Lecture 15 September 29 Proteins 1. The genetic code has 64 triplet nucleotide codons that encode the 20 amino acids in a redundant fashion. Translation of the messenger RNA actually occurs on the ribosomes , which are a large complex of proteins and ribosomal RNAs . It is facilitated by another type of RNA called transfer RNA , which brings each amino acid to the correct codon. The first codon of each gene is AUG, which codes for methionine.
Mutations are changes in the DNA sequence.
Changes in third codon positions, or introns, or spacers will have little noticeable effect or are neutral . Changes in first and second codon positions will commonly lead to an amino acid substitution , which can be devastating, as in sickle cell anemia. Insertions and deletions of DNA can also occur. If they are multiples of three then the reading frame is retained, but amino acids are added or lost, which can again be serious, witness cystic fibrosis. Finally, if not multiples of three, then the reading frame is changed, causing a frameshift , which generally destroys a protein.