Biotechnology and Bioinformatics (1+1)

1.2.7.1. Properties of the Genetic code

1. The code is highly de gene rate, meaning that most of the amino acids are coded for more than one amino acids. Leucine, serine and arginine have 6 different codon s. Proline, threonine and alanine, have four. Isoleucine has three. Methionine and tryptophan have only one codon.

1. The code is not overlapping. There is no punctuation or spacing between different codons. The starting signal for protein synthesis is the codon AUG (for methionine).
2. The code appears to be highly universal that is, it is the same for various different kind of organisms. Coding regions can be transferred from one organism to another and the correct protein produced. However, a few exceptions to this are known. For example, in yeast mitochondria, UGA codes for tryptophan instead of stop. In Paramecium, UAA and UAG code for glutamine instead of stop codon.
3. Point Mutation will cause change in the amino acid sequence .

	1	2	3	4	5
Normal gene frame	BIG	FAT	CAT	ATE	RAT
Delete 1 base (F)	BIG	ATC	ATA	TER	AT-
Add 1 base (X)	BIG	FAT	CXA	TAT	ERA +

The universality provides strong evidence that life on earth started only once. When the first living forms appeared some 3 billion years ago, the genetic code was established and it has not changed since then through out the evolution of living organisms. Once the initial code was established, there were strong selective pressures to maintain it invariant because the change in a single codon would change amino acids in a great many proteins at the same time and these multiple mutations would in all likelihood be lethal.

The selective pressure has been less strict in mitochondrial DNA. Mitochondria code only for few proteins and have their own protein synthetic machinery. The overall code has been maintained.