biology note : genetic code

Genetic code

• Gene (DNA coded for a polypeptide/protein) is a unit of inheritance
• 4 nucleotides : UCGA
• 20 standard amino acids
• triplet combination : 4x4x4 = 64 different sequences / codons
• base sequences of mRNA; reading 5' to 3'

Features of genetic code

• 3 nonsense / terminator codons (UAA, UAG, UGA) - specify termination of protein synthesis.
• 2 amino acids represented by a single codon (Met, Trp)
• start codon is AUG (Met)
• degenerate - 2 or more synonymous codons code for any particular amino acids
• universality - the same codons stand for the same amino acids in viruses and all organisms from bacteria to higher eukaryotes.
• non-overlapping coding

degeneracy and wobble hypothesis

• 1st 2 bases are constant. 3rd vary.
• do not require 61 aminoacyl-tRNAs
• Wobble hypothesis - formation of nonstandard base pair between the 3' terminal position of codons and the 5' terminal positions of anticodons.

thanx.do come again.