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Evolution and proteins
• You can see the effects of evolution, not
only in the whole organism, but also in its
molecules - DNA and protein
• For a mutation to have an effect on the
phenotype (and be subject to selection) it
must (usually) affect the structure or
function of a protein
• You can learn a lot about evolution by
studying the structure of proteins
Chapter 26 Purves
th
7
• Figures 26.2, 26.3, 26.5, 26.9
edition
Reminder - protein structure
• The primary structure of a protein is its sequence
of amino acids, e.g. Glu-Asp-Gly-Leu-Asp---• The secondary structure is how the chain of AAs
coils up into helices, loops and sheets
• The tertiary structure is the 3-dimensional folding
of the secondary structures
• The quaternary structure is the way in which some
proteins are made of 2 or more separate subunits
(e.g. haemoglobin, a tetramer)
Some protein structures
Protein sequence alignments
• How can you show 2 proteins (e.g. from 2
different species) are homologous (i.e. have
the same evolutionary origin?
• Make an alignment: write the 2 sequences
side-by-side so they match up as far as
possible (you may need to introduce gaps):
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How often do changes occur?
• Mutations in the DNA can either be in the
parts that code for a protein (coding
sequences) or in the parts that don’t (noncoding sequences)
• Mutations in coding DNA can be either
synonymous (“neutral”, do not change an
amino-acid) or non-synonymous (changes
an amino-acid)
Amino-acids are not equally
“swappable”
• If we compare many examples of homologous
proteins, we can count how many times each
amino-acid can be substituted by any of the others
• The degree to which this happens, depends on
how similar the amino-acids are
• Glutamate and aspartate both have acidic sidechains and often “swap”
• The position in the protein structure also makes a
difference - some positions are always the same
A molecular clock
• Plot the number of changes in amino-acids
between the same protein in different
species (such as cytochrome C) against the
time since the species diverged
• Gives a straight line - so evolution of a
protein sequence proceeds at a constant rate
and therefore can be used as a clock
The origin of new proteins
• Genomes are full of “paralogues” - two or more
homologous versions of a gene and protein,
forming a gene (or protein) “family”
• These arose by a duplication of that part of the
genome
• Once duplicated, the 2 genes can evolve
independently
• This may lead to the evolution of a new protein
function, e.g. haemoglobin and myoglobin
The homeobox gene family
• Homeobox (Hox) proteins are “master switch”
proteins that control development in all metazoan
organisms
• The number of Hox genes is from one (in
sponges) up to 13 (in vertebrates)
• All Hox genes are homologous. The Hox system
was created once only in early evolution
• You’ll get more lectures on this later
Homeobox protein