3.3 meiosis

meiosis 1

reductional division

meiosis 2

equational division

interphase

same as mitosis: dna replication, organelle replication, prep atp

prophase 1

synapsis - homologous chroms form bivalents

crossing over at chiasma: exchange of genetic material → non-identical sister chromatids, recombinant chromatids

metaphase 1

bivalents align at metaphase plate

random orientation / independent assortment occurs

anaphase 1

homologous chroms separate, pulled by kinetochore spindle fibres to opposite poles of the cell

bivalents are split

polar spindle fibres slide past e/o → elongate cell

telophase 1

new nuclei (now haploid) form

chromosomes decondense

cleavage furrow forms, cells divide

prophase 2

same as mitosis — chroms condense, microtubules form/ spindle fibres

no crossing over

centrioles moves to opposing poles: perpendicular to direction in meiosis 1

metaphase 2

independent assortment

chroms align at metaphase plate

anaphase 2

spindle fibres contract → centromere splits → sister chromatids / chromosomes split/ separate

move to opposite poles

form daughter chromosomes

polar spindle fibres elongate → elongate cell

telophase 2

same as mitosis: nuclear membrane reforms, chroms decondense

cytokinesis begins

end result: 4 haploid gametes that are genetically different

mitosis vs meiosis

meiosis prophase 1: crossing over, bivalents

metaphase 1: chroms align vs bivalents align

anaphase 1: chroms (sister chromatids) separate & centromeres divide vs homologous chroms separate (sister chromatids don’t, centromeres don’t)

end: 2 diploid vs 4 haploid daughter cells

genetic variation

prophase 1: bivalents, crossing over → recombinant chromosomes

metaphase 1 & 2: independent assortment of bivalents or chromosomes

non-disjunction

when chroms fail to separate at either anaphase 1 or 2

can cause down syndrome, other syndromes (klinefelter, turner)

anaphase 1: homologous chroms don’t separate properly

anaphase 2: sister chromatids