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The Genetic Code

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The Standard Genetic Code

This table provides the way that you can translate a strand of messenger RNA's into a sequence of amino acids that make up a polypeptide. To use the table find the first codon. Usually that's going to be AUG. The corresponding amino acid is methionine or MET and you can get this by noting that the column of the left tells you which main row in the table to go to. Since A is the first letter, the corresponding main row is the third row on the left. U is the second letter and so the first column of the table has the amino acids corresponding to all codons starting with AU. Next go to the right hand side of the column and look within the third main row to find the G sub row and where the intersection with the first column is is the amino acid methionine. I've high lighted the appropriate, main row, column and sub row in red for you.

First letter\second letter	U	C	A	G	Third
U	phenylalanine	serine	tyrosine	cysteine	U
	phenylalanine	serine	tyrosine	cysteine	C
	leucine	serine	stop	stop	A
	leucine	serine	stop	tryptophan	G
C	leucine	proline	histadine	arginine	U
	leucine	proline	histadine	arginine	C
	leucine	proline	glutamine	arginine	A
	leucine	proline	glutamine	arginine	G
A	isoleucine	threonine	asparagine	serine	U
	isoleucine	threonine	asparagine	serine	C
	isoleucine	threonine	lysine	arginine	A
	methionine, start codon	threonine	lysine	arginine	G
G	valine	alanine	aspartate	glycine	U
	valine	alanine	aspartate	glycine	C
	valine	alanine	glutamate	glycine	A
	valine	alanine	glutamate	glycine	G

Properties of the genetic code

1. The code is read in non overlapping groups of three mRNA nucleotides. Each group is called a codon.

2. There are no spaces or commas separating neighboring codons. This is like having a sentence in English consisting entirely of 3 letter words where there are no spaces between the words. This property is especially important in understand the effects of mutations on proteins.

3. The genetic code is redundant. There are 64 possible codons but only 20 amino acids.

4. There is a start codon corresponding to the amino acid methionine. When translation begins the first amino acid is always methionine. After translation this amino acid is removed as part of editing the protein. Note though that once translation has started, methionine can occur in the protein.

5. There are three non coding stop or nonsense codons. These tell the machinery of translation that the end of the protein has been reached.

6. Not all amino acids have an equal number of codons coding for it. Observe that tryptophan has one codon while arginine has six codons!

7. The code is almost universal. The table shown above shows the standard genetic code shared by most organisms on the planet. However, certain bacteria, mitochondria and protista have minor variations in their codes. The near universality of the code suggests that the code arose very early in the evolution of life.

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pgd 3/13/03