CDNA sequencing studies

A protein consists of a long chain built up of twenty different amino acids (Table 5.1), folded on itself in a rather complicated way. Its properties depend critically on the arrangement of the folds, which is determined by the exact order in which its constituent amino acids are strung together. This in turn is specified by the sequence of the nucleotide bases that make up the DNA molecules which constitute the genetic material of the cell. There are only four different bases, and each of the twenty amino acids corresponds according to a universally obeyed triplet code to a specific group of three of them. The information embodied in the base sequence of a DNA molecule is transcribed on to an intermediary messenger RNA, and is then translated during the synthesis of the protein to yield the correct sequence of amino acids. Rapid sequencing methods for nucleotides were perfected by Sanger and his colleagues, and modern recombinant DNA technology makes possible the cloning of DNA so that the quantity required for the determination can be prepared from a single gene. Hence the amino acid sequences of proteins are

Table 5.1.

The amino acids found in proteh

is. Amino acids have thegt

meralformula R—CH(NH2)COOH, where R is the side chain or

residue. Pn

'dine is actually an amino acid, M

'rile cystine is two cysteines,

linked by a disulphide bridge. The standard abbreviations are given in

three- and t

me-letter codes. The hydropathy in

'dex is taken from Kyte am

d Doolittle (1982)

Hydropathy

Type

Amino acid

Side chain

Abbreviations index

Non-polar

Isoleucine

Valine

Leucine

Phenylalanine

Methionine

Alanine

Tryptophan

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