Introduction to Meiosis Bio 3201 (p. 470)

The process of Meiosis A type of cell division that results in gametes (sperm and egg) being created with half the number of chromosomes (haploid) as the parent cell (diploid). Located in the gonads Testes (male) : Spermatogenesis Ovaries (female) : Oogenesis

What is the purpose of Meiosis???

Purpose of Meiosis: Genetic continuity (that is to pass on genetic material) Genetic variability ( main difference from mitosis!)

Meiosis is a 2 step process!

Step 1: Meiosis 1 -> has four (4) Stages: PMAT Prophase 1 Metaphase 1 Anaphase 1 Telophase 1

Step 2: Meiosis 2 -> Has four (4) stages: PMAT Prophase 2 Metaphase 2 Anaphase 2 Telophase 2

Meiosis 1 / Reduction Division The process of reducing the number of chromosomes in a cell by half. (½) Going from diploid (2n) to haploid (n)

Prophase 1 During Prophase 1 of meiosis, homologous chromosomes (similar chromosomes) line up next to each other as pairs. Pairs of Homologous chromosomes are TETRADS. Crossing over of nonsister chromatids occurs

Crossing over: the overlapping of non sister chromatids resulting in the exchanges of genes.

Metaphase 1 During metaphase 1, Homologous chromosomes line up in the middle of the cell. The spindle from one end of the cell attaches to one pair of sister chromatids while a spindle at the other end/pole attaches to the other pair of sister chromatids

Anaphase 1 During anaphase 1, the spindle pulls homologous chromosomes to separate ends of the cell.

Telophase 1 Nuclear membrane reappears around each set of separated chromosomes Spindle disappears Cytokinesis occurs 2 new cells are created with ½ (haploid or n) the chromosome number of the original cell (diploid or 2n)

Meiosis 2 **Same as Mitosis!!**

Prophase 2 Nuclear membrane disappears Spindle forms Chromosomes become visible

Metaphase 2 Chromosomes line up at the middle of the cell Spindle attaches to centromere of each chromosome

Anaphase 2 Spindle pulls one chromatid from each chromosome to opposite ends of cell

Telophase 2 Nuclear membrane reappears around each group of chromatids ( now called SS chromosomes) Spindle disappears Two new nuclei are created, each having the haploid number of organisms … and Cytokinesis!

Comparison of Mitosis & Meiosis Occurs in growth and asexual reproduction Occurs in production of gametes in animals and spores in plants and organisms Homologous chromosomes not paired off during prophase. There is no exchange of parts between homologous chromosomes Homologous chromosomes paired off during prophase of the first division. Involves ONE cell division Involves TWO cell divisions As a result of mitosis each daughter cell receives the same number of chromosomes as the original cell. Mitosis maintains the chromosome number. As a result of meiosis, each daughter cell receives only one member of each pair of homologous chromosomes. (It only has one half the number of chromosomes as the original cell

Oogenesis Meiosis that occurs in the ovaries of females. Ends with ONE egg with half the number of chromosomes (haploid n)

Oogenesis in humans Within the ovaries of females, a diploid (2n) cell called a Oognium undergoes meiosis. Before Meiosis begins (during interphase) the oogonium replicates (doubles) its chromosomes so that is has 46 chromosomes During the first Meiotic division, the homologous (pairs) chromosomes are separated. During the stage cytokinesis occurs unevenly and as a result one large and one small cell are created. The large cell is called a Primary Oocyte and the small cell is called a Polar Body Each of these cells will have 23 DS chromosomes or 46 chromosomes (chromatids) During the second meiotic division, the cells divide again creating 4 cells, but this time the new cells created each have 23 chromosomes . ( we have un equal distribution after cytokinesis again!

Note: the three (3) polar bodies will die because they do not have enough cytoplasm to keep them running

Spermatogenesis Meiosis that occurs in the testes of males Results in the creation of FOUR (4) sperm, which each have half the number of chromosomes (haploid)

Spermatogenesis in humans Within the testicles of a male, a spermatogonium, replicates its chromosomes to contain 46 DS chromosomes. During the first meiotic division, the cells divide and the homologous chromosomes (pairs) separate. This results in 2 new cells each with 23 chromosomes. During the second division, two new cells are created, each with 23 chromosomes. These cells are called primary spermatids, will mature into the sperm

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