1. form of cell division by which gametes, with half the number of chromosomes, are produced. 2n n

2.  sexual reproduction  Two divisions (meiosis I and meiosis II)  Diploid (2n)  haploid (n)  divide to produce gametes (sperm or egg)  Gametes have half the number of chromosomes  Occurs only in gonads (testes or ovaries)

3.  Similar to mitosis interphase.  Chromosomes replicate (S phase).  Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.  Centriole pairs also replicate.

4.  Interphase is divided into three phases: Growth 1 (G 1 ) phase:  This is a very active period, where the cell synthesizes its vast array of proteins,  including the enzymes and structural proteins it will need for growth.  In G 1 stage each of the chromosomes consists of a single (very long) molecule of DNA.  In humans, at this point cells are 46 chromosomes, 2N, identical to somatic cells.

5. Synthesis (S) phase:  The genetic material is replicated: each of its chromosomes duplicates,  so that each of the 46 chromosomes becomes a complex of two identical sister chromatids.  The cell is still considered diploid because it still contains the same number of centromeres.  The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope.  This will take place during prophase I in meiosis.

6. Growth 2 (G 2 ) phase:  G 2 phase as seen before mitosis is not present in Meiosis.  Actually, the first four stages of prophase I in many respects correspond to the G 2 phase of mitotic cell cycle.

7.  four phases: a.prophase I b.metaphase I c.anaphase I d.telophase I

8.  Longest and most complex phase (90%).  Chromosomes condense.  Synapsis occurs: homologous chromosomes come together to form a tetrad.  Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

9. Homologous chromosomes sister chromatids Tetrad

10.  Pair of chromosomes (maternal and paternal) that are similar in shape and size.  Homologous pairs (tetrads) carry genes controlling the same inherited traits.  Each locus (position of a gene) is in the same position on homologues.  Humans have 23 pairs of homologous chromosomes. a.22 pairs of autosomes b.01 pair of sex chromosomes

11. eye color locus eye color locus hair color locus hair color locus PaternalMaternal

12.  Crossing over (variation) may occur between non-sister chromatids at the chiasmata.  Crossing over: segments of non-sister chromatids break and reattach to the other chromatid.  Chiasmata (chiasma) are the sites of crossing over.

13. nonsister chromatids chiasmata: site of crossing over Tetrad variation

14. centrioles spindle fiber aster fibers

15.  Shortest phase  Tetrads align on the metaphase plate.  INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2 n Example:2n = 4 then n = 2 2 2 thus 2 2 = 4 combinations

16. metaphase plate OR metaphase plate

17.  Homologous chromosomes separate and move towards the poles.  Sister chromatids remain attached at their centromeres.

19.  Each pole now has haploid set of chromosomes.  Cytokinesis occurs and two haploid daughter cells are formed.

21.  Go from 1 cell to 2 cells  Each daughter cell contains 1 copy of each chromosome  they are haploid  with the chromosomes still having 2 chromatids each  For humans: start with one cell containing 46 chromosomes (23 pairs) to 2 cells containing 23 chromosomes.  As a result of crossing over, each chromosome is the mixture of the original homologues

22.  Meiosis 2 is just like mitosis  No replication of DNA between meiosis 1 and meiosis 2.  Chromosomes line up individually on the equator of the spindle at metaphase.  At anaphase the centromeres divide, splitting the 2 chromatids.  The one-chromatid chromosomes are pulled to opposite poles.  No interphase II

24. metaphase plate

26.  Nuclei form.  Cytokinesis occurs.  Remember:four haploid daughter cells produced. gametes = sperm or egg

28.  Start with 2 copies of each chromosome (homologues), each with 2 chromatids.  In meiosis I, crossing over in prophase mixes alleles between the homologues.  In metaphase of meiosis I, homologues pair up, and in anaphase the homologues are separated into 2 cells.  Meiosis II is just like mitosis. The centromeres divide in anaphase, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.