Transcript Slide 1
PEPTIDE BIOSYNTHESIS Peptide/protein mRNA DNA = string of amino acids in specific sequence, specified by sequence of nucleotides in the gene (DNA) DNA bases RNA bases RNA Codonsamino acids A U UUU (phenylalanine) UUA (leucine) T C A G UGG (tryptophan) AUG (start) UAA (stop) G C DNARNAPeptide DNA ……TAC, AAA, AAT, ACC, ATT….. RNA …..AUG, UUU, UUA, UGG, UAA….. Peptide …start phenylalanine--leucine--tryptophan stop THE GENETIC CODE IS REDUNDANT Sequences of 3 bases in RNA code for a single amino acid There are 64 possible ‘triplets’ that can be formed from the 4 different bases, but there are only 20 amino acids (AA) In most cases, more than one type of triplet codes for a given AA For example, CAA and CAG both code for the same AA, glutamine A substitution of a G for the second A in CAA would constitute a neutral mutation, since the triplet would still code for glutamine (i.e., CAA becomes CAG). Thus, natural selection will not act directly to alter the frequency of the ‘mutant’ codon Such neutral mutations may serve as useful genetic markers “Evo-Devo:” Genes that control development play a major role in evolution From egg to embryo ? Genotype Ed Lewis bithorax The homeodomain • Homeobox: a conserved stretch of 180 bps found in the genes affected by homeotic mutations • Homeodomain: a 60 aa domain that binds to DNA • Homeobox genes code for the production of transcription factors (peptides) that control downstream gene expression Homeotic mutants • Homeotic mutants transform one segment into another – Antennapedia: Leg -> wing – Bithorax Complex: T3 haltere -> T2 wing – Effect segment identity • Found in gene complexes • Order of genes reflects order of segments influenced • Homeotic genes are selector genes selecting identity of entire segment or region • Paedomorphosis, Chimpanzee fetus Chimpanzee adult Human fetus Human adult Figure 15.7A, B