Nucleic acids

Heredity is carried by chromosomes in the cell nucleus, made of proteins and nucleic acids. The two nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Both are long chains of nucleotides, so they are also called polynucleotides.

Complete hydrolysis gives three things: a pentose sugar, phosphoric acid and nitrogen-containing heterocyclic bases. The sugar is β-D-2-deoxyribose in DNA and β-D-ribose in RNA.

Bases: DNA's four are A (adenine), G (guanine), C (cytosine) and T (thymine). RNA shares A, G and C but carries U (uracil) where DNA has T.

A nucleoside is a base joined to C1′ of the sugar; the primes keep sugar carbons apart from base atoms. A nucleotide is a nucleoside with phosphoric acid on C5′ of the sugar.

Nucleotides join through phosphodiester links between the 5′ and 3′ carbons of neighbouring sugars, so the backbone alternates sugar and phosphate with a base on each sugar.

Primary structure is the sequence of nucleotides. Watson and Crick proposed DNA's secondary structure: two strands wound as a double helix, held by hydrogen bonds between bases.

Pairing is specific: adenine with thymine, cytosine with guanine. So the two strands are complementary, not identical.

RNA is a single-stranded helix that sometimes folds back on itself. Its three types are messenger (m-RNA), ribosomal (r-RNA) and transfer (t-RNA).

DNA is the chemical basis of heredity. It copies itself during cell division so each daughter cell gets identical DNA, and it keeps a species' identity over millions of years. It carries the message for each protein, which the RNAs then build.

DNA fingerprinting uses the base sequence, unique to a person and the same in every cell: in forensics, for paternity, to identify accident victims by comparing with parents or children, and to study racial groups and evolution.