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 7 th edition Figures 26.2, 26.3, 26.5, 26.9

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): ASDFGFGHRTED * *** *** * TS-FGFSHRTDD

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 dont (non- coding 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 side- chains 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 Youll get more lectures on this later

Homeobox protein