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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 Codonsamino acids
A
U
UUU (phenylalanine)
UUA (leucine)
T
C
A
G
UGG (tryptophan)
AUG (start)
UAA (stop)
G
C
DNARNAPeptide
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