A1.2 Nucleic acidsIB Biology HL: Revision notes

DNA is the genetic material of all living organisms; some viruses use RNA, but viruses are not considered living. A nucleotide is a phosphate, a pentose sugar and a nitrogenous base (drawn as circle, pentagon, rectangle). Nucleotides join by condensation: phosphate to sugar, forming a strong, covalently bonded sugar–phosphate backbone.

DNA bases: A, T, G, C; RNA has U instead of T. DNA is a double helix of two antiparallel strands joined by hydrogen bonds between complementary bases (A–T, G–C). DNA vs RNA: two strands vs one; thymine vs uracil; deoxyribose vs ribose (deoxyribose lacks the O at carbon 2).

Complementary base pairing allows replication and expression. Any base sequence of any length is possible (4ⁿ sequences), giving a limitless, economical information store. The conserved genetic code is evidence of universal common ancestry.

Carbons in the pentose are numbered 1' to 5'. In the backbone, a phosphate links carbon 5' of one sugar to carbon 3' of the next, so each strand has a 5' end (phosphate) and a 3' end (free OH). DNA strands are antiparallel.

Significance: nucleotides are only added to the 3' end, so

• replication makes new strands 5'→3';
• transcription builds mRNA 5'→3';
• translation: ribosomes read mRNA 5'→3'.

Purines (A, G) have two rings; pyrimidines (C, T, U) have one. Each base pair joins a purine to a pyrimidine, so A–T and C–G pairs are the same length. The helix therefore has the same width and three-dimensional structure whatever the base sequence, contributing to its stability.

A nucleosome is DNA wrapped around a core of eight histone proteins, held in place by an additional histone attached to the linker DNA between nucleosomes. Nucleosomes package long DNA molecules into the nucleus. Molecular visualisation software lets you see how positively charged histones associate with the negatively charged DNA.

Hershey–Chase: phages labelled with ³⁵S (protein) or ³²P (DNA) infected bacteria. After blending and centrifuging, ³²P was found inside the bacteria and in new phages; ³⁵S stayed outside with the coats. So DNA is the genetic material. NOS: the experiment only became possible once radioisotopes were available — technology opens up new experiments.

Chargaff: base proportions vary between species, but A ≈ T and G ≈ C, and purines ≈ pyrimidines. This falsified the tetranucleotide hypothesis (a repeating sequence of four bases, predicting 25% of each). NOS: the problem of induction — no number of confirming observations proves a generalisation — is addressed by the certainty of falsification: one contradicting result is enough to reject a hypothesis.