1. Meiosis

2. Overview of Meiosis Sexual reproduction
Gametes, with half the # of chromosomes, are produced
2n1n
Ensures that the next generation will have a combination of traits from both parents

3. Overview of Meiosis
Homologous chromosomes: similarly constructed chromosomes with the same shape and that contain genes for the same traits (homologues)
Just like in mitosis, meiosis occurs after interphase, when the cell grows, DNA is replicated, and chromosomes are duplicated.

4. Genetic recombination: production of offspring with combinations of traits that differ from those found in either parent.
Crossing over
Independent assortment

5. Meiosis I Prophase I Synapsis occurs (pairing of
homologous chromosomes)
Spindle appears
Nuclear membrane fragments
Nucleolus dissappears

6. Meiosis I Prophase I Synapsis
Homologous chromosomes line up side by side
Crossing-over: exchange of genetic material between non-sister chromatids
Result= chromatids held together by centromere are no longer identical
Daughter cells will receive chromosomes with recombined genetic material

7. Meiosis I Metaphase I Homologous pairs line up at equator
Maternal and paternal member may be oriented toward either pole.
In humans, results in 223 (8,388,608) combinations of maternal and paternal chromosomes in gametes

8. Anaphase I Homologous chromosomes separate, pulled to opposite poles
by spindle fibers
Independent assortment: all
possible combinations of
chromosomes occur in the
daughter nuclei.

9. Telophase I Nuclear membrane reforms Nucleoli appear
Cytokinesis produces two
haploid daughter cells

11. Interkinesis: between meiosis I and II
No DNA replication since chromosomes are already dublicated

12. Prophase II: spindle appears, nuclear membrane disassembles, nucleolus disappears
Metaphase II: chromosomes attach to spindles and align at equator
Anaphase II: sister chromatids separate becoming daughter chromosomes
Telophase II: spindle disappears, nuclei form, nuclear membrane reforms
Cytokinesis occurs
Meiosis results in four haploid daughter cells

14. Nondisjunction
Failure of homologous chromosomes to separate during anaphase I, or sister chromatids to separate at anaphase II
Results in gametes with abnormal number of chromosomes
Causes various syndromes
Trisomy 21 (Down Syndrome)—extra copy of chromosome 21
Turner Syndrome—absence of second X chromosome

16. Daughter cells mature into gametes (eggs, sperms) that fuse during fertilization
Restores the diploid number of chromosomes in a cell that will develop into a new individual.