Unit 4 – Lecture 2

DiscussReview the stages of Mitosis [cell cycle]

with your partner.

Mitosis – cell division in somatic [body] cells[I]PMAT

[interphase]prophasemetaphaseanaphasetelophase

MeiosisMeiosis – cell division to create gametes

sex cells / reproductive cells / egg&spermMeiosis I –

IPMATMeiosis II –

PMAT

Meiosis IMeiosis I – similar to mitosis [diff in red]

[interphase…]Prophase I

chromosomes condensenuclear envelope disappearshomologous chromosomes pair

instead of chromatidscrossing over occurs

Crossing OverCrossing over – exchange of genetic

information between homologous chromosomescontributes to genetic diversity of resulting

gameteshomologous chromosome pair = tetrad

tetra = four not required:

chiasma(ta) – place at which crossing over occurs

Meiosis I – cont’d[Meiosis I]

Metaphase Ispindle fibers move tetrads to cell equator

[similar to metaphase plate / equatorial plate in mitosis]

Meiosis I – cont’d[Meiosis I]

Anaphase Ihomologous chromosomes are each

pulled by spindle fibers to cell poles

Meiosis I – cont’d [Meiosis I]

Telophase Ihomologous chromosomes each

arrive at cell polesnuclear envelopes reformspindle fibers break downcytoplasm divides [cytokinesis]

Meiosis I – cont’dMeiosis I: End Result –

2 diploid cells with:same amount of informationdifferent combination of information

Meiosis IIMeiosis II –

process is just like mitosisNO SECOND INTERPHASE. [PMAT]

Discuss

What processes happen during interphase?

If there is no interphase, what processes are NOT going to happen in between the first stages of meiosis and the second meiosis?

Meiosis II – cont’d[Meiosis II]

Prophase IIspindle fibers formnuclear envelopes dissolve

Meiosis II – cont’d[Meiosis II]

Metaphase II – chromosomes line up at equator

Meiosis II – cont’d[Meiosis II]

Anaphase IIchromosomes pulled apart at

centromeres; [form chromatids]

chromosomes [chromatids] pull toward poles

Meiosis II – cont’d[Meiosis II]

Telophase II – nuclear envelopes reformspindle fibers break downcytokinesis

Meiosis II – cont’dMeiosis II: End Result –

4 haploid cells with:half of info as original cell different combination of information

Meiosis – cont’dCells produced from meiosis are genetically

differentdue to:

crossing over [in prophase I]independent assortment [in metaphase I]

random distribution of homologues in metaphase I

Meiosis – cont’dMore than 8 million [223] combinations

of genes in humans PER PARENT [w/ only IA]

total…with 2 parents combining genes…= >70 trillion combinations [223 * 223]