1 ONE LAST TIME : What is the difference between MITOSIS and MEIOSIS? HOW ARE THEY SIMILAR? HOW ARE THEY DIFFERENT?

2 Mitosis and Meiosis are both forms of dividing the nucleus

3 QUIZ Question 1 I II****II**** P PP******PP*******M*A* T TT******TT****** C*

4 MitosisMeiosis WHAT it is used for How many daughter cells Are the daughters identical to the parent? If the parent is diploid with 16 chromosomes, what will the daughter be? Does the DNA need to replicate before division? Does crossing over occur?

5 Mitosis Cell Division Used for growth and repair Used for growth and repair Occurs in somatic (body) cells Occurs in somatic (body) cells Produces two new cells identical to the parent cell Produces two new cells identical to the parent cell

6 Chromosomes

7 Chromosomes Chromosomes store genetic information Human cells have 46 chromosomes: 23 pairs

8 Chromosomes Duplicated chromosomes are called chromatids chromatids held together by centromere Called Sister Chromatids

9 Body cells are diploid = 2n The pairs are called homologs

10 The Problem: How can the cell divide and cut its chromosome number in ½? How can we go from 2n to 1n?

11 Meiosis

12 Facts About Meiosis Two divisions- Meiosis I and II Two divisions- Meiosis I and II reduction- division reduction- division Original cell is diploid (2n) Original cell is diploid (2n) Four daughter cells are haploid (1n) Four daughter cells are haploid (1n)

13 Facts About Meiosis Produces gametes (eggs & sperm) Produces gametes (eggs & sperm) Occurs in the testes in males (Spermatogenesis) Occurs in the testes in males (Spermatogenesis) Occurs in the ovaries in females (Oogenesis) Occurs in the ovaries in females (Oogenesis)

14 Here is the problem: How do you cut the chromosome number in ½? How do you divide and get the right chromosomes into each cell?

15 A major difference between the divisions of mitosis and meiosis: The HOMOLOGS pair up before dividing

16 Meiosis: Two Cell Divisions Homologsseparate Sisterchromatidsseparate Diploid Meiosis I Meiosis II Diploid Haploid

17 Prophase I Early prophase Homologs pair. Homologs pair. Crossing over occurs Crossing over occurs. Late prophase Chromosomes condense. Chromosomes condense. Spindle forms. Spindle forms. Nuclear envelope fragments. Nuclear envelope fragments.

18 Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids) Homologous chromosomes (each with sister chromatids) Join to form a TETRAD Called Synapsis

19 Crossing-Over Homologous chromosomes in a tetrad cross over each other Homologous chromosomes in a tetrad cross over each other Pieces of chromosomes or genes are exchanged Pieces of chromosomes or genes are exchanged Produces Genetic recombination in the offspring Produces Genetic recombination in the offspring

20 Crossing-over multiplies the already huge number of different gamete types produced by independent assortment Crossing-Over

21 Metaphase I Homologous pairs of chromosomes align along the equator of the cell

22 Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres attached at their centromeres.

23 Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two.

24 Prophase II Nuclear envelope fragments. Spindle forms.

25 Metaphase II Chromosomes align along equator of cell.

26 Anaphase II Sister chromatids separate and move to opposite poles Sister chromatids separate and move to opposite poles. Equator Pole

27 Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two.

28 Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

29 Gametogenesis Oogenesis or Spermatogenesis

30 Spermatogenesis Occurs in the testes Occurs in the testes Two divisions produce 4 spermatids Two divisions produce 4 spermatids Spermatids mature into sperm Spermatids mature into sperm Men produce about 250,000,000 sperm per day Men produce about 250,000,000 sperm per day

31Spermatogenesis

32 Oogenesis Occurs in the ovaries Occurs in the ovaries Two divisions produce 3 polar bodies that die and 1 egg Two divisions produce 3 polar bodies that die and 1 egg Polar bodies die because of unequal division of cytoplasm Polar bodies die because of unequal division of cytoplasm Immature egg called oocyte Immature egg called oocyte Starting at puberty, one oocyte matures into an ovum (egg) every 28 days Starting at puberty, one oocyte matures into an ovum (egg) every 28 days

33 Oogenesis Oogonium(diploid)Mitosis Meiosis I Ovum (egg) a A X X a X A X a X a X Matureegg A X A X

34 Comparing Mitosis and Meiosis

35 MitosisMeiosis WHAT it is used for How many daughter cells Are the daughters identical to the parent? If the parent is diploid with 16 chromosomes, what will the daughter be? Does the DNA need to replicate before division? Does crossing over occur?

36 MitosisMeiosis Number of divisions Number of daughter cells Genetically identical? Chromosome # Where When Role Comparison of Divisions

37 MitosisMeiosis Number of divisions 12 Number of daughter cells 24 Genetically identical? YesNo Chromosome # Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction Comparison of Divisions