Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2017 / S12 / 1 / Solution

DNA is a directional polymer with a sugar-phosphate backbone linked by phosphodiester bonds. Its deoxyribose sugars carry adenine, thymine, guanine or cytosine. In the usual cellular duplex, antiparallel strands form a double helix with complementary A–T and G–C pairing, stabilized by base stacking as well as hydrogen bonding. The sequence can vary without changing the basic backbone, allowing a large information capacity. Complementarity provides a template for copying and repair; the two strands are not merely two redundant lists stored without a copying mechanism. Major and minor grooves permit proteins to recognize sequence-dependent chemical patterns. Eukaryotic chromosomes package DNA around histones, while bacterial chromosomes are usually supercoiled and associated with other proteins; not all cellular DNA is nuclear or linear.

RNA uses ribose with a reactive 2-prime hydroxyl and usually uracil in place of thymine. It is often a single chain, but intramolecular complementary segments form stems, loops and elaborate three-dimensional folds. Messenger RNA conveys coding information to ribosomes; transfer RNA combines an anticodon with an amino-acid attachment site; ribosomal RNA has structural and catalytic roles in translation. Regulatory and catalytic RNAs show why it is inadequate to describe RNA solely as a transient DNA copy. RNA's local pairing and folding let sequence encode both information and molecular machinery. DNA's chemical stability and duplex repair make it especially suitable for long-term inheritance.

Replication is highly faithful but imperfect. Heritable sequence changes, recombination and gene duplication create variation, and selection, Genetic drift and population history determine its spread. Thus the same chemistry explains both continuity of lineages and their divergence; mutation is not directed toward what an organism needs. Conserved genes, homologous sequences and the widely shared genetic code support common ancestry. RNA catalysis makes an early RNA-based stage of evolution a plausible hypothesis, not a direct historical observation or a deduction that all modern RNA preceded DNA in a demonstrated sequence. Structure constrains function and evolutionary possibilities without uniquely determining a species' history.^[ref]