Meiosis Molecular Biology Sumner HS Boo-Yah Biology!
Quick Mitosis Review Mitosis important in growth & repair Produces genetically identical daughter cells. DNA replicated in S phase Copies divided into daughter cells during mitosis (PMAT) & cytokinesis. Daughter cells have the diploid number of chromosomes. (humans = 46, or 23 pair)
Meiosis Making Reproductive Cells (Sexual Reproduction)
Meiosis Fundamentals Meiosis is a process of cell division that reduces the number of chromosome sets from diploid to haploid, producing 4 cells. Takes place in two sets of divisions, meiosis I and meiosis II It is this process that organisms use to make sperm and egg cells
Meiosis Fundamentals Chromosomes replicate once. Two consecutive nuclear divisions (similar stages to mitosis) Meiosis 1 Meiosis 2 Results in four haploid cells. Reduces chromosome number in half.
Meiosis Fundamentals Why do we want reproductive cells to be haploid?
Two stages of Meiosis Meiosis I Homologous chromosomes separate, sister chromatids together. Reduces the number of chromosomes from diploid (2n) to haploid (n) producing 2 cells. Meiosis II Sister chromatids separate Produces four haploid (n) daughter cells
Chromosome Accounting
Chromosome Accounting Regular human body cell = 46 total chromosomes The total number of chromosomes in a regular cell is the diploid number (46 in humans). Or 2n = 46 Or 23 pairs The number of chromosomes received from each parent is the haploid number (23 in humans) Or n = 23 Or 23 total 9
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Organism Life Cycles At sexual maturity, the ovaries and testes produce haploid gametes by meiosis. n = 23 Not genetically identical to parent cell due to only having half of the total chromosome set During fertilization these gametes (sperm and ovum) fuse, forming a diploid zygote The zygote develops into an adult organism
Life cycle
Homologous Chromosome What is a homologous chromosome? The pair of chromosomes from each parent Ex: Homologous chromosome 1: 1 from Mom and 1 from Dad
Miller s Biology Phases of Meiosis
The Steps of Meiosis I Interphase Chromosomes duplicate (sister chromatids made) Prophase I Homologous replicated chromosomes pair forming tetrads (four chromatids) and exchange gene segments = crossing over
More examples of Crossing Over of tetrads
Mix and Match Swapping pieces of the chromosome gives a new mix of genes to each of the chromatids.
Mixing Advantage Imagine a population with the exact same (lack of) resistance to a disease. What would the disease do to that population? Extinction does not help a population continue. Genetic (& trait) diversity makes a population more resilient in the face of survival challenges.
Steps of Meiosis I Metaphase 1 Tetrads line up on cell equator Spindles attached to centromere
Steps of Meiosis I Anaphase I Homologous chromosome pairs separate 23 each way Sister chromatids still attached
Steps of Meiosis I Telophase I/Cytokinesis New nuclei form Cytokinesis produces two non-identical haploid cells Only 23 chromosomes Chromosomes still as sister chromatids.
Steps of Meiosis II Prophase II Much like prophase in mitosis Preparation for division
Steps of Meiosis II Metaphase II Chromosomes as sister chromatids line up on the metaphase plate (equator) Spindle attaches to centromere
Steps of Meiosis II Anaphase II Sister chromatids separate Pulled to opposite ends of the cell.
Steps of Meiosis II Telophase II/ Cytokinesis New nuclei forming New cells are haploid, not diploid Cells divide Four non-identical daughter cells formed.
Meiosis Accounting During G1 = 2n = 46 (chromosomes as single chromatids) During S & G2 = 2n = 46 chromosomes with sister chromatids (92 chromatids total) After Cytokinesis I = n = unpaired chromosomes with sister chromatids (46 chromatids total) After Cytokinesis II = n = 23 chromosomes as single chromatids
Meiosis animation http://www.cellsalive.com/meiosis.htm
The Big Picture
Comparison