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. The halving of the chromosome number counteracts a fusion of cells later in the life cycle of the organism.  In animals , meiosis produces haploid reproductive cells called GAMETES. Human gametes are sperm and egg cells, each which contains 23(1n) chromosomes.  The fusion of sperm and egg results in a zygote that contains 46 (2n) chromosomes

MEIOSIS IS DIFFERENT FROM MITOSIS A. The process of meiosis results in the production of Daughter Cells that have HALF THE NUMBER OF CHROMOSOMES OF THE PARENT CELL (HAPLOID CELL).     B. Daughter Cell produced by meiosis ARE NOT ALL ALIKE.  THE DAUGHTER CELLS MAY HAVE DIFFERENT CHROMOSOMES FROM EACH OTHER.     C. The NUMBER OF CELLS PRODUCED BY MEIOSIS IS DIFFERENT.        (1) Mitosis - One Parent Cell PRODUCES TWO DIPLOID DAUGHTER CELLS.        (2) Meiosis - One Parent Cell PRODUCES FOUR HAPLOID DAUGHTER CELLS.

STAGES OF MEIOSIS Like Mitosis – Cells go through G1, S, G2 Cells begin with 2N # of Chromosomes THE PROCESS OF MEIOSIS SEPARATES THE PAIRS OF CHROMOSOMES IN A DIPLOID CELL TO FORM HAPLOID CELLS. ONE PARENT CELL DIVIDES TWICE TO PRODUCE FOUR HAPLOID DAUGHTER CELLS. DURING MEIOSIS, THE NUMBER OF CHROMOSOMES IN EACH CELL IS REDUCED FROM DIPLOID TO HAPLOID BY SEPARATING HOMOLOGOUS PAIRS OF CHROMOSOMES.

MEIOSIS I  AT THE START OF MEIOSIS I EACH CHROMOSOME CONSIST OF TWO STRANDS OF SISTER CHROMATIDS CONNECTED AT THE CENTROMERE. HOMOLOGOUS PAIRS OF CHROMOSOMES COME TOGETHER BEFORE MEIOSIS BEGINS, AN EVENT THAT DOES NOT OCCUR IN MITOSIS. THIS EVENT IS CALLED SYNAPSIS . Each Pair of Homologous Chromosomes is called a TETRAD .

PROPHASE I Chromosomes become thick and visible, the chromosomes of each homologous pair are tangled together. Portions of Chromatids may break off and attach to adjacent Chromatids on the homologous Chromosome - a process called CROSSING-OVER (genes are exchanged between homologues) Crossing-Over results in Genetic Recombination by producing a new mixture of genetic material. Chiasmata – location on homologue where crossing over occurs

PROPHASE I cont. Each pair consists of FOUR CHROMATIDS, BECAUSE EACH CHROMOSOME IN THE PAIR HAD REPLICATED BEFORE MEIOSIS BEGAN. The centrosomes move to opposite poles, Nucleolus and the Nuclear Envelope disappear and the spindle fibers form.

METAPHASE I Homologous pairs (Tetrads) are still together and arrange along the equator of the cell (metaphase plate) Kinetichore fibers attach to each chromosome Ensures tetrad will be separated

ANAPHASE I The homologous pairs of chromosomes separate from each other, spindle fibers pull one member from each pair to opposite ends of the cell. The random separation of the homologous chromosomes is called INDEPENDENT ASSORTMENT. (random separation of Male and Female chromosomes)

TELOPHASE I AND CYTOKINESIS I Cytokinesis takes place; each new cell is haploid, containing one chromosome from each pair.

Results of Meiosis I Formation of 2 new daughter cells – but unlike mitosis, these daughter cells are haploid cells (1N) not diploid cells (2N) The cells have ½ the # of chromosomes as parent cell.

MEIOSIS II OCCURS AFTER MEIOSIS I (may go through a short interphase – cells grows slightly) CHROMOSOMES DO NOT REPLICATE BEFORE BEGINNING THE SECOND PHASE MEIOSIS II WILL DIVIDE CHROMOSOMES INTO HAPLOID CELLS CALLED GAMETES. Meiosis II just like the phases of Mitosis Each new haploid Cell from Meiosis I will go through a second division, forming the FOUR GAMETES  HAPLOID CELL.

PROPHASE II Spindle fibers form again (if cell goes through brief interphase) and begin to move the chromosomes toward the midline of the dividing cell

METAPHASE II The chromosomes move to the midline of the dividing cell, with each chromatid facing opposite poles of the dividing cell.

ANAPHASE II The chromatids separate and move to opposite poles of the cell

TELOPHASE II AND CYTOKINESIS II A nuclear membrane forms around the chromosomes in each of the four new cells, resulting in four new cells, each with half of the original number of chromosomes.

FORMATION OF GAMETES In Animals, meiosis produces haploid reproductive cells called GAMETES. Meiosis occurs within the Reproductive Organs, in the TESTES or OVARIES

SPERMATOGENESIS In the Testes, meiosis is involved in the production of Male Gametes known as Sperm Cells or Spermatozoa. In the development of Sperm Cells, a Diploid Reproductive Cell divides Meiotically to form FOUR Haploid Cells called SPERMATIDS. Each Spermatid then develops into a Mature Sperm Cell. The production of Sperm Cells is called SPERMATOGENESIS .

OOGENESIS OOGENESIS is the production of Mature Egg Cells or OVA. Notice that the Female only produces ONE EGG (OVUM) under normal circumstances. Although creating 4 Haploid Cells through meiosis, only One Becomes the Egg, the other Three products of meiosis are called POLAR BODIES ,and Degenerate.  This is due to the unequal dividing of the cytoplasm during Cytokinesis I & II.

Meiosis I Meiosis II