Lecture11+notes

Lecture11+notes - Recombination Why recombination? Kinds of...

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Lecture 11 Recombination 1 Recombination Why recombination? Kinds of recombination: homologous vs nonhomologous Homologous: recA, an amazing protein Holliday Junctions and branch migration resolution: two ways, two distinct outcomes Nonhomologous: recombinases transposases mechanisms
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Lecture 11 Recombination 2 Why recombination? Probably originally evolved as a repair mechanism Double stranded breaks are deadly. If two DNA copies are present, recombination can be used to repair a ds break in one, if the other is not broken. This requires a mechanism for identifying DNA sequences that are copies of each other, since repair would imply that the original sequence was restored. Ultimately recombination allows new arrangements of information as well, and these are evolutionarily advantageous.
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Lecture 11 Recombination 3 Recombination First observed in genetic crosses whereby new organisms appear to have a new combination of traits found separately in their parents. Molecular understanding first began with the observation of chromosome dynamics during meiosis in diploid plants and animals. Under microscope can see parts of chromosomes being exchanged. Since chromosomes have DNA and DNA is the genetic material, DNA must be exchanged. How does this work? Holliday proposed based on genetic results that at some point during recombination, two 2 duplexes join together to produce a four way junction (now called a Holliday junction, or a branch).
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Lecture 11 Recombination 4 Sex and the single cell ( E. coli ) Some E. coli strains have “F”, or fertility factor, a circular DNA molecule that replicates autonomously (a plasmid). F has genes necessary to transfer its DNA from one cell to another. F exists in two forms: F+, and Hfr. F+ is the plasmid form. F+ cells can only transfer a copy of F itself. The Hfr form occurs when the F plasmid inserts into the chromosome. This allows F to transfer a copy of a large piece of the chromosome. Once the donor DNA is present in the recipient cell, it must recombine with the recipient's DNA in order to become permanently part of the recipient's genome.
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5 The E. coli genome codes for proteins that carry out the events necessary to recombine DNA that is present in the cell. In order for recombination to be an orderly process that is good for the genome, it must satisfy two criteria: 1. It must only occur between homologous DNA sequences , that is only between pieces of DNA that have nearly the same sequence of bases. 2.
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Lecture11+notes - Recombination Why recombination? Kinds of...

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