1Where the information is kept

Prokaryotes (bacteria and archaea) typically have a single circular chromosome: one closed loop of double-stranded DNA lying in the cytoplasm, in a region with no membrane around it. It carries the genes the cell needs to live.

Eukaryotes typically have multiple linear chromosomes, each one long DNA molecule with two ends, kept inside the nucleus. A human cell holds about two metres of DNA in a nucleus a few micrometres across, so the DNA is packed. It is wound around clusters of positively charged proteins called histones; DNA is negatively charged (its phosphate groups), so the two attract. One cluster of histones with DNA wrapped round it is a nucleosome, and a string of nucleosomes is folded further, with other proteins, into chromatin. Figure 1 shows the first level of packing.

Packing also affects which genes can be read: tightly wound DNA is hard to reach, an idea you meet again in topic 6.5.

Plasmids are extra-chromosomal circles of DNA: separate from the main chromosome, smaller, and copied on their own. They are common in bacteria and are found in some eukaryotes too (baker's yeast carries one). A plasmid usually holds a handful of genes the cell can live without in normal conditions but which help in special ones, such as a gene for resistance to an antibiotic. Bacteria can pass plasmids to each other, which is one reason resistance spreads (topic 6.7), and biologists use plasmids to carry new genes into bacteria (topic 6.8).

The CED says typically: a few bacteria break the pattern, so choose the typical case unless the stem says otherwise.

RNA as hereditary material. In cells, RNA is a working copy, never the master copy. But some viruses keep their genes only as RNA: influenza and the coronaviruses are examples. When they infect a cell, that RNA is what is copied and passed to new virus particles. Retroviruses such as HIV are RNA viruses with a twist you meet in topic 6.4: they copy their RNA into DNA.

2Base pairing: the property that makes DNA hereditary

The four bases of DNA come in two shapes.

• Purines, adenine (A) and guanine (G), have a double ring: a six-membered ring fused to a five-membered ring.
• Pyrimidines, cytosine (C), thymine (T) and uracil (U), have a single ring.

In a double strand, a purine on one strand always faces a pyrimidine on the other: A pairs with T (two hydrogen bonds) and G pairs with C (three hydrogen bonds). In RNA, uracil takes the place of thymine and pairs with A. Figure 2 shows why the rule is purine-to-pyrimidine: a purine and a pyrimidine together span the same distance whichever pair it is, so the two backbones stay a constant distance apart all along the helix. Two purines would be too wide to fit, and two pyrimidines too narrow to meet.

Why this makes DNA a good hereditary molecule, in three steps a free-response answer can use:

1. The information is in the order of bases. The backbone of sugar and phosphate is the same all the way along, so it carries no message. The sequence of A, T, G and C does.
2. Each strand specifies the other. If one strand reads 5′-ATGCCA-3′, the partner must read 3′-TACGGT-5′. So when the strands separate, each one is a template for a new partner (topic 6.2), and the same rule lets a gene be copied into RNA (topic 6.3).
3. The rule is the same everywhere. Bacteria, oak trees and people all pair A with T and G with C. The CED calls this pairing conserved through evolution: a feature so basic that it has not changed since the common ancestor of all life.

The two strands run in opposite directions, which is called antiparallel: one strand runs 5′ to 3′ and its partner 3′ to 5′. The hydrogen bonds between bases are individually weak, so the strands can be separated for copying and reading, while the bonds within each strand's backbone are covalent and strong, so the sequence itself is stable.

3Working with base composition

Pairing has a number you can use. In double-stranded DNA every A is matched by a T and every G by a C, so:

• %A = %T and %G = %C;
• purines (A + G) = pyrimidines (C + T) = 50% of all bases.

Worked example (invented data). Double-stranded DNA from a newly described species of lichen fungus is 21% thymine. What percentage is guanine?

• A = T, so A = 21%. Together A and T are 42%.
• That leaves 100 − 42 = 58% for G and C, shared equally.
• G = 58 ÷ 2 = 29%.

The rule fails when the molecule is single-stranded, because there is no partner strand to match the bases. Data like these, from an invented study, show how an exam uses that.

• P has T and no U, so it is DNA, and A = T, G = C, so it is double-stranded.
• Q has U and no T, so it is RNA, and A ≠ U, G ≠ C, so it is single-stranded.
• R has T, so it is DNA, but A ≠ T, so it cannot be double-stranded: it is single-stranded DNA.

The claim, the evidence and the reasoning are all there: claim (R is single-stranded DNA), evidence (it contains T; A is 33% but T is 17%), reasoning (in a double strand each A pairs with a T, so their percentages must match).

4DNA and RNA compared

Both are built the same way: nucleotides joined by covalent bonds between the sugar of one and the phosphate of the next, giving a sugar-phosphate backbone with the bases sticking out. Both read 5′ to 3′. The single strand of RNA is not a weakness: it lets RNA fold into shapes, which is why tRNA and rRNA can do jobs that depend on shape (topic 6.3).

In double-stranded DNA a purine always pairs with a pyrimidine, A with T and G with C, so one strand fixes the sequence of the other.