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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): 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 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