Nucleic acids

Nucleic acids (DNA and RNA) are long chains of nucleotides, joined by phosphodiester bonds. Each nucleotide has three parts: a nitrogen base, a five-carbon sugar (deoxyribose in DNA, ribose in RNA) and a phosphate. A base joined to just the sugar, without the phosphate, is a nucleoside.

The bases are of two kinds. Purines (two rings): adenine (A) and guanine (G). Pyrimidines (one ring): cytosine (C), found in both DNA and RNA; thymine (T), only in DNA; and uracil (U), only in RNA. The bases absorb ultraviolet light strongly near 260 nm, which is used to measure DNA.

Each chain has two different ends: a 5' end (with a phosphate) and a 3' end (with a free -OH group), and is written 5' → 3'. Watson and Crick (1953) showed that DNA is two chains running in opposite directions (antiparallel), twisted into a right-handed double helix, with the sugar-phosphate backbones outside and the bases inside. A pairs with T (held by two hydrogen bonds) and G pairs with C (held by three), so G-C pairs hold tighter.

DNA can take several shapes. B-DNA, Watson and Crick's form, is the most stable. A-DNA is also right-handed but wider, with 11 base pairs per turn. Z-DNA is left-handed, with 12 base pairs per turn. Heating DNA separates its two strands (the temperature at which half has separated is its Tm); slow cooling lets them rejoin (annealing).

RNA comes in three main types:

• mRNA (messenger RNA): carries the message from DNA to the ribosome. In eukaryotes it has a 5' cap, a coding region and a 3' poly-A tail.
• rRNA (ribosomal RNA): builds the ribosomes (16S, 23S and 5S in prokaryotes; 18S, 28S, 5.8S and 5S in eukaryotes).
• tRNA (transfer RNA): shaped like a clover leaf, with a CCA end that carries an amino acid and an anticodon that reads the mRNA.