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Lecture 11

Lecture 11.docx

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Biology 1002B
Tom Haffie

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Lecture 11: Intro to Prokaryotic Gene Structure Modern Chlamydomonas have three different genomes…  Have mitochondria and chloroplasts, as well as a nucleus Modern endoymbiont genomes are greatly diminished  Chlamy has 120,000 kilobases (1000 base pairs) in the nuclear genome  17 linear chromosomes  Chloroplast has 200 kilobases, and mitochondrion has 16 kilobases  Chromosomes in mitochondrion & chloroplasts are circular chromosomes as opposed to linear in the nucleus  Present in many copies in each organelle  A typical prokaryotic organism: E. coli  Should assume that it’s kind of similar in size to that of the ancient prokaryotes that have given rise to mitochondria & chloroplasts, but E. coli has about 5000 genes in about 5000 kb  If there’s 5000 genes, and 5000 kb, 1 gene is about 1000 base pairs; each amino acid needs 3 bases to code each codon, so there’s about 300 amino acids in a typical average protein  Chloroplast genome in Vaucheria (algae)  Has only 169 genes in only 115 kilobases of circular cpDNA  This genome has gone from 5000 kb down to 115 kb over evolutionary time  Humans have only 37 genes in only 16 kb of circular mtDNA  Went from 5000 to 16 kb  Only about a dozen proteins made by mitochondria  Why does it have 37 genes, but make only a dozen proteins?; Why is there a disconnect between the number of genes & proteins  All RNAs are made by transcription of genes; so a lot of the genes are coding for RNA  Why are organelle genomes so small?  Evolved to be very specific, they only carry out certain processes, so they don’t need any more genes  Why did the organelle put its DNA into the nucleus?  If you have an entire genome in both the mitochondria and nucleus, there’s a lot of redundancy  Genes can be safely lost from mitochondria/chloroplasts  Genes that code for flagella, etc.  Hexokinase, and other glycolysis genes aren’t needed in both the mitochondria and the nucleus  Hosts in which their mitochondria have lost genes have genomes that are easier to replicate; they have a selective advantage  Genes can be removed either by mutation/deletion or lateral gene transfer  Why the nucleus?  So the nucleus could have more control; coordinated control  Nucleus is subverting cellular immune systems so the cell will welcome the organelles into the cell; subverting cellular rejection system  Want to get the DNA out of mitochondria & chloroplasts because the organelles are involved in electron passport & oxygen metabolism, it generates reactive oxygen species  Oxygen + electron = reactive oxygen species  Organelles are sites of ROS production -> ROS is reactive and very mutagenic -> creates damage in DNA -> get DNA out of organelles, into the nucleus to get it away from the reactive oxygen  DNA in the nucleus can go through sexual recombination, but DNA in mitochondria & chloroplasts cannot  Want to get DNA out of organelles so it can participate in sexual recombination and generate diversity Why haven’t all organelle genes moved to the nucleus?  Why is it a select
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