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Lecture 3


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BIOL 1090
Andrew Bendall

Lecture 3 Meiosis N number of chromosomes = the haploid state 2N chromosomes = the diploid state Homologous both from chromosomes 1 (example) Heterologues chromosome 2 and 3 (example) Meiosis 1 Produces two HAPLOID daughter cells that are genetically distinct Involves DNA replication and TWO cell divisions Phrophase 1 o Chromosomes compactcondense o Maternal 5 and paternal 5 physically interact, get together o Become fully paired but then start to separate except at chiasmta o Pairing of homologous chromosomes is called SYNAPSIS and is often facilitated by formation of a SYNAPTONEMAL COMPLEX o Pachynema (stage of prophase 1) Homologous chromosomes are fully paired BIVALENTS or TETRADS Cross over and break chromatids and the exchange of the broken pieces between homologous chromosomes (non sister chromatids) After crossing over, homologous start to pull apart but remain joined at the cross over junction called CHIASMATA Location of CHIASMATA is random Crossover happens at slightly different location on each chromosome o Diplotene (stage of prophase 1) May persist for entire reproductive life of the individual (more that 40 years in female humans) o Diakinesis (end of prophase 1) Paired chromosomes condense further and becomes attached to spindle fibres Metaphase 1 o Paired chrom. Align on the equatorial plane in the cell Anaphase 1 o Homologous chromosomes disjoin and move to opposite poles of the cell Telophase 1 o Chrom. Movement is completed and new nuclei begin to form Meosis 2 Prophase 2 o Chromosomes, each consisting of 2 sister chromatid, condense and become attached to spindle fibres Metaphase 2 o Kinetochores must be on opposite sides of centromere so the sister chromatids can be pulled apart Anaphase 2 Telophase 2 Cytokenisis Kinetochores positions change between prophase 1 and 2 o Attachement is not random o Steriospecific attachment so the right things separate at the right times Spermatogenisis in mamals 1: Speratogonia (diplod cells capable of undergoing mitosis) (2n) make 4 cells A lot of mitosis in testes Individual spermatocyte undergoes meiosis Spermatogenisis in mammals 2:
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