A1.2 Nucleic acidsIB Biology SL: Revision notes

DNA is the genetic material of all living organisms. Some viruses use RNA as their genetic material, but viruses are not considered to be living, so this does not contradict the statement.

A nucleotide has three parts: a phosphate group, a pentose (five-carbon) sugar and a nitrogenous base. In diagrams, show the phosphate as a circle, the sugar as a pentagon and the base as a rectangle, with the phosphate and base both attached to the sugar.

RNA is a polymer formed by condensation of nucleotide monomers: the phosphate of one nucleotide bonds to the sugar of the next, releasing water. Repeated sugar–phosphate bonds make a continuous chain of covalently bonded atoms, the strong sugar–phosphate backbone, with the bases projecting sideways.

The bases form the basis of a code. DNA bases: adenine (A), thymine (T), guanine (G), cytosine (C). RNA has uracil (U) instead of thymine.

DNA is a double helix of two antiparallel strands (running in opposite directions) linked by hydrogen bonds between complementary base pairs: A with T and G with C. When drawing DNA, show the strands antiparallel; you do not need to draw the helix.

• Strands: DNA is usually double-stranded; RNA is usually single-stranded.
• Bases: DNA has thymine; RNA has uracil.
• Sugar: DNA has deoxyribose; RNA has ribose. Deoxyribose has one fewer oxygen atom: at carbon 2 it has H where ribose has OH.

Examples of nucleic acids: chromosomal DNA, plasmid DNA, mRNA, tRNA and rRNA.

Complementarity is based on hydrogen bonding between specific bases. It allows genetic information to be replicated (each strand is a template for a new complementary strand) and expressed (a complementary RNA copy of a gene is made, and base pairing between codons and anticodons is used in translation).

Any base can follow any other, so any sequence of any length is possible. With four bases, a sequence of n bases has 4ⁿ possible sequences, giving DNA an almost limitless capacity to store information with great economy of space.

The genetic code is conserved: the same codons code for the same amino acids in almost all organisms. The best explanation is universal common ancestry — all life inherited the code from a shared ancestor.