Evolution and proteins

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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

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Chapter 26 Purves 7th edition

• Figures 26.2, 26.3, 26.5, 26.9

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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)

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Some protein structures
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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

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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 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)

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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

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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

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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

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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

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Homeobox protein