McCarty	Transforming principle is DNA (1933-44)
Hershey and Chase	Unequivocal proof that DNA is the genetic material (1952)
Meselson and Stahl	Semiconservative replication in E. coli (1958)
Taylor and colleagues	Semiconservative replication of chromosomes in Vicia faba
N-glycosidic linkage	Base to the 1′ C of the sugar
Phosphoester linkage	Phosphate to the 5′ C of a nucleoside
3′-5′ phosphodiester linkage	Nucleotide to nucleotide
DNA ligase	Joins discontinuously synthesised fragments
Origin of replication	Definite region where replication starts

Exceptions

RNA, not DNA, is the genetic material in some viruses: Tobacco Mosaic Virus and Q $β$ bacteriophage.
Information flows from RNA to DNA (reverse transcription) only in some viruses.
Thymine is found only in DNA and uracil only in RNA; cytosine is common to both.
Only RNA carries an -OH at the 2′ position of every sugar.
Proteins fail the very first criterion of a genetic material (replication).
$^{15} N$ is a heavy isotope but not radioactive, unlike $^{32} P$ and $^{35} S$ .
DNA polymerase cannot initiate replication and cannot polymerise in the $3^{'} \to 5^{'}$ direction.

Numbers to Remember

$ϕ$ X174: 5386 nucleotides; lambda: 48502 bp; E. coli: $4.6 \times 1 0^{6}$ bp; human: $3.3 \times 1 0^{9}$ bp (haploid), $6.6 \times 1 0^{9}$ bp (diploid).
A with T: 2 H-bonds; G with C: 3 H-bonds.
Pitch 3.4 nm; about 10 bp per turn; 0.34 nm between base pairs; 1 nm = $1 0^{- 9}$ m.
1869 Miescher; 1926 search at molecular level; 1928 Griffith; 1933-44 Avery's group; 1952 Hershey-Chase; 1953 Watson-Crick; 1958 Meselson-Stahl and Taylor.
E. coli divides every 20 minutes; replicates its DNA in about 18 minutes at about 2000 bp per second.

Examples to Remember

Organism	Why it is named
Bacteriophage $ϕ$ X174	DNA of 5386 nucleotides
Bacteriophage lambda	DNA of 48502 bp
Escherichia coli	$4.6 \times 1 0^{6}$ bp; host in the Hershey-Chase experiment; used by Meselson and Stahl
Streptococcus pneumoniae	Griffith's transformation experiment (with mice)
Tobacco Mosaic Virus, Q $β$ bacteriophage	RNA is the genetic material
Vicia faba (faba bean)	Taylor's experiment on chromosomes

10. Quick Revision

DNA is the genetic material of most organisms; RNA is the genetic material of some viruses and mostly a messenger.
Nucleotide: base + pentose sugar + phosphate; purines A, G; pyrimidines C, U, T.
N-glycosidic (base-sugar), phosphoester (phosphate-sugar), 3′-5′ phosphodiester (nucleotide-nucleotide).
Chain ends: 5′ phosphate and 3′ OH; backbone of sugar and phosphate.
RNA has a 2′-OH and uracil; thymine is 5-methyl uracil.
Miescher 1869 (nuclein); Watson and Crick 1953, using Wilkins and Franklin's X-ray data and Chargaff's rule.
Double helix: anti-parallel, A=T (2 H-bonds), G with C (3), right-handed, 3.4 nm pitch, 10 bp per turn, 0.34 nm per bp, stacking.
Central dogma: DNA $\to$ RNA $\to$ protein; reverse transcription in some viruses.
Griffith 1928: heat-killed S + live R killed mice; transforming principle.
Avery, MacLeod, McCarty: DNase alone blocked transformation; DNA is the transforming principle.
Hershey and Chase 1952: $^{32} P$ (DNA) entered E. coli; $^{35} S$ (protein) did not.
Genetic material must replicate, be stable, mutate slowly and express; DNA stores, RNA transmits; RNA came first.
Semiconservative replication: Meselson and Stahl 1958 ( $^{15} N$ , CsCl); Taylor on Vicia faba.
DNA polymerase: $5^{'} \to 3^{'}$ only, about 2000 bp/s, dNTPs as substrate and energy, cannot initiate; origin of replication; ligase.
Eukaryotes replicate DNA in S phase; failure to divide after replication gives polyploidy.

11. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. Griffith, B. Avery, MacLeod and McCarty, C. Hershey and Chase, D. Meselson and Stahl
List II: I. Semiconservative replication, II. DNA is the transforming principle, III. Transforming principle, IV. Unequivocal proof using bacteriophages
Choose the correct answer:
(A) A-III, B-II, C-IV, D-I
(B) A-II, B-III, C-IV, D-I
(C) A-III, B-IV, C-II, D-I
(D) A-III, B-II, C-I, D-IV

Solution:

Answer: (A). Griffith discovered the transforming principle (III); Avery's group showed it is DNA (II); Hershey and Chase used phages for the unequivocal proof (IV); Meselson and Stahl proved semiconservative replication (I).

Solved Example 2

Read the statements about the DNA double helix.
A. The two chains have anti-parallel polarity.
B. Adenine forms three hydrogen bonds with thymine.
C. The helix is right-handed with a pitch of 3.4 nm.
D. The bases form the backbone and the sugars project inside.
E. Base stacking adds to the stability of the helix.
Choose the correct answer:
(A) A, C and E only
(B) A, B and C only
(C) B, D and E only
(D) A, C, D and E only

Solution:

Answer: (A). B is wrong: A and T share two H-bonds (G and C share three). D is wrong: the sugar-phosphate backbone is outside and the bases project inside.

Solved Example 3

Arrange the steps of the Hershey-Chase experiment in the correct order.
A. Viral coats removed in a blender
B. Phages grown on media with radioactive phosphorus or sulfur
C. Phages separated from bacteria by centrifugation
D. Radioactive phages allowed to infect E. coli
Choose the correct answer:
(A) B, D, A, C
(B) D, B, A, C
(C) B, A, D, C
(D) B, D, C, A

Solution:

Answer: (A). The phages are labelled first (B), then they infect the bacteria (D), the coats are blended off (A), and the mixture is centrifuged (C).

Solved Example 4

A double-stranded DNA has 32 per cent adenine. What is the percentage of guanine?
(A) 32%
(B) 18%
(C) 36%
(D) 68%

Solution:

Answer: (B). A = T = 32%, so A + T = 64%. G + C = 36%, and G = C, so G = 18%.

Solved Example 5

E. coli fully labelled with

^{15} N

is moved to a

^{14} N

medium. What fraction of DNA molecules is hybrid after 60 minutes?
(A) 1/2
(B) 1/4
(C) 1/8
(D) None

Solution:

Answer: (B). 60 minutes is three generations, giving $2^{3}$ = 8 molecules. Only 2 keep an original $^{15} N$ strand, so 2/8 = 1/4 are hybrid.

Solved Example 6

Which of the following is NOT a reason why DNA is a better genetic material than RNA?
(A) RNA has a reactive 2′-OH group
(B) RNA is catalytic and hence reactive
(C) DNA has thymine in place of uracil
(D) DNA can directly code for proteins

Solution:

Answer: (D). It is RNA that can directly code for protein synthesis; DNA depends on RNA. The other three explain DNA's greater stability.

12. Practice Questions

Practice Questions

Match List I with List II.
List I: A. N-glycosidic linkage, B. Phosphoester linkage, C. Phosphodiester linkage, D. Hydrogen bond
List II: I. Joins two nucleotides, II. Joins bases of opposite strands, III. Joins base to sugar, IV. Joins phosphate to nucleoside
(A) A-III, B-IV, C-I, D-II (B) A-IV, B-III, C-I, D-II (C) A-III, B-I, C-IV, D-II (D) A-II, B-IV, C-I, D-IIIAnswer: (A). Base-sugar is N-glycosidic, phosphate-nucleoside is phosphoester, nucleotide-nucleotide is 3′-5′ phosphodiester, and H-bonds pair bases.
Read the statements.
A. Heat-killed S strain bacteria alone killed the mice.
B. The S strain has a polysaccharide coat.
C. Living S bacteria were recovered from mice given heat-killed S and live R.
D. DNase did not affect transformation.
Choose the correct answer: (A) B and C only (B) A, B and C only (C) B, C and D only (D) A and D onlyAnswer: (A). A is wrong: heat-killed S alone did not kill mice. D is wrong: DNase inhibited transformation.
Arrange the Meselson-Stahl procedure in order.
A. Transfer to $^{14} N H_{4} Cl$ medium
B. Growth for many generations in $^{15} N H_{4} Cl$
C. CsCl density gradient centrifugation of each sample
D. Samples taken at definite intervals
(A) B, A, D, C (B) A, B, D, C (C) B, D, A, C (D) B, A, C, DAnswer: (A). Label with heavy nitrogen, transfer to light nitrogen, sample at intervals, then measure density on CsCl gradients.
Statement I: RNA mutates at a faster rate than DNA.
Statement II: Viruses with an RNA genome and a short life span evolve faster.
(A) Both Statement I and Statement II are correct (B) Statement I is correct, Statement II is incorrect (C) Statement I is incorrect, Statement II is correct (D) Both are incorrectAnswer: (A). RNA is unstable, so it mutates faster, and RNA viruses therefore mutate and evolve faster.
Which statement about the replication fork is NOT correct? (A) DNA polymerase polymerises only $5^{'} \to 3^{'}$ (B) Synthesis is continuous on the template with polarity $3^{'} \to 5^{'}$ (C) DNA polymerase can initiate replication at any point (D) DNA ligase joins the discontinuous fragmentsAnswer: (C). DNA polymerases cannot initiate replication; it begins at a definite origin of replication.
If E. coli grows for 80 minutes in $^{14} N$ medium after full $^{15} N$ labelling, what are the proportions of light and hybrid DNA?Answer: 87.5% light and 12.5% hybrid (14 : 2, or 7 : 1), since four generations give 16 molecules and only 2 are hybrid.
Group the following as nitrogenous bases and nucleosides: adenine, cytidine, thymine, guanosine, uracil and cytosine.Answer: Nitrogenous bases: adenine, thymine, uracil, cytosine. Nucleosides: cytidine, guanosine.
If a double-stranded DNA has 20 per cent cytosine, calculate the per cent of adenine.Answer: 30%. G = C = 20%, so A + T = 60%, and A = T = 30%.
The sequence of one strand of DNA is 5′-ATGCATGCATGCATGCATGCATGCATGC-3′. Write the complementary strand in the $5^{'} \to 3^{'}$ direction.Answer: 5′-GCATGCATGCATGCATGCATGCATGCAT-3′. The complement is 3′-TACG...-5′; read from its 5′ end, it is GCAT repeated seven times.
Which property of the DNA double helix led Watson and Crick to propose the semiconservative mode of replication?Answer: Complementary base pairing between the two strands. Each strand can act as a template for a new complementary strand, so each daughter keeps one parental strand.
How did Hershey and Chase tell DNA and protein apart in their experiment?Answer: They labelled DNA with radioactive phosphorus ( $^{32} P$ ), since only DNA has phosphorus, and protein with radioactive sulfur ( $^{35} S$ ), since only protein has sulfur. Only $^{32} P$ entered the bacteria.

Common Mistakes to Avoid

Watch out

Writing that $^{32} P$ labels protein. Correct: $^{32} P$ labels DNA, and $^{35} S$ labels protein.
Saying Griffith proved DNA is the genetic material. Correct: he found a transforming principle; Avery's group showed it is DNA; Hershey and Chase gave the unequivocal proof.
Giving A and T three hydrogen bonds. Correct: A and T share two; G and C share three.
Calling $^{15} N$ radioactive. Correct: it is a heavy isotope, separated only by density.
Saying synthesis is continuous on the template with polarity 5′ to 3′. Correct: it is continuous on the 3′ to 5′ template.
Placing a free phosphate at the 3′ end. Correct: the 5′ end has the free phosphate; the 3′ end has a free OH.
Mixing up nucleoside and nucleotide. Correct: nucleoside = base + sugar; nucleotide = nucleoside + phosphate.
Saying DNA is better for transmitting genetic information. Correct: DNA is better for storage; RNA is better for transmission.

Frequently Asked Questions

What is the difference between a nucleoside and a nucleotide?

A nucleoside is a nitrogenous base joined to the 1′ carbon of a pentose sugar by an N-glycosidic linkage, such as adenosine. A nucleotide is a nucleoside with a phosphate group joined to the 5′ carbon by a phosphoester linkage. Nucleotides are the monomers of DNA and RNA.

What are the main features of the Watson-Crick double helix?

DNA has two polynucleotide chains with an outer sugar-phosphate backbone and bases inside. The chains are anti-parallel and complementary, with A pairing T by two hydrogen bonds and G pairing C by three. The helix is right-handed, with a 3.4 nm pitch and about 10 base pairs per turn.

What is Chargaff's rule and why does it matter?

Erwin Chargaff found that in double-stranded DNA the ratios of adenine to thymine and guanine to cytosine are constant and equal to one. This supported base pairing in the Watson-Crick model and lets you calculate all four base percentages from one known value.

How did the Hershey-Chase experiment prove that DNA is the genetic material?

Hershey and Chase labelled phage DNA with radioactive phosphorus and phage protein with radioactive sulfur. After infection, blending and centrifugation, the bacteria carried radioactive phosphorus but not sulfur. So DNA, not protein, entered the bacteria and acted as the genetic material.

Why is DNA a better genetic material than RNA?

DNA is chemically less reactive and structurally more stable. RNA has a reactive 2′-OH group on every nucleotide and is catalytic, so it is labile and easily degraded. Thymine in place of uracil adds further stability. So DNA is preferred for storing information, and RNA for transmitting it.

What is meant by the RNA world?

The RNA world is the idea that RNA was the first genetic material. RNA could store information and also act as a catalyst, and processes such as metabolism, translation and splicing evolved around it. Because RNA was reactive and unstable, DNA later evolved from RNA as a more stable molecule.

How did Meselson and Stahl show that DNA replicates semiconservatively?

They grew E. coli in heavy nitrogen-15, moved it to normal nitrogen-14 and separated the DNA on a caesium chloride density gradient. After one generation all DNA was hybrid, and after two generations half was hybrid and half light, exactly as semiconservative replication predicts.

Why is DNA replication continuous on one strand and discontinuous on the other?

DNA polymerase can add nucleotides only in the 5′ to 3′ direction, and the two template strands are anti-parallel. On the template with polarity 3′ to 5′ the new strand grows continuously; on the other it is made in short pieces that DNA ligase later joins.

Previous year questions on Nucleic acids (DNA and RNA)

6 questions from past papers, each with a step-by-step solution.

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