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Lecture 21.docx

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Biology 1002B
Denis Maxwell

LeCTURE 21: Experimental Evolution 1. Potentiation, Actualization and Refinement  For new ability to appear, the bacterial populations went through three successive evolutionary steps:  Potentiating mutations (of unclear nature) were required for cells to acquire actualizing mutations that consisted of a specific rearrangement of a few genes and that allowed some growth — although poor — on citrate in the presence of oxygen.  Further ‘refining’ mutations, which involved duplications of the rearranged DNA sequence, were needed for robust growth under such conditions 2. Characteristics of model systems that can be used for experimental evolution  Subject cells and organisms to selective pressure and look at the outcome  Viruses, bacteria, chlamy, drosophila, yeast  Model systems have very fast life cycles o Short generation time to look at evolution in real time 3. Origins of genetic novelty (variation)  Gene duplication o One of the two copies usually gets lost through deletion or degeneration o If it gets retained, only one gets to stay the same  Neo-functionalization: mutate faster than the other gene by changing structure  Sub-functionalization: change promoter/regulation (expressed in different conditions)  Or change both structure and promoter  Genome rearrangement o Shift promoter closer to another gene 4. Design of Lenski's long term evolutionary experiment (LEE) with E. coli  Can evolution produce adaptation if it depends on random mutations (most of which are harmful)?  Spontaneous mutation usually either bad or neutral  E. Coli has a short generation time and a huge population o Asexual (no recombination)  12 isogenetic identical populations created from a single cell o Transfer 0.1 mL (5 million cells) of culture into 9.9 mL media daily o Freeze every 500 generations/75 days  Compare frozen ancestor with evolved generation 5. Value of cryopreservation to LEE  Low temperatures without causing additional damage caused by the formation of ice during freezing 6. Where citrate enters metabolism  After 30 000 generations Ara – 3 changed in turbidity (more cells per mL culture) o Acquired the ability to use citrate as carbon (Cit +)  Of a genome
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