Meiosis (Eukaryotes) PDF

PART I-C 1. Meiosis (eukaryotes)  4 sperm cells Males Continuous after puberty Meiosis (mammals) 1 oocyte (2 polar bodies) Females...

PART I-C 1. Meiosis (eukaryotes)  4 sperm cells Males Continuous after puberty Meiosis (mammals) 1 oocyte (2 polar bodies) Females Begins in fetus, ends with fertilization MEIOSIS Reductive division to generate genetically distinct haploid gametes  5 Chromosome terminology Homologous chromosomes: in a diploid organism, the pair of chromosomes that has: Almost identical DNA sequence Same genes in the same location (locus) Same size Centromere in the same place To the left we see un unreplicated pair of chromosomes. We have: 2 chromosomes (one of maternal origin, the other of paternal origin) 2 dsDNA molecules (one per chromosome) Chromosome 3  6 Chromosome terminology To the left we see a REPLICATED pair of chromosomes (after DNA synthesis_. We have: 2 replicated chromosomes (one of maternal origin, the other of paternal origin). 4 dsDNA molecules (two per chromosome) Since chromosomes are replicated, each is made up of two sister chromatids. Chromosome 3 Structure of the Synaptonemal Complex Early Prophase I: Chromosomes condense, synapse and recombination begins Replicated chromatin is packed onto the axial element (AE/LE). Recombination hotspots are identified, DSBs are made, and associate with the AE. Telomeres anchor to the nuclear envelope for homologue pairing and bringing chromosomal axes together.  Prophase I: Homologues pair and chromosomal axes are brought together  Synapsis of homologs proceeds through the length of the chromosome (down to the bp!).  DSB repairs allow recombination:  Most breaks = non- crossover events.  Minority = crossover events Transverse (CO). Central filament element  The transverse filament (green), forms on the AE of each homolog and binds the central element (CE, red  Prophase I: Chromosomes are fully synapsed and recombination is finalized  Deposition of CE is complete = chromosomes are fully synapsed:  Recombination is finalized = all DSB are repaired.  Proteins that aided recombination via DSB and repair are Prophase I: Chromosomes desynapse and prepare for the first meiotic division  The synaptonemal complex is disassembled.  Homologous chromosomes are held together by the chiasmata.  Meiosis I: Reductional division Replicated homologues Homologous pairs of Homologues separate and Cytoplasm divides. Nuclear chromosomes condense chromosomes are moved move towards opposite envelope may reform. and synapse. by the spindle fibers to the poles of the cell Chromosomes may Crossing over occurs. metaphase plate decondense. Nuclear envelope breaks down. 2n = 6 6 6 6 C value 4C 4C 4C 2C Number of chromosomes 6 6 6 3 Number of DNA 12 12 12 6 molecules  Meiosis II: Equational division Chromosomes Replicated chromosomes Sister chromatids Chromosomes arrive at recondense. New spindle lineup at the metaphase separate and pole. The nuclear envelope forms. Nuclear envelope plate chromosomes move forms around each set of breaks down. towards opposite poles of chromosomes. The the cell. cytoplasm divides. 2n = 6 6 6 6 C value 2C 2C 2C 1C Number of chromosomes 3 3 6 3 Number of DNA 6 6 6 3 molecules  15 Meiosis: generating unique cells Product = 4 cells (some exceptions) Chromosome number is reduced by half (2n >> n) Cells produced are genetically different from one another and from the parent cell: 1. Recombination (crossing over) DNA strands of non-sister chromatids break and are then repaired. Allows allele ‘shuffling’ 2. Random alignment of chromosomes during metaphase I & II defines segregation of chromosomes in anaphase I & II Random distribution of maternal and paternal chromosomes into daughter cells  16 Random alignment of chromosomes on metaphase plate THANK YOU ADDITIONAL SLIDES Cell cycle (eukaryotes) Mitosis DNA replication