1. Meiosis

2. Homologous chromosomes – identical chromosomes
…same length, centromere, same staining pattern
…exception…males sex chromosome
1-22 autosomes
sex chromsomes
23 of the 46 chromosomes are paternal
23 of the 46 are maternal

3. Homologous chromosomes
Paired chromosomes
both chromosomes of a pair carry “matching” genes
control same inherited characters
homologous = same information
single stranded homologous chromosomes
diploid 2n
2n = 4
double stranded homologous chromosomes

4. How do we make sperm & eggs?
Must reduce 46 chromosomes  23 23 46 23 46
egg 23
meiosis 46 23
Gametes – single set of 22 chromosomes
1 sex chromosome…(n)=23
zygote
fertilization
sperm
Gametes – single set of 22 chromosomes
1 sex chromosome…(n)=23
gametes

5. Meiosis: production of gametes
Happens in gonads
Conserves chromosome #
diploid  haploid
2n  n
humans: 46  23
meiosis reduces chromosome number
makes gametes
fertilization restores chromosome number
haploid  diploid
n  2n

6. Meiosis I – Division of homologous pairs
Interphase I : -G1, S, G2 *replicates only once!

7. Meiosis I – Division of homologous pairs
Prophase I : -chromosomes condense & form homologous
chromosomes called tetrads or bivalents
-crossing over occurs (exchange genes between
homologous pairs
-synapsis allows for maternal chromsomes to pair
up w/ its paternal homolog
-centrosomes move to poles, meiotic spindle forms
-nuclear membrane disappears
-90% of time
At the start of prophase I, the chromosomes have already duplicated. During prophase I, they coil and become shorter and thicker and visible under the light microscope. • The duplicated homologous chromosomes pair, and crossing-over (the physical exchange of chromosome parts) occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids. The sites of crossing-over are seen as crisscrossed nonsister chromatids and are called chiasmata (singular: chiasma). • The nucleolus disappears during prophase I. • In the cytoplasm, the meiotic spindle, consisting of microtubules and other proteins, forms between the two pairs of centrioles as they migrate to opposite poles of the cell. • The nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus. • Prophase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions.

8. Meiosis I – Division of homologous pairs
Metaphase I : -homologous pairs align at the middle/equator
-spindle fibers attach to kinetochore

9. Meiosis I – Division of homologous pairs
Anaphase I : -homologous chromosomes move toward
opposite poles
*separate the pair

10. Meiosis I – Division of homologous pairs
Telophase I & Cytokinesis : -two new nuclei form
-homologous chromosomes form
into chromatin
-nuclear membrane reappears
-divide cytoplasm

11. Meiosis I Animation

12. Meiosis II – Division of sister chromatids
Interphase II : -G1, G2 * No replication!!
Prophase II : -spindle apparatus forms
-sister chromatids become visible
-nuclear membrane disappears

13. Meiosis II – Division of sister chromatids
Metaphase II : -sister chromatids at metaphase plate/midline
-spindle fibers attach to kinetochores on
each sister chromatid

14. Meiosis II – Division of sister chromatids
Anaphase II : -sister chromatids move toward opp poles

15. Meiosis II – Division of sister chromatids
Telophase II : -nuclear membrane reforms
-chromatin forms
-cytokinesis produces 4 haploid (n) cells

16. Meiosis II Animation

17. Meiosis I and II

18. Meiosis 1

19. Meiosis 2

20. Spermatogenesis Oogenesis
Spermatogenesis Oogenesis
= 4 haploid sperm =1 egg and 3 polar bodies

21. One way meiosis generates genetic variability is through the different ways in which maternal and paternal chromosomes are combined in the daughter cells.
The number of possible chromosome combinations in the haploid nuclei is potentially very large. In general, the number of possible chromosome combinations is 2n, where n is the number of chromosome pairs. For example, in fruit flies, which have 4 chromosome pairs, the number of possible combinations is 2n, or 16. For humans, with 23 chromosome pairs, there are over 8 million metaphase arrangements. 2 parents = zygote : 223 x 223 = over 70 trillion

22. 3 Sources of Genetic Variability
Independent Assortment of homologous chromosome in meiosis I
Crossing over during prophase I
Random fertilization of egg and sperm

23. Differences across kingdoms
Not all organisms use haploid & diploid stages in same way
which one is dominant (2n or n) differs
but still alternate between haploid & diploid
must for sexual reproduction

24.
Meiosis Square Dance