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BIOL 239
Christine Dupont

Video 4 due Feb 3rd Meiosis Germ Cells - undergo meisos - but undergo mitosis to make many germ cells (in men only) - women already born with all eggs. does not produce more gametes - round 1 (meiosis 1)- duplicated DNA. takes from diploid state (two pairs), to single pairs - round 2 (meiosis 2) - pulling apart sister chromatids to give gametes Meiosis 1 - centromeres remain intact - homologous chromosomes pair up. - recombination of genetic material to shift alleles around - segregate into 2 nuclei - 4 stages: Prophase 1: leptonema - dna starting to condese - spindle fibers start to form Prophase 1: zygonema - synapsis - synaptomenal complex forms - sister chromatids pair up and attach - loop dna with same squence and attach it together. (zipper effect) - called tetrads Prophase 1: pachynema - exchange of genetic material - recombination enzymes travels along complex and take snip of dna from one homologue and dna from the other and switch them between homologues. - bit of maternal dna will be on paternal chromosome and vise vera crossing over: mixing of genetic material from maternal and paternal chromasomes (nonsister chromotids) happens during every meiosis. # of different progeny nuclei is very large average crossing over is 3 or 4 per chromosome. over 9 million different possibilities (creates genetic diversity) Prophase 1: diplonema - nulcues h=nothing has happened - chromosomes start to pull away from eachother - still remained joined at spots that crossing over occurs - chiasma is spot where crossing over occured - can see switches in dna when they seperate (all 4 chromatids will go into seperate gametes) Prophase 1: diakinesis - spindle fibers attach to centromeres and pull homologues apart Metaphase 1: - pulled to one end and other Anaphase 1: centromeres remain intact Telophase 1: - went from 2n to n. - end with two cells Meiosis 2 - go from 2 haploids to 4 hapoilds Prophase 2: - spindle fibers start to form and attach to each chromatid Telophase 2: - cytokenisis - end up with 4 individual cells - will occasionally be wrapped in me
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