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A1.2 Nucleic acids

SL & HL+ AHL sectionsSL and HL, with AHL content · Molecules · Theme A

Big questions

  • How can a chain of just four kinds of building block hold the instructions for a whole organism?
  • What is it about DNA's shape that lets it be copied so faithfully?

Sections framed in purple are AHL (Higher Level only). SL students can hide them in Display settings.

DNA is the genetic material of life

SL & HLA1.2.1

Every living organism, from bacteria to whales and oak trees, stores its genetic information in DNA (deoxyribonucleic acid). Genes are sections of DNA that carry the instructions a cell uses to build and run itself, and DNA is copied and passed on when cells divide and when organisms reproduce. This shared use of DNA is one of the clearest examples of the unity of life.

DNA and RNA (ribonucleic acid) are the two types of nucleic acid. Both are polymers of nucleotides, and both can store information as a sequence of bases.

What about viruses?

Some viruses use RNA rather than DNA as their genetic material. These RNA viruses include coronaviruses (such as SARS-CoV-2, which causes COVID-19), influenza viruses and HIV. Viruses are not usually considered to be living, however:

  • they are not made of cells;
  • they cannot carry out metabolism or reproduce on their own, only inside a host cell, using the host’s ribosomes and enzymes.

So the statement “DNA is the genetic material of all living organisms” still holds. Other viruses, such as the T2 bacteriophage used by Hershey and Chase (see below), use DNA.

Nucleotides and the sugar–phosphate backbone

SL & HLA1.2.2 · A1.2.3 · A1.2.4

DNA and RNA in 3D
A single nucleotideA circle (phosphate) is joined by a line to carbon 5′, which projects from the pentagon (pentose sugar). A rectangle (nitrogenous base) is joined to carbon 1′ on the other side of the pentagon. Carbon 3′ carries an –OH group.1 · A single nucleotidePO1′2′3′4′5′OHbasephosphatepentose sugarnitrogenous basebase on C1′,phosphate on C5′Ribose and deoxyriboseTwo pentagons, each a five-carbon sugar with an oxygen in the ring. Ribose, in RNA, has an –OH group on carbon 2′. Deoxyribose, in DNA, has only a hydrogen on carbon 2′: it has one fewer oxygen atom. Both have –OH on carbon 3′.2 · Ribose vs deoxyriboseCH₂OHbaseOHOHO1′2′3′4′Ribose (RNA)CH₂OHbaseOHHO1′2′3′4′Deoxyribose (DNA)A short section of DNATwo strands of nucleotides side by side. Each strand has a backbone of alternating phosphates and deoxyribose sugars joined by covalent bonds, with the bases pointing inwards. The left strand runs 5′ at the top to 3′ at the bottom; the right strand is upside down, running 5′ at the bottom to 3′ at the top, so the strands are antiparallel. From top to bottom the base pairs are A–T, G–C, T–A and C–G, each linked by hydrogen bonds (two between A and T, three between G and C).3 · Part of a DNA moleculeOHOHATGCTACGPPPPPPPP5′3′5′3′antiparallel: the strands run 5′→3′ in opposite directionsdashed lines: hydrogen bonds between complementary basessugar–phosphate backbonesugar–phosphate backbone

Key: A adenine T thymine G guanine C cytosine · circle = phosphate · pentagon = deoxyribose · rectangle = base. Purines (A, G) are drawn longer than pyrimidines (T, C), so every pair is the same width.

Text description of this visual

A section of DNA 20 base pairs long (two full turns), lying on its side. Two strands of nucleotides wind round each other in a right-handed double helix with about 10 base pairs per turn. On the outside of each strand is a sugar–phosphate backbone of alternating phosphate groups (dark spheres) and deoxyribose sugars (grey pentagons) joined by covalent bonds. The bases point inwards and pair across the middle: adenine (blue) with thymine (orange), and guanine (green) with cytosine (purple). Purines (A and G) are drawn as a pentagon fused to a hexagon and pyrimidines (T and C) as a single hexagon, so every pair is one purine plus one pyrimidine and every rung is the same length. Dashed sticks show the hydrogen bonds: two in each A–T pair and three in each G–C pair. The strands are antiparallel: the upper-front strand runs 5′ (left) to 3′ (right) and its partner runs 5′ (right) to 3′ (left); each 3′ end finishes with a free –OH group (red). Twisting creates a wide major groove and a narrow minor groove. A second model shows a single strand of RNA, with ribose (an extra red –OH on carbon 2′) and uracil (amber) in place of thymine. A third model (higher level) shows three nucleosomes: DNA wound about 1.65 times round a core of eight histone proteins, linker DNA between the beads, and an H1 histone where the DNA enters and leaves.

DNA double helix

5′ end (strand 1)
This strand starts at the left with a phosphate group attached to carbon 5′ of the first deoxyribose. Reading along it from here, the strand runs 5′ → 3′ from left to right.
3′ end (strand 1)
The strand finishes at the right with a free –OH group (red) on carbon 3′ of the last deoxyribose. New nucleotides can only be added at a 3′ end.
5′ end (strand 2)
The partner strand starts with its 5′ phosphate at the right, so it runs 5′ → 3′ from right to left. The two strands run in opposite directions — they are antiparallel.
3′ end (strand 2)
The partner strand's 3′ end, with its free –OH, is at the left — the same end of the molecule as the 5′ end of strand 1. Each end of a DNA molecule has one 5′ end and one 3′ end.
Sugar–phosphate backbone
Alternating phosphate groups (dark spheres) and deoxyribose sugars joined by covalent bonds form a continuous chain on the outside of each strand. It is strong, so the base sequence is held stably.
Deoxyribose
The pentose (five-carbon) sugar of DNA. Its carbon 1′ is bonded to a base, carbon 3′ to the phosphate of the next nucleotide and carbon 5′ to its own phosphate. It has –H, not –OH, on carbon 2′.
Adenine (A, a purine)
A double-ring base (purine), shown blue. Adenine forms hydrogen bonds with thymine on the other strand — two of them.
Thymine (T, a pyrimidine)
A single-ring base (pyrimidine), shown orange. Thymine pairs with adenine. It is found only in DNA; RNA has uracil instead.
Guanine (G, a purine)
A double-ring base (purine), shown green. Guanine pairs with cytosine on the other strand, forming three hydrogen bonds.
Cytosine (C, a pyrimidine)
A single-ring base (pyrimidine), shown purple. Cytosine pairs with guanine. Cytosine is found in both DNA and RNA.
Hydrogen bonds
Dashed sticks between the bases. There are two in each A–T pair and three in each G–C pair. Each one is weak, so the strands can be separated, but thousands together hold the two strands firmly.
Major groove
The wider of the two spiral grooves, where the edges of the base pairs are most exposed. Many proteins that bind specific base sequences, such as transcription factors, read the sequence here.
Minor groove
The narrower groove. It is narrow because the two sugars of each base pair are attached on the same side of the pair, so the backbones are closer together on that side.

RNA strand

5′ end
The strand starts with a phosphate on carbon 5′ of the first ribose. RNA, like each DNA strand, has a 5′ end and a 3′ end, and is made in the 5′ → 3′ direction.
3′ end
The last ribose has a free –OH on carbon 3′. During transcription, each new nucleotide is joined here by a condensation reaction.
One strand only
RNA is usually a single strand. Its bases are not paired with a second strand, although they can pair with complementary bases elsewhere (for example codon with anticodon).
Ribose
The pentose sugar of RNA. Like deoxyribose it has five carbons and a ring containing oxygen, but it carries an –OH group on carbon 2′.
2′ –OH group
The extra hydroxyl (red) on carbon 2′ is the difference between ribose and deoxyribose ("de-oxy" = one oxygen fewer). It makes RNA less chemically stable than DNA.
Phosphate and backbone
As in DNA, phosphate groups link the 3′ carbon of one ribose to the 5′ carbon of the next by covalent bonds, forming a sugar–phosphate backbone with the bases projecting from it.
Uracil (U)
A single-ring pyrimidine found in RNA in place of thymine. Uracil pairs with adenine.
Adenine (A)
The same purine as in DNA. In RNA, adenine pairs with uracil (or with thymine when RNA pairs with DNA during transcription).
Guanine (G)
A purine found in both DNA and RNA. It pairs with cytosine.
Cytosine (C)
A pyrimidine found in both DNA and RNA. It pairs with guanine.

Nucleosomes (AHL)

Nucleosome
One "bead" of chromatin — a histone octamer with DNA wound round it, plus an H1 histone. Nucleosomes package the long DNA molecule of a eukaryotic chromosome so it fits in the nucleus.
Histone octamer
A disc-shaped core of eight histone proteins — two each of H2A, H2B, H3 and H4 (shown in four colours, two layers of four).
One histone protein
Histones are small proteins rich in the positively charged amino acids lysine and arginine. This one is H3; its partner copy is in the other layer of the octamer.
DNA wound round the core
About 147 base pairs of the double helix are wrapped round the octamer in about 1.65 turns of a left-handed coil. The DNA stays a double helix as it bends round the core.
Protein–DNA attraction
Positively charged lysine and arginine side chains on the histone surface attract the negatively charged phosphate groups of the DNA backbone, holding the DNA on the core without bonding to particular bases.
Histone tails
Flexible N-terminal tails of the histones (here H3 and H4) project out between the turns of DNA. Chemical changes to these tails help control how tightly DNA is packed (see D2.2).
H1 histone
A ninth histone that binds on the outside of the nucleosome where the DNA enters and leaves, attached to the linker DNA. It holds the DNA in place on the core.
Linker DNA
A stretch of DNA between one nucleosome and the next. Its length varies. Under the electron microscope, nucleosomes and linker DNA look like beads on a string.

Parts of a nucleotide

A nucleotide has three parts:

  • a phosphate group, which is negatively charged and makes nucleic acids acidic;
  • a pentose sugar, with five carbon atoms, four of which are in a ring with an oxygen atom — deoxyribose in DNA and ribose in RNA;
  • a nitrogenous base, a molecule containing nitrogen with one or two rings.

The carbons of the sugar are numbered 1′ to 5′ (“one prime” to “five prime”). The base is attached to carbon 1′ and the phosphate to carbon 5′, which projects from the ring. In simple diagrams the phosphate is drawn as a circle, the sugar as a pentagon and the base as a rectangle.

The sugar–phosphate backbone

Nucleotides link when the phosphate on carbon 5′ of one nucleotide bonds covalently to carbon 3′ of the sugar of the adjacent nucleotide. Repeating this gives a chain in which sugar and phosphate alternate: the sugar–phosphate backbone. Every link is a strong covalent bond, so the backbone is a continuous chain of atoms. This makes each strand stable, so the information it carries is not easily lost, and a nucleotide can always be added to the free end, so a strand can have any length.

The bases

There are four bases in each nucleic acid:

DNA RNA
Adenine (A) ✓ ✓
Guanine (G) ✓ ✓
Cytosine (C) ✓ ✓
Thymine (T) ✓ —
Uracil (U) — ✓

The bases project from the backbone, and the order in which they occur, the base sequence, is what carries genetic information, just as the order of letters carries the meaning of a sentence.

RNA: a polymer built by condensation

SL & HLA1.2.5

RNA is a polymer made from nucleotide monomers. Each RNA nucleotide contains ribose, a phosphate and one of the bases A, U, G or C.

Condensation

Nucleotides are joined by condensation reactions:

  1. The phosphate group of a free nucleotide approaches the –OH on carbon 3′ of the last nucleotide in the chain.
  2. A covalent bond forms between them, linking the two sugars through the phosphate.
  3. The atoms of a molecule of water are removed, so this is a condensation reaction. (Strictly, in cells each incoming nucleotide carries two extra phosphate groups, as ATP does; these are split off as the bond forms and supply the energy for it.)

Repeating this adds one nucleotide at a time, building a single strand with a sugar–phosphate backbone and bases projecting to one side. The enzyme that does this during transcription is RNA polymerase (see D1.2 Protein synthesis).

Drawing an RNA polymer

A short RNA strand is drawn as a row of pentagons linked by circles: each circle joins carbon 3′ of one pentagon to carbon 5′ of the next. Every pentagon carries a rectangle labelled A, U, G or C, all on the same side, and there is no second strand. Breaking the bonds between nucleotides needs water: this is hydrolysis, the reverse of condensation.

Switch the 3D model to RNA strand to see a single chain of 12 nucleotides. Notice the extra –OH group on carbon 2′ of every ribose.

DNA: a double helix of antiparallel strands

SL & HLA1.2.6

DNA usually consists of two strands of nucleotides. The bases of one strand form hydrogen bonds with the bases of the other, linking the strands like the rungs of a ladder, with the two sugar–phosphate backbones as the sides.

Complementary base pairs

Only two combinations fit and form hydrogen bonds properly:

  • adenine (A) with thymine (T);
  • guanine (G) with cytosine (C).

These are complementary base pairs. (A–T pairs are held by two hydrogen bonds and G–C pairs by three, as the 3D model shows, but you do not need to memorize these numbers.)

Antiparallel strands

The two strands run in opposite directions: they are antiparallel. In a diagram this means one strand is drawn the right way up and the other upside down, so at each end of the molecule the end of one strand faces the opposite kind of end on the other. (These are called the 5′ and 3′ ends; see the higher level section on directionality.)

The double helix

The two strands are wound round each other to form a double helix, the overall shape of a DNA molecule. The backbones spiral round the outside and the base pairs are stacked inside, protected from the surrounding water. The twisted backbones leave two spiral grooves of different widths, the major groove and minor groove.

Comparing DNA and RNA

SL & HLA1.2.7

DNA and RNA are built in the same way, but differ in three ways.

Feature DNA RNA
Number of strands Usually two (double helix) Usually one
Pentose sugar Deoxyribose Ribose
Bases A, T, G, C A, U, G, C

Ribose and deoxyribose

Both sugars have five carbon atoms, four of them in a five-membered ring with one oxygen atom. The difference is on carbon 2′:

  • ribose has an –OH group on carbon 2′;
  • deoxyribose has just an –H there. “Deoxy” means one oxygen atom fewer.

The extra –OH makes RNA more reactive and less stable than DNA, which suits RNA’s short-term roles and DNA’s role as a long-term store.

Ribose compared with deoxyriboseTwo five-membered rings, each with four carbons numbered 1′ to 4′ and an oxygen at the top, and carbon 5′ projecting from carbon 4′. Both have –OH on carbon 3′. Ribose has –OH on carbon 2′; deoxyribose has only –H on carbon 2′.5′ CH₂OHOHOHOHO1′2′3′4′Ribose (in RNA)5′ CH₂OHOHOHHO1′2′3′4′Deoxyribose (in DNA)differ onlyat carbon 2′
Ribose (left) and deoxyribose (right). Both have an oxygen in the ring and –OH on carbon 3′; only ribose has –OH on carbon 2′.

Examples of nucleic acids

  • DNA: chromosomes in the nucleus; also the circular DNA of prokaryotes, mitochondria and chloroplasts.
  • Messenger RNA (mRNA): a copy of a gene’s base sequence, carried to ribosomes.
  • Transfer RNA (tRNA): carries amino acids to the ribosome and pairs with codons.
  • Ribosomal RNA (rRNA): part of the structure of ribosomes, where it catalyses peptide bond formation.
  • Viral genomes: DNA in some viruses, RNA in others.

Complementary pairing: copying and expressing information

SL & HLA1.2.8

Complementary base pairing depends on hydrogen bonding. The bases A and T (and G and C) have hydrogen-bond donors and acceptors arranged so that they line up with each other, while other combinations do not. Hydrogen bonds are individually weak, so strands can be separated and re-paired without breaking the covalent backbones.

Pairing lets the sequence of one strand specify the sequence of another, which is used in three processes.

Replication

The two strands of DNA separate, and each acts as a template. Free nucleotides pair with the exposed bases (A with T, G with C), so each new strand is complementary to its template, and two identical copies of the DNA are produced. This lets a cell pass its genetic information accurately to its daughter cells (see D1.1 DNA replication).

Transcription

A section of DNA is unzipped and one strand is used as a template for RNA. RNA nucleotides pair with the DNA bases (A with T, U with A, G with C, C with G), so the mRNA carries a copy of the gene’s base sequence.

Translation

At a ribosome, each three-base codon on mRNA pairs with the complementary anticodon of a tRNA molecule, which carries a specific amino acid. In this way the base sequence decides the order of amino acids in a polypeptide (see D1.2 Protein synthesis).

Simulation Complementary base-pairing builder

Template: 5′-ATGGCATTCGAC-3′

Complementary DNA: 3′-TACCGTAAGCTG-5′ (read 5′→3′: GTCGAATGCCAT)

Complementary RNA: 3′-UACCGUAAGCUG-5′

Base pairs
12
A–T pairs
6
G–C pairs
6
Hydrogen bonds
30
G + C content
50%

12 base pairs: 6 A–T pairs × 2 + 6 G–C pairs × 3 = 30 hydrogen bonds.

Each A–T pair is held by two hydrogen bonds and each G–C pair by three. Hydrogen bonds are weak individually, so the strands can be separated for replication and transcription, but many together hold the double helix firmly. Strands with more G–C pairs need more energy (a higher temperature) to separate.

Text description of this simulation

A text box for a template DNA strand written 5′ to 3′, with buttons that add A, T, G or C, delete the last base, clear the strand, or load an A–T-rich, a G–C-rich or a random example. Below it the strand is shown as a ladder: under each base is its complementary DNA base (A with T, G with C), forming a strand that reads 3′ to 5′ from left to right, and dashes show two hydrogen bonds for each A–T pair and three for each G–C pair. A third row shows the complementary RNA, in which uracil replaces thymine. Counters give the number of base pairs, A–T and G–C pairs, total hydrogen bonds and the percentage of G + C. Typing U or another letter gives a warning.

Storing information: sequence diversity and a universal code

SL & HLA1.2.9 · A1.2.10

A limitless store

Any base can follow any other along a DNA strand: there are no rules restricting the sequence, and pairing only fixes the partner strand. A strand of n nucleotides can therefore have 4n different sequences:

Length (bases) Possible sequences
1 4
3 64
10 1 048 576 (over a million)
100 about 1.6 × 1060

A typical gene is over a thousand bases long and the human genome has about 3 billion base pairs, so the number of possible sequences is effectively limitless. DNA molecules can also be any length. This diversity of base sequences is why DNA can store all the information needed to build any organism.

DNA stores this information with great economy: a diploid human cell holds about 6.4 billion base pairs, and at about 0.34 nm per base pair that is roughly 2 m of DNA (6.4 × 109 × 0.34 nm ≈ 2.2 m), yet it is packed into a nucleus only about 6 µm across. Experiments storing computer files in synthetic DNA have reached densities of about 200 million gigabytes per gram, far beyond any electronic medium.

A code shared by all life

Information in DNA is read in triplets (codons). The genetic code is the set of rules linking each of the 64 codons to one of the 20 amino acids or to “stop”. There is no chemical reason why a particular codon must code for a particular amino acid, so a huge number of different codes would be possible. Yet almost all organisms use the same code, with only a few minor variations (for example in mitochondria).

The best explanation is that the code was established in a common ancestor of all living things and has been conserved ever since, because any change to it would alter many proteins at once and would almost always be harmful. The universality of the code is therefore strong evidence for universal common ancestry. It also explains why a human gene can be expressed in a bacterium in genetic engineering.

Directionality: 5′ and 3′ ends

AHLA1.2.11

The backbone of each strand has a direction, because each sugar is linked to its neighbours through two different carbons: carbon 3′ on one side and carbon 5′ on the other. Every phosphate in the backbone joins carbon 3′ of one sugar to carbon 5′ of the next, a 5′→3′ linkage.

  • The 5′ end of a strand has a phosphate group attached to carbon 5′ of the last sugar.
  • The 3′ end has a free –OH on carbon 3′ of the last sugar.

In a double helix the strands are antiparallel, so each end of the molecule has the 5′ end of one strand and the 3′ end of the other. In the 3D model, strand 1 runs 5′→3′ from left to right and strand 2 runs 5′→3′ from right to left.

Why direction matters

Nucleotides can only be added to the free –OH at the 3′ end of a chain: the phosphate of the incoming nucleotide bonds to carbon 3′. Strands therefore always grow in the 5′→3′ direction. This has consequences for each process:

  • Replication: DNA polymerase adds nucleotides only to the 3′ end of a new strand, and the new strand is antiparallel to its template. One new strand can be made continuously, but the other has to be made in short fragments (see D1.1 DNA replication).
  • Transcription: RNA polymerase builds mRNA 5′→3′, moving along the template strand from its 3′ end towards its 5′ end.
  • Translation: ribosomes read mRNA from its 5′ end towards its 3′ end, so the start codon is near the 5′ end, and codons are read in order.

Purine–pyrimidine pairs and helix stability

AHLA1.2.12

The four DNA bases fall into two chemical groups:

  • purines, adenine and guanine, have two rings (a six-membered ring fused to a five-membered ring);
  • pyrimidines, cytosine and thymine (and uracil in RNA), have one ring.

Each base pair in DNA consists of one purine and one pyrimidine: A (purine) with T (pyrimidine), and G (purine) with C (pyrimidine). Two purines together would be too wide to fit between the backbones, and two pyrimidines would be too narrow to reach each other and form hydrogen bonds.

As a result, A–T and C–G pairs are the same length. The distance between the two sugar–phosphate backbones is the same at every point, so the double helix keeps the same shape and width along its whole length, regardless of which bases are present. This matters because:

  • the helix is regular and stable, with base pairs stacked neatly on top of each other;
  • any sequence of bases can be accommodated, so the structure places no limit on the information that can be stored;
  • enzymes such as DNA polymerase can work on any part of any DNA molecule.

In the 3D model, purines are drawn as a pentagon fused to a hexagon and pyrimidines as a hexagon: every rung is the same length, but in some the double-ring base is on one strand and in others on the other.

Nucleosomes: packaging DNA

AHLA1.2.13

The DNA molecule in a human chromosome can be several centimetres long, yet it must fit into a nucleus only a few micrometres across and still be accessible for replication and transcription. In eukaryotes, DNA is packaged with proteins called histones into nucleosomes.

Structure

  • A core of eight histone proteins, the histone octamer, made of two copies each of four histones (H2A, H2B, H3 and H4).
  • DNA wound round the core: about 147 base pairs of double helix, in about 1.65 turns.
  • An additional histone, H1, attached to the linker DNA where it enters and leaves the nucleosome, holding the DNA in place.
  • Linker DNA, of variable length, connecting each nucleosome to the next.

In electron micrographs, chromatin that has been spread out looks like beads on a string: the beads are nucleosomes and the string is linker DNA. Nucleosomes can be packed together into thicker fibres and further coiled during cell division, when chromosomes condense.

How the DNA and proteins associate

Histones contain many positively charged amino acids (lysine and arginine). DNA is negatively charged because of its phosphate groups. The attraction between these opposite charges holds the DNA on the octamer. Because the contacts are with the backbone, not the bases, histones can bind DNA of any sequence. Flexible histone tails project out from the core; chemical changes to them (such as acetylation) loosen or tighten packing and so help regulate gene expression (see D2.2 Gene expression).

Select Nucleosomes (AHL) in the 3D viewer above to explore this structure.

Hershey–Chase: DNA is the genetic material

AHLA1.2.14

In the early 1950s it was still unclear whether genes were made of protein or DNA. Proteins seemed a good candidate, because there are 20 amino acids and so far more possible sequences than with four bases. In 1952, Alfred Hershey and Martha Chase tested this using T2 bacteriophage, a virus that infects the bacterium E. coli. T2 consists only of DNA inside a protein coat, so its genes must be made of one or the other.

The key idea: label each molecule differently

  • Protein contains sulfur (in the amino acids cysteine and methionine) but almost no phosphorus.
  • DNA contains phosphorus (in its phosphate groups) but no sulfur.

So one batch of phage was grown with radioactive sulfur, ³⁵S, to label the protein coats, and another with radioactive phosphorus, ³²P, to label the DNA.

The Hershey–Chase experimentAHL
= radioactive labelproteinDNA³⁵S labels the protein coatE. coliblender shakes coats offsupernatant:phage coatspellet:bacteriasupernatant 0%pellet 0%³²P labels the DNAE. coliblender shakes coats offsupernatant:phage coatspellet:bacteriasupernatant 0%pellet 0%DNA, not protein, entered the bacteria: DNA is the genetic material
Text description of this animation

Two versions of the experiment side by side. On the left, T2 bacteriophages have protein coats labelled with radioactive sulfur-35; on the right, phages have DNA labelled with radioactive phosphorus-32. In six steps the phages attach to E. coli and inject their DNA, a blender shakes the empty protein coats off the bacteria, a centrifuge separates the heavier bacteria into a pellet from the lighter phage coats in the liquid supernatant, and the radioactivity in each fraction is measured. Most ³⁵S stays in the supernatant with the coats, while most ³²P ends up in the pellet with the bacteria, showing that DNA, not protein, entered the cells.

  1. Label the phage. T2 phage consist only of a protein coat and DNA. One batch is grown with ³⁵S, which goes into protein (in the amino acids cysteine and methionine) but not DNA. Another is grown with ³²P, which goes into DNA (in its phosphates) but almost none into protein.
  2. Infect E. coli. Each batch is mixed with E. coli. The phages attach to the bacteria and inject their genetic material, while the empty protein coats stay on the outside of the cells.
  3. Blend. After a few minutes, the mixture is agitated in a blender. This shears the phage coats off the surface of the bacteria without breaking the cells open.
  4. Centrifuge. Spinning in a centrifuge separates the mixture. The bacteria, which are larger and heavier, form a pellet at the bottom; the much lighter phage coats stay in the liquid supernatant.
  5. Measure radioactivity. With ³⁵S-labelled phage, most radioactivity (about 80%) is in the supernatant with the coats. With ³²P-labelled phage, most (roughly 70%) is in the pellet with the bacteria. The small amounts in the "wrong" fraction come from coats still stuck to cells and DNA in unattached phage.
  6. Conclusion. DNA, not protein, entered the bacteria, and the bacteria went on to make new phages. So DNA is the genetic material of T2. In further work, ³²P (but almost no ³⁵S) was passed on to the next generation of phage.

Results and conclusion

Label Labels Mostly found in Interpretation
³⁵S Protein coat Supernatant (about 80%) Protein stayed outside the bacteria
³²P DNA Pellet (roughly 70%) DNA entered the bacteria

The bacteria in the pellet went on to produce new phages, so whatever entered them carried the genetic information: that was DNA. In addition, some ³²P, but almost no ³⁵S, was found in the next generation of phages. The small amount of ³⁵S in the pellet came from coats that were not shaken off.

Chargaff's data and the tetranucleotide hypothesis

AHLA1.2.15

In the early twentieth century, the tetranucleotide hypothesis (proposed by Phoebus Levene) suggested that DNA was a simple repeating chain of the four nucleotides, such as …ATGC ATGC ATGC…. If so, DNA would contain equal amounts (25%) of each base and could not carry varied information, which was one reason why many scientists thought genes were made of protein.

What Chargaff found

From the late 1940s Erwin Chargaff measured the proportions of the bases in DNA from many species and tissues. He found that:

  • the amounts of the four bases are not equal;
  • the proportions are the same in different tissues of one species but vary between species;
  • in every species, A = T and G = C (within experimental error), so purines = pyrimidines (A + G = T + C).
Simulation Chargaff's data explorerAHL
Percentage of each base in the DNA of the selected organism010203040% of bases29.3A30.0T20.7G20.0CA and TG and Ctetranucleotide prediction: 25% each

Human (mammal)

A ÷ T
0.98
G ÷ C
1.03
Purines ÷ pyrimidines
1.00
G + C
40.7%

A ≈ T and G ≈ C, but the four bases are not equal (A and T are about 30% each), so this sample does not fit the tetranucleotide hypothesis.

G + C content of each source of DNA

  • Human40.7%
  • Chicken43.6%
  • Octopus35.2%
  • Sea urchin35.0%
  • Wheat45.5%
  • Maize46.0%
  • Yeast35.8%
  • E. coli51.7%
  • φX174 *44.8%

The marker shows 50%. * φX174 is a virus whose DNA is single-stranded, so its bases are not paired.

Show data table
Source% A% T% G% CA ÷ TG ÷ C
Human29.330.020.720.00.981.03
Chicken28.028.422.021.60.991.02
Octopus33.231.617.617.61.051.00
Sea urchin32.832.117.717.31.021.02
Wheat27.327.122.722.81.011.00
Maize26.827.222.823.20.990.98
Yeast31.332.918.717.10.951.09
E. coli24.723.626.025.71.051.01
φX17424.031.223.321.50.771.08

Real measurements (mole % of each base), from Chargaff's laboratory and others in the late 1940s and 1950s, as compiled by Bansal (2003). Small differences between A and T, or G and C, are measurement error.

Text description of this simulation

Buttons choose a source of DNA: human, chicken, octopus, sea urchin, wheat, maize, yeast, the bacterium E. coli, or the virus φX174. A bar chart shows the percentage of adenine, thymine, guanine and cytosine, with a dashed line at 25%, the amount of each base predicted by the tetranucleotide hypothesis. For every double-stranded DNA, the amount of A is close to the amount of T and the amount of G is close to the amount of C, so the ratios A ÷ T and G ÷ C are about 1 and purines equal pyrimidines. But the four bases are not equal, and the proportion of G + C varies between species, from about 35% in octopus and sea urchin to about 52% in E. coli, as a second chart shows. The single-stranded DNA of φX174 does not show A = T. A data table lists all the values.

The equalities A = T and G = C were explained in 1953 by Watson and Crick’s model: every A pairs with a T and every G with a C, a purine always pairing with a pyrimidine. The single-stranded DNA of the virus φX174 does not follow the rule, because its bases are not paired.

Must-know definitions

Nucleotide
The monomer of DNA and RNA, made of three parts joined by covalent bonds: a phosphate group, a pentose sugar (five carbons) and a nitrogenous base. The base is attached to carbon 1′ of the sugar and the phosphate to carbon 5′.
Sugar–phosphate backbone
The continuous chain of covalently bonded atoms along each strand of a nucleic acid, formed by covalent bonds between the phosphate on carbon 5′ of one nucleotide and carbon 3′ of the sugar of the adjacent nucleotide. It is strong and has the bases projecting from it.
Condensation (polymerization of nucleotides)
Nucleotides join when the –OH on carbon 3′ of one sugar reacts with the phosphate of another nucleotide, releasing a molecule of water. Repeating this builds a polynucleotide of any length.
Complementary base pairing
Specific pairing between bases on opposite strands by hydrogen bonds: adenine with thymine (or uracil in RNA) and guanine with cytosine. Each base sequence therefore fixes the sequence of its partner strand.
Double helix
The structure of DNA: two polynucleotide strands, linked by hydrogen bonds between complementary bases, wound round each other with the sugar–phosphate backbones on the outside and the base pairs inside.
Antiparallel
The two strands of a DNA molecule run alongside each other in opposite directions, so one end of the molecule has the 5′ end of one strand and the 3′ end of the other.
Universal genetic code
The same set of codons (base triplets) codes for the same amino acids in almost all organisms. This conservation is evidence that all life shares a single common ancestor.
5′ → 3′ directionalityAHL
Each strand has a 5′ end (a phosphate on carbon 5′) and a 3′ end (a free –OH on carbon 3′). New nucleotides are only ever added to the 3′ end, so DNA and RNA are made 5′ → 3′, and mRNA is translated 5′ → 3′.
Purines and pyrimidinesAHL
Purines (adenine, guanine) have two rings; pyrimidines (cytosine, thymine, uracil) have one. Every base pair in DNA is one purine plus one pyrimidine, so all pairs are the same length.
NucleosomeAHL
A unit of DNA packaging in eukaryotes: DNA wound round a core of eight histone proteins, with an additional histone (H1) attached to the linker DNA where it enters and leaves.

Common exam mistakes

  • Loses marks Drawing the phosphate attached to the base, or the base attached next to the phosphate on the same carbon.

    Write instead In a nucleotide the phosphate is bonded to carbon 5′ (outside the sugar ring) and the base to carbon 1′, on opposite sides of the pentagon. The next nucleotide's phosphate bonds to carbon 3′.

  • Loses marks Drawing both DNA strands with the sugars the same way up (parallel strands).

    Write instead The strands are antiparallel: draw one strand upside down relative to the other, so the 5′ end of one strand is beside the 3′ end of the other.

  • Loses marks The bases in DNA are held together by covalent bonds between the strands.

    Write instead Bases on opposite strands are linked by hydrogen bonds, which are weak enough to break for replication and transcription. The covalent bonds are within each strand, in the sugar–phosphate backbone.

  • Loses marks RNA contains thymine and deoxyribose.

    Write instead RNA contains uracil instead of thymine and ribose instead of deoxyribose, and is usually single-stranded.

  • Loses marks Deoxyribose has no oxygen atoms (or lacks the oxygen in the ring).

    Write instead Deoxyribose lacks only the oxygen of the –OH on carbon 2′: it has –H there, where ribose has –OH. Both sugars still have a ring oxygen and –OH on carbon 3′.

  • Loses marks Writing a complementary strand by pairing A with U, or by pairing A with G.

    Write instead In DNA, A pairs with T and G with C. U pairs with A only when RNA is involved, for example when RNA is made using a DNA template.

  • Loses marks Saying that viruses are living organisms that use RNA as their genetic material.

    Write instead All living organisms use DNA as their genetic material. Some viruses, such as coronaviruses, have RNA as their genetic material, but viruses are not classed as living organisms because they cannot carry out the processes of life outside a host cell.

  • Loses marks New nucleotides are added to the 5′ end of a growing strand.

    Write instead Nucleotides are added only to the free –OH on carbon 3′ at the 3′ end, so strands grow in the 5′ → 3′ direction.

    AHL

  • Loses marks Hershey and Chase concluded that DNA is the genetic material because the ³²P was found in the supernatant.

    Write instead Most ³²P (in DNA) was found in the pellet with the bacteria, showing that DNA entered the cells; most ³⁵S (in protein) stayed in the supernatant with the coats.

    AHL

  • Loses marks Chargaff found that DNA contains equal amounts of all four bases.

    Write instead Chargaff found that A = T and G = C, but that the four bases are not equal and their proportions vary between species. This falsified the tetranucleotide hypothesis.

    AHL

Self-test

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Multiple-choice questions

  1. Question 1In a nucleotide, to which carbon atoms of the pentose sugar are the nitrogenous base and the phosphate group attached?
  2. Question 2Which feature is found in RNA but not in DNA?
  3. Question 3A section of double-stranded DNA is 120 base pairs long. 30% of its bases are adenine. How many guanine bases does it contain?
  4. Question 4A DNA template strand has the bases A T T G C. Which RNA base sequence would form by complementary base pairing with it during transcription?
  5. Question 5How many different base sequences are possible for a single strand of DNA six nucleotides long?
  6. Question 6Why is the near-universality of the genetic code evidence that all living organisms share a common ancestor?
  7. Question 7AHLA new strand of DNA is made using the template 3′-TACGGA-5′. Which describes the new strand and how it grows?
  8. Question 8AHLWhy does a DNA double helix have the same width along its whole length, whatever its base sequence?
  9. Question 9AHLIn the Hershey–Chase experiment, which result in the ³⁵S experiment would have supported the hypothesis that protein is the genetic material of T2 phage?
  10. Question 10AHLDNA from one species contains 31% adenine, 31% thymine, 19% guanine and 19% cytosine. What do these data show about the tetranucleotide hypothesis?

Flashcards

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Prompt

What is the genetic material of all living organisms, and what is the exception?

Last reviewed 19 October 2026. Spotted a mistake? Report it.

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