The genetic code

Messenger RNA carries information (copied from a DNA template) in the form of a genetic code that directs the synthesis of a particular protein. The nature of this code was worked out in the early 1960s by Marshall Nirenberg, Har Gobind Khorana and others (Box 11.4). The message encoded in a gene takes the form of a series of triplets (codons). Of the 64 possible three-letter combinations of A, C, G and U, 61 correspond to specific amino acids, while the remaining three act as 'stop' messages, indicating that reading of the message should cease at that point (Figure 11.6). It is essential that the reading of the message starts at the correct place, otherwise the reading frame (groups of

Messenger RNA (mRNA) is formed from a DNA template (transcription) and carries its encoded message to the ribo-some, where it directs synthesis of a polypeptide (translation).

Box 11.4 The genetic code is (almost) universal

The genetic code was first worked out using E. coli, but soon found to apply to other organisms too. It seemed reasonable to assume that the code was universal, i.e. applicable to all life forms. It has been shown, however, that certain genes such as those found in the mitochondria of some eucaryotes employ a slight variation of the code. Mitochondria have their own transcription enzymes, ribosomes and tRNAs and so are able to use a modified system.

uuu

phe

ucu

ser

UAU

tyr

UGU

cys

uuc

phe

ucc

ser

UAC

tyr

UGC

cys

UUA

leu

UCA

ser

UAA

STOP

UGA

STOP

UUG

leu

UCG

ser

UAG

STOP

UGG

trp

CUU

leu

CCU

pro

CAU

his

CGU

arg

CUC

leu

CCC

pro

CAC

his

CGC

arg

CUA

leu

CCA

pro

CAA

gin

CGA

arg

CUG

leu

CCG

pro

CAG

gin

CGG

arg

AUU

ile

ACU

thr

AAU

asn

AGU

ser

AUC

ile

ACC

thr

AAC

asn

AGC

ser

AUA

ile

ACA

thr

AAA

lys

AGA

arg

AUG

met

ACG

thr

AAG

lys

AGG

arg

GUU

val

GCU

ala

GAU

asp

GGU

gty

GUC

val

GCC

ala

GAC

asp

GGC

giy

GUA

val

GCA

ala

GAA

glu

GGA

giy

GUG

val

GCG

ala

GAG

glu

GGG

giy

Figure 11.6 The genetic code. Apart from methionine and tryptophan, all amino acids can be coded for by more than one triplet codon (for some, e.g. leucine, there may be as many as six). The code is thus said to be degenerate. Three of the triplet sequences are stop codons, and represent the point at which translation of the mRNA message must end. Translation always begins at an AUG codon, meaning that newly-synthesised proteins always begin with a methionine residue. See Box 2.4 for full names of amino acids

Figure 11.6 The genetic code. Apart from methionine and tryptophan, all amino acids can be coded for by more than one triplet codon (for some, e.g. leucine, there may be as many as six). The code is thus said to be degenerate. Three of the triplet sequences are stop codons, and represent the point at which translation of the mRNA message must end. Translation always begins at an AUG codon, meaning that newly-synthesised proteins always begin with a methionine residue. See Box 2.4 for full names of amino acids three nucleotides) may become disrupted. This would lead to a completely inappropriate sequence of amino acids being produced. Frameshift mutations (see below) have this effect. Since there are only 20 amino acids to account for, it follows that the genetic code is degenerate, that is, a particular amino acid may be coded for by more than one triplet. Amino acids such as serine and leucine are encoded by as many as six alternatives each, whilst tryptophan and methionine are the only amino acids to have just a single codon (Figure 11.6).

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