A1.2 Nucleic acids

Every cell that has ever lived needs a way to store instructions and pass them on, and life solved this problem once, very early, with nucleic acids. The fact that the same two molecules — DNA and RNA — carry information in bacteria, oak trees and humans is some of the strongest evidence for the common ancestry of all life. For A1.2 the central idea is elegantly simple: information is stored as a sequence of bases along a stable chemical backbone, and the rules of complementary base pairing allow that information to be copied and read accurately. Master the structure first and the function follows directly.

Nucleotides: the building blocks

Both DNA and RNA are polymers built from repeating monomers called nucleotides. Each nucleotide has three parts joined together: a pentose (five-carbon) sugar, a phosphate group, and a nitrogenous base. The sugar and phosphate are the same in every nucleotide of a given polymer; only the base varies, and it is that variation that carries information.

Nucleotides are linked into a chain by condensation reactions. The phosphate of one nucleotide bonds to the sugar of the next, forming a sugar–phosphate backbone with the bases projecting from it. Because the backbone is identical along its whole length, it is chemically stable and gives no information away — rather like the spine of a book. The order of the bases is the message.

This monomer–polymer logic is worth stating clearly in exams: the same small set of subunits can be arranged in an essentially unlimited number of sequences, so a simple chemistry can encode huge diversity.

How DNA and RNA differ

The syllabus expects you to distinguish DNA from RNA on three precise points. Learn them as a set:

• Sugar: DNA contains deoxyribose; RNA contains ribose (which has one more oxygen atom).
• Number of strands: DNA is normally double-stranded; RNA is normally single-stranded.
• Bases: both use adenine (A), guanine (G) and cytosine (C), but DNA uses thymine (T) while RNA uses uracil (U) in its place.

There are four possible bases at each position. They fall into two chemical classes: the larger purines (adenine and guanine, double-ring) and the smaller pyrimidines (cytosine, thymine and uracil, single-ring). You do not need to memorise the ring structures in detail, but knowing that a purine always pairs with a pyrimidine helps explain why the DNA double helix has a constant width.

The double helix: base pairing and antiparallel strands

In DNA the two strands wind around each other as a double helix, held together by hydrogen bonds between bases on opposite strands. Pairing is not random: it follows the rule of complementary base pairing — A pairs with T (two hydrogen bonds) and G pairs with C (three hydrogen bonds). Because a large purine always pairs with a small pyrimidine, every rung of the ladder is the same width, keeping the helix uniform.

The two strands are antiparallel: they run in opposite directions, one oriented 5′→3′ and the other 3′→5′. (The numbers refer to carbon atoms on the sugar; the key point for SL is simply that the strands are oppositely oriented.) This arrangement matters because the enzymes that copy and read DNA work in only one direction.

Complementarity is the heart of the molecule’s power: if you know the sequence of one strand, you automatically know the other. This is what makes accurate replication possible — each strand can act as a template for building a new partner.

Storing information — and the scale of it

Genetic information is stored as the sequence of bases. With four possible bases at each of millions of positions, the number of possible sequences is astronomically large, so even a simple four-letter alphabet can specify every protein an organism needs.

The syllabus highlights how vast genomes can be. Methods of determining base sequences (DNA sequencing) have advanced so quickly that whole genomes are now read routinely, revealing differences in size: some organisms have far more DNA than others, and genome size does not match an organism’s apparent complexity. A useful application is the comparison of base sequences between species — the more similar two sequences are, the more recently the species shared a common ancestor, which is why DNA evidence is central to modern classification.

Finally, note that the same backbone chemistry that stores information also protects it: the bases that carry the message are tucked inside the helix, shielded by the stable sugar–phosphate backbone on the outside.