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MEIOSIS.

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Presentation on theme: "MEIOSIS."— Presentation transcript:

1 MEIOSIS

2 1. The division of the nucleus of a diploid germ cells
D. Meiosis 1. The division of the nucleus of a diploid germ cells * Produces haploid gametes = egg or sperm * Reduces the chromosomes in the gametes to half of what is found in the somatic cells of the organism. (= reduction division)

3 2. Why is meiosis necessary?
* For SEXUAL REPRODUCTION! * = egg sperm zygote * The union of egg and sperm is called Fertilization.

4 * Without meiosis, the chromosome number would double with each generation.

5 3. Meiosis Overview 1st Division 2nd Division 2nd Division

6 Meiosis Bozeman - MEIOSIS

7 A Germ Cell In Interphase

8 4. Germ Cell Life Cycle: Interphase = Meiosis Stage I begins:
*G1 = diploid germ cell is growing in the testes or ovaries *S = DNA Replication occurs (the only time this happens during the entire process) *G2 = Cell prepares to divide Meiosis Stage I begins: * Homologous pairs of chromosomes will be separated into different cells.

9 Prophase I

10 MEIOSIS STAGE I 1. PROPHASE I: * Chromosomes become visible
* Homologous chromosomes pair = Synapsis * Crossing over occurs= a portion of a chromatid breaks off and is exchanged with corresponding chromatid portions on the other homologous chromosome.

11 Crossing -Over

12 Metaphase I

13 MEIOSIS STAGE I METAPHASE I * Homologous pairs of chromosomes
line up on the equator of the cell. * Spindles attach to the centromeres

14 Anaphase I

15 MEIOSIS STAGE I 3. ANAPHASE I * Homologous pairs separate and
move to opposite centrioles * Nondisjunction could occur. (The failure of homologous chromosomes to separate during Anaphase I or failure of sister chromatids to separate during Anaphase II of meiosis which can result in a gamete having an abnormal chromosome number)

16

17 Telophase I followed by Cytokinesis I

18 MEIOSIS STAGE I TELOPHASE I then Cytokinesis
* Chromosomes gather at the centrioles. * Cytokinesis = The cytoplasm divides.

19 Prophase II

20 MEIOSIS STAGE II PROPHASE II * New spindles form in each of
the two new cells.

21 Metaphase II

22 MEIOSIS STAGE II METAPHASE II * Individual chromosomes line
up on the equator in each of the two cells. * Spindles attach to each chromosome in each cell.

23 Anaphase II

24 MEIOSIS STAGE II ANAPHASE II * Chromatids (copies of the DNA) are
pulled apart and move to opposite centrioles in each of the two cells. * Nondisjunction could occur.

25

26 Telophase II and Cytokinesis II
OR

27 MEIOSIS STAGE II TELOPHASE II and cytokinesis
* A nuclear envelope forms around each set of chromosomes in each of the two cells *Chromosomes return to chromatin *Nucleoli reappear *Cytokinesis = The cytoplasm divides *Gametes are formed (1 egg females; 4 sperm in males) *Each gamete is haploid.

28 F. Spermatogenesis vs Oogenesis
Diploid Germ Cells Meiosis I Meiosis II * Uneven distribution of the cytoplasm during Meiosis I and Meiosis II results in the formation of 1 haploid egg cell and 3 polar bodies that will be destroyed by the body and not used for reproduction. *Results in the formation of 4 haploid sperm cells.

29 Comparison of Mitosis to Meiosis
MEIOSIS ANIMATION Bozeman Biology Meiosis Video Bozeman animation – meiosis

30 Begins with a diploid cell
Allows for growth and to maintain structure A cell with 16 chromosomes divides to produce 2 cells each having 16 chromosomes. Is preceded by DNA Replication Synapsis occurs Starts with a somatic cell Forms gametes. Results in 2 diploid cells A cell with 8 chromosomes divides to produce 4 cells each having 4 chromosomes Starts with a germ cell Occurs only in the testes or ovaries Crossing over occurs Results in haploid cells Produces somatic cells


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