PEPTIDE BIOSYNTHESIS
DNA
mRNA
Peptide/protein
= 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 A UGG (tryptophan)
C G 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
wildtyp
e
bithorax
antennapedia
Ed Lewis
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,
Figure 15.7A, B
Chimpanzee fetus Chimpanzee adult
Human fetus Human adult