1. Meiosis
Chapter 13

2. You must know The differences between asexual and sexual reproduction
The role of meiosis and fertilization in sexually reproducing organisms
The importance of homologous chromosomes to meiosis
How the chromosome number is reduced from diploid to haploid in meiosis
Three events that occur in meiosis but not mitosis
The importance of crossing over, independent assortment, and random fertilization to increasing genetic variability

3. Genes – segments of DNA that code for proteins
Transmitted from one generation to the next by gametes
Locus – location of a gene on a chromosome
Ex. CF - 7q31.2
7 = chromosome #7
q = long arm
Region 3
Band 1
Sub-band 2

4. Sexual vs. Asexual Reproduction
Asexual – single parent, offspring is an exact copy/clone
Sexual – two parents contribute genes to offspring
Results in greater genetic variability
Life Cycle – generation to generation sequence of events from conception to reproduction of offspring

5. Homologous chromosomes
Similar in length and centromere position, carry genes for the same traits in the same loci and therefore have similar staining patterns
Inherit one from each parent
Karyotype – picture of an
individual’s complete set of
chromosomes arranged in
homologous pairs

6. Sex Chromosomes
XX = female, homologous
XY = male, not homologous
Non-sex chromosomes are called autosomes

7. Meiosis
Cell division that reduces the number of chromosomes from diploid to haploid in order to maintain chromosome number across generations
Consists of 2 divisions
Results in 4 haploid cells

8. Interphase
Same as in mitosis (G1, S, G2)
For meiosis occurs in cells called oogonia and spermatogonia, found in ovaries and testes

9. Synapsis – homologues find each other and join to form a tetrad
Prophase I
Chromosomes condense
Synapsis – homologues find each other and join to form a tetrad
Perfectly align gene to gene

10. Prophase I cont.
Crossing over – homologues exchange portions of themselves – very exact
Chiasmata – point where
chromosomes cross – help
hold homologues together
Increases genetic variability

11. Prophase I cont.
Centrioles move to poles
Nuclear membrane disintegrates
Spindle fibers attach to kinetochores

12. Homologous pairs are pulled to metaphase plate
Metaphase I
Homologous pairs are pulled to metaphase plate
Lined up in pairs!

13. Anaphase I
Homologous pairs are separated and pulled to opposite sides of the cell
Note: Sister chromatids still attached

14. Each pole contains a haploid set of duplicated chromosomes
Telophase I
Each pole contains a haploid set of duplicated chromosomes
Cytokinesis – results in 2 haploid daughter cells
Note: Meiosis I is called a “reduction division”

15. Prophase II
Spindle forms and attaches to chromosomes at kinetochores

16. Chromosomes line up single file at the metaphase plate
Metaphase II
Chromosomes line up single file at the metaphase plate
Due to crossing over in prophase I, the sister chromatids are not identical

17. Anaphase II
Sister chromatids are separated and pulled to opposite poles

18. Telophase II
Nuclear membranes form around sets of chromosomes
Cytokinesis occurs

19. Results of Meiosis
4 haploid daughter cells, each genetically different from each other and from the parent cell

20. Differences from mitosis
Synapsis and crossing over
Homologous pairs line up during metaphase I
Homologous pairs separated in anaphase I

21. Genetic variation
Crossing over – shuffling of genes from parents so that no offspring receives an exact copy of a chromosome from their grandparent
Independent Assortment – separation of chromosomes during metaphase I & II is random – some parental and some maternal chromosomes go to each side
Random fertilization – which sperm happens to reach which egg in order to produce an offspring