How DNA Makes a Copy of Itself Before Every Cell Division

Every cell in your body carries a long molecule of DNA holding the instructions for building and running you. Before a cell divides, it must copy that molecule accurately, and it must pack it tightly enough to fit inside a nucleus far too small to see.

This lesson covers what makes DNA a good genetic material, the structure and packaging of DNA, and how DNA replicates.

A genetic material must replicate, stay chemically and structurally stable, change slowly by mutation and express itself as traits, and experiments on bacterial transformation and bacteriophage infection showed that DNA — not protein — carries genetic information.

Properties of a genetic material:

- Able to replicate itself
- Stable chemically and structurally
- Able to mutate slowly, allowing evolution
- Able to express itself as characters

DNA versus RNA. DNA is more stable, because its sugar lacks the reactive 2'-OH group found in RNA and it uses thymine instead of uracil. RNA is more reactive and mutates faster, so DNA is the better store of information, while RNA is better at expressing it.

Transformation experiment:

- Streptococcus pneumoniae S strain, with a capsule, kills mice; R strain, without a capsule, does not
- Heat-killed S strain alone does not kill mice, but heat-killed S strain mixed with live R strain does — the R strain has been transformed
- When the transforming principle was purified, DNase destroyed its effect but protein-digesting and RNA-digesting enzymes did not, identifying it as DNA

Hershey-Chase experiment:

- Bacteriophages were grown with radioactive phosphorus, which labels DNA, or radioactive sulphur, which labels protein
- After infection and blending, radioactive phosphorus was found inside the bacteria, while radioactive sulphur stayed outside with the empty viral coats
- So viral DNA, not protein, entered the bacteria and directed the making of new viruses

An everyday example. Forensic laboratories in India use DNA from blood or hair to identify people, relying on DNA being a stable, inherited record.

The substance. In some viruses, such as the tobacco mosaic virus, RNA is the genetic material — so DNA is the predominant genetic material, not the only one.

DNA is a double helix of two antiparallel polynucleotide chains held together by hydrogen bonds between complementary bases — adenine with thymine and guanine with cytosine — and in eukaryotes it is wound around histone proteins into nucleosomes that coil further into chromatin and chromosomes.

Nucleotides. Each has a nitrogenous base, a deoxyribose sugar and a phosphate group, and neighbouring nucleotides are joined by phosphodiester bonds.

The double helix:

- The two chains run antiparallel, one 5' to 3' and the other 3' to 5'
- Sugar-phosphate backbones lie outside, and the bases stack inside
- A pairs with T through two hydrogen bonds, and G pairs with C through three
- The right-handed helix has a pitch of 3.4 nm with about 10 base pairs per turn

Chargaff's rule. In double-stranded DNA, A equals T and G equals C.

Worked example. If DNA has 20 per cent adenine, it has 20 per cent thymine, leaving 60 per cent shared equally as 30 per cent guanine and 30 per cent cytosine.

Packaging in eukaryotes:

- Positively charged histones, rich in lysine and arginine, form an octamer around which negatively charged DNA wraps
- DNA with a histone octamer forms a nucleosome, containing about 200 base pairs
- Nucleosomes look like beads on a string and coil into chromatin fibres, which condense into chromosomes during division
- Loosely packed, active euchromatin stains lightly; densely packed, inactive heterochromatin stains darkly

An everyday example. A long phone charger cable wound neatly around a spool fits into a small space, just as DNA wound around histones fits into a nucleus.

The substance. The DNA in one human cell is about 2.2 metres long — only this packaging lets it fit inside a nucleus a few micrometres wide.

In semi-conservative replication the two strands separate and each acts as a template for a new complementary strand, so every daughter molecule has one old and one new strand; DNA polymerase builds new strands in the 5' to 3' direction, and DNA ligase joins the short fragments.

Evidence for semi-conservative replication:

- E. coli was grown with heavy nitrogen, ${}^{15}\mathrm{N}$, so its DNA became dense
- The cells were moved to normal ${}^{14}\mathrm{N}$ medium, and DNA was separated by density in a caesium chloride gradient
- After one generation all the DNA had intermediate density; after two generations, half was intermediate and half was light
- This matches the semi-conservative model and rules out the conservative model

Steps of replication:

- Replication begins at a fixed point, the origin of replication
- The helix unwinds to form a replication fork, using energy from deoxyribonucleoside triphosphates
- DNA polymerase adds nucleotides only in the 5' to 3' direction
- On the leading strand, synthesis is continuous
- On the lagging strand, synthesis is discontinuous, producing short Okazaki fragments
- DNA ligase joins the Okazaki fragments into a continuous strand

Worked example. One DNA molecule with ${}^{15}\mathrm{N}$ in both strands, replicated twice in ${}^{14}\mathrm{N}$ medium, gives 4 molecules: 2 intermediate and 2 light.

An everyday example. A PCR test at a diagnostic laboratory copies a small piece of DNA again and again using the same base-pairing rules and a heat-stable DNA polymerase.

The substance. Replication takes place in the S phase of the cell cycle — if it is not followed by cell division, the cell ends up with extra chromosome sets.