MeiosisGamete Production Mr. Young
Cells do not increase in size. They increase in number. • Meiosis = process of nuclear division that reduces the chromosome number by half in new cells (gametes) • cells undergo interphase, grow, copy DNA just like mitosis, but in meiosis the cells divide twice • two Diploid (2n) cells divide twice to produce four Haploid (1n) cells • 2 stages = Meiosis I and Meiosis II
Meiosis • Interphase • Meiosis I • Prophase I • Metaphase I • Anaphase I • Telophase I & Cytokinesis (results in 2 haploid cells) • Goes directly into Meiosis II (no interphase, no dna copying) • Prophase II • Metaphase II • Anaphase II • Telophase II & Cytokinesis (results in 4 haploid cells)
Meiosis • Creates four haploid cells (gametes) • Two nuclear divisions occur without DNA being copied in between
Crossing over - portions of chromatids break off and attach to adjacent chromatids on homologous chromosome Crossing over allows for the random mixing of genes (genetic recombination) which adds genetic variety to a species so that no two individuals are exactly the same. Crossing Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.
Prophase I: Tetrad formation/ crossing over Metaphase I Telophase I Telophase II Anaphase I Crossing Over Because of crossing over, every gamete receives a unique set of genetic information.
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Stages Of Meiosis • Meiosis resembles mitosis except that it is actually two divisions not one • These divisions are called Meiosis I and Meiosis II • Meiosis I results in haploid cells with chromosomes made up of two chromatids • Meiosis II is essentially mitosis on haploid cells
Stages Of Meiosis - Meiosis I • Prophase I - The beginning phase - • DNA which was unraveled and spread all over the nucleus is condensed and packaged • Homologous chromosomes (each made of two identical chromatids) come together and form tetrads (4 chromatids) • Crossing over, in which chromatids within tetrads exchange genetic material, occurs • Metaphase I - Middle stage - Tetrads line up along the equator of the cell
Stages Of Meiosis - Meiosis I • Anaphase I - One copy of each chromosome still composed of two chromatids moves to each pole of the cell • Telophase I - End stage - New nuclear membranes are formed around the chromosomes and cytokinesis (cytoplasm division) occurs resulting in two haploid daughter cells
NOTE: There is NO interphase between meiosis I and meiosis II. Why is this important? So that each new cell will only have half (haploid) the number of chromosomes as the original parent cell. Meiosis II is not preceded by the copying of DNA
Stages Of Meiosis - Meiosis II • Prophase II - Cells do not go into interphase between meiosis I and II, thus chromosomes are already condensed • Metaphase II - Chromosomes line up at the equator of the two haploid cells produced in meiosis I • Anaphase II - Chromosomes made up of two chromatids split to make chromosomes with one chromatid which migrate to the poles of the cells • Telophase II - Cytokinesis and reformation of the nuclear membrane in four haploid cells each with one set of chromosomes made of one chromatid
Mother cell Interphase Prophase I: Condensing Chromosomes Prophase I: Tetrad formation/ crossing over Metaphase I Meiosis II Anaphase I Telophase I Stages Of Meiosis: Meiosis I
Metaphase II Anaphase II Telophase II Stages Of Meiosis: Meiosis II Telophase I The products of meiosis are 4 haploid cells each with a unique set of chromosomes. Prophase II The products of mitosis are 2 diploid cells with identical chromosomes.
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Gametes Are Haploid • Gametes = haploid reproductive cells (sperm, egg cells) one set of chromosomes (1n) • Gametes must have half the genetic material of a normal cell • If the genetic material in the gametes was not halved, when they combined the zygote would have more genetic material than the parents • Meiosis is specialized cell division resulting in cells with half the genetic material of the parents
Spermatogenesis - diploid reproductive cell divides through meiosis to form 4 sperm cells Spermatogenesis
Oogenesis - Production of mature egg cells through meiosis. Only one of the four develops into a mature egg cell, the other 3 are called ‘polar bodies’ and degenerate Oogenesis
What Meiosis is About Meiosis allows the creation of unique individuals through sexual reproduction.