Lect22_mitosis_Muller

Lect22_mitosis_Muller - Mitosis & Limits on Genome Size...

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1 Mitosis & Limits on Genome Size Biology 2A Chapter 22 How multiple chromosomes help cells escape Muller’s Ratchet Serial Endosymbiosis a Fossil evidence suggests that Eukaryotes arose ca. 2.1 billion years ago; prokaryotes ca. 3.8 bya. ` All eukaryotic cells have mitochondria (or the genetic remnants of them) ` Not all eukaryotes have plastids a Thus, early eukaryotes first engulfed free- living aerobic cells (mitochondria) Serial Endosymbiosis a Then a subset of these cells also engulfed a free-living photosynthetic bacteria (plastids) ` Engulfing photosynthetic bacteria appears to have happened multiple times in the history of eukaryotic cells ` The chloroplast does appear to have been engulfed by a predator-like eukaryote Chloroplasts perform photosynthesis Inheriting information a When cells divide, each resulting “daughter” cell must receive a complete set of DNA instructions in order to survive. a Thus, DNA replication must precede cell division a When cells reproduce, the DNA in the daughter cells is identical to the DNA that was in the parent cell ` Unless there were errors in DNA replication
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2 Mechanisms of Cell Division a Cell division is pretty straightforward with only one chromosome – ` Copy the DNA ` Separate the two copies ` Separate the cells a = Binary fission, used by bacteria ` and, in modified form, by some eukaryotes 1. DNA replication – the double-stranded DNA is ‘unzipped’ to provide two template strands that are replicated to produce two identical chromosomes Binary Fission (Reproduction in Bacteria) 1.DNA replication 2. Origins of replication are moved toward opposite ends of the cell Binary Fission (Reproduction in Bacteria) 1.DNA replication 2. Origins of replication separated 3. New cell membrane and cell wall are added to separate the two copies of the chromosome. Binary Fission (Reproduction in Bacteria) Development of multiple chromosomes requires a new mechanism for Cell Division a One chromosome – simply separate the two copies, then separate the cells (binary fission in bacteria) a Two or more chromosomes – must keep track of which chromosomes go to which daughter cell ` Two copies of one chromosome can be harmful ` No copies of a chromosomes (and its genes) is certainly harmful! More genes, more chromosomes a Why might a larger genome be favored? a All genes are competing to get passed on ` Genes that do not reproduce, do not get passed on ` Any strategy that increases a gene’s likelihood of getting passed to the next generation is favored, at least in the short run ` Having more genes can lead to more options in response to a changing environment a But, there are costs to having a large genome ` Replication, maintenance and repair of DNA ` Greater accumulation of errors
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3 Review: Muller’s Ratchet a DNA is copied millions of times down
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This note was uploaded on 03/08/2011 for the course BIS 2A taught by Professor Grossberg during the Fall '08 term at UC Davis.

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Lect22_mitosis_Muller - Mitosis & Limits on Genome Size...

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