Meiosis-An Introduction Unit 8: Cell Reproduction Quiz 3 Meiosis

Meiosis  Meiosis is a process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell.

Why is there a need for MEIOSIS?  Most eukaryotes reproduce sexually.  Sexual reproduction involves the union of two reproductive cells to produce a new individual.  The reproductive cells must be haploid so that when they unite to form the new individual the chromosome number is restored.

Overview  Start with a diploid cell, with 2 copies of each chromosome, one form each parent. The two copies are called homologues. Chromosomes each with 2 chromatids attached at the centromere.  Use 2 cell divisions:  Meiosis 1. First separate the homologues  Meiosis 2. Then separate the 2 chromatids.  The stages of meiosis have the same names as in mitosis: prophase, metaphase, anaphase, telophase. Each of the 2 cell divisions has all of these stages.  Meiosis 1 is unusual and needs a bit of study, but meiosis 2 is just like mitosis

Meiosis 1  Two important events in meiosis 1: crossing over in prophase, and the pairing of homologues in metaphase.  Crossing over. Homologues break at identical locations, then rejoin opposite partners. This creates new combinations of the alleles on each chromosome. Occurs randomly several times on every chromosome. Results in mixing of the genes you inherited from your parents.

More Meiosis 1  The main event in meiosis 1 is the pairing and separation of the homologues.  At metaphase, the pairs of homologous chromosomes line up side by side. This does not happen in mitosis of meiosis 2, but only in meiosis 1.  At anaphase of meiosis 1, the pairs of homologues are pulled to opposite poles by the spindle.  Note: the centromeres do NOT divide; the chromosomes remain in the 2-chromatid X- shaped state.

Result of Meiosis 1  Go from 1 cell to 2 cells. Each daughter cell contains 1 copy of each chromosome: they are haploid, with the chromosomes still having 2 chromatids each.  For humans: start with one cell containing 46 chromosomes (23 pairs) to 2 cells containing 23 chromosomes.  As a result of crossing over, each chromosome is the mixture of the original homologues.

Meiosis 2  Meiosis 2 is just like mitosis  No replication of DNA between meiosis 1 and meiosis 2.  Chromosomes line up individually on the equator of the spindle at metaphase.  At anaphase the centromeres divide, splitting the 2 chromatids.  The one-chromatid chromosomes are pulled to opposite poles.

More Meiosis 2

Summary of Meiosis  2 cell divisions.  Start with 2 copies of each chromosome (homologues), each with 2 chromatids.  In meiosis 1, crossing over in prophase mixes alleles between the homologues.  In metaphase of meiosis 1, homologues pair up, and in anaphase the homologues are separated into 2 cells.  Meiosis 2 is just like mitosis. The centromeres divide in anaphase, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.

Meiosis I: 1.Chromosomes are reduced from diploid (2N) to haploid (1N). 2.Four stages Prophase ISimilar to prophase of mitosis, except that homologous chromosomes pair and cross- over. Spindle apparatus begins to form, and nuclear envelope disappears. Metaphase IChromosome pairs (bivalents) align across equatorial plane. Random assortment of maternal/paternal homologs occurs (different from metaphase of mitosis). Anaphase IHomologous chromosome pairs separate and migrate toward opposite poles. Telophase IChromosomes complete migration, and new nuclear envelopes form, followed by cell division.

Meiosis II: 1.Similar to mitotic division. 2.Also four stages: Prophase IIChromosomes condense.. Metaphase IISpindle forms and centromeres align on the equatorial plane. Anaphase IICentromeres split and chromatids are pulled to opposite poles of the spindle (one sister chromatid from each pair goes to each pole). Telophase IIChromatids complete migration, nuclear envelope forms, and cells divide, resulting in 4 haploid cells. Each progeny cell has has one chromosome from each homologous pair, but these are not exact copies due to crossing-over.

Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings. Fig The stages of meiosis in an animal cell Crossing-over Random Assortment

Significant results of meiosis: 1.Haploid cells are produced because two rounds of division follow one round of chromosome replication. 2.Alignment of paternally and maternally inherited chromosomes is random in metaphase I, resulting in random combinations of chromosomes in each gamete. Number of possible chromosome arrangements = 2 n-1. 3.Crossing-over between maternal and paternal chromatids during meiosis I provides still more variation. Moreover, the crossing-over sites vary from one meiosis to another.