Meiosis Biology

Review: Mitosis Results in 2 identical daughter cells Occurs in somatic cells Body cells Ex: skin, stomach, muscle # of chromosomes in parent cell = # of chromosomes in daughter cells Resulting cells are diploid (2N)

What is meiosis? Process of making gametes (sperm/egg) Chromosomes number is reduced in half (haploid) When gametes combine, offspring has correct # of chromosomes Haploid + haploid = diploid

Example: Humans

Because of meiosis…

For the remaining slides… Pay attention to what is happening to the DNA in each phase – not the details of what goes on in each Red text in the remaining slides = important information – focus on this!

Meiosis Occurs in 2 phases: Understand homologous chromosomes! Meiosis I Meiosis II Understand homologous chromosomes! Pair of identical chromosomes containing copies of genes One from each parent

Meiosis I Interphase Prophase I DNA is copied Crossing over occurs Pieces of chromosomes break off and switch places on sister chromatids Leads to genetic variation

Metaphase I Anaphase I Telophase I Cytokinesis I Homologous chromosome line up double file at equator Anaphase I Homologous chromosomes separate Telophase I Cytokinesis I Results in 2 haploid daughter cells containing one set of replicated chromosomes

Meiosis II NO interphase (why??) Prophase II Metaphase II Anaphase II Chromosomes line up single file at equator Anaphase II Sister chromatids separate Telophase II Cytokinesis II Results in 4 haploid daughter cells containing ONE copy of each homologous chromosome

Meiosis Parent cell – chromosome pair Chromosomes copied 1st division – homologous pairs split 2nd division – sister chromatids split; produces 4 gamete cells with ½ the original number of chromosomes

Think you are 1 in 1,000,000? Don’t sell yourself short! Humans with 23 pairs of chromosomes, a gamete (egg or sperm) could have 223 or 8,388,604 possible combinations of chromosomes from that parent. Any couple could have 223 × 223 or 70,368,744,177,644 (70 trillion) different possible children, based just on the number of chromosomes, not considering the actual genes on those chromosomes. Thus, the chance of two siblings being exactly identical would be 1 in 70 trillion. In addition, crossing over can add further variation to an individual’s genetic make-up.

Meiosis Animation Meiosis Cartoon