1. Meiosis
Is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells (gametes).

2. Meiosis
Involves two divisions of the nucleus – meiosis I and Meiosis II
Starts with one diploid cell, and ends with four haploid cells.

3. Review of Terms Haploid (n)-- one set chromosomes
Diploid (2n)-- two sets chromosomes
(46 chromosomes)
Chromatid – Two exact copies of DNA (each single copy called a chromatid) makes up a chromosome
Centromere – point at which the two chromatids are attached
Homologous chromosomes – chromosomes similar in size, shape, and genetic content.

4. Meiosis

5. Meiosis I

6. Prophase I Crossing – over occurs Homologous Chromosomes synapse (pair
together)
This group of 2 chromosomes together is called a tetrad (because there are 4 chromatids)
Crossing – over occurs
When portions on one homologous chromosome are broken and exchanged with the corresponding portion on one of the chromatids of the other homologous chromosome.

7. Crossing - Over

8. Metaphase I
Paired Homologous Chromosomes line up in the middle of the cell.

9. Anaphase I Paired Homologus Chromosomes separate
Daughter cells are now HAPLOID

10. Telophase I Nuclear membrane forms around each set of chromosomes
Cytokinesis begins -division of cytoplasm
Left with two haploid daughter cells.

11. Between Meiosis I & II
The chromosomes do NOT replicate between Meiosis I and Meiosis II.
Start of Meiosis I Start of Meiosis II
DNA copy made during Interphase

12. Meiosis II – Same as Mitosis
Prophase II Metaphase II Anaphase II Telophase II

13. Prophase II A new spindle forms around the chromosomes
Spindles – are cell structures made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division
Centrosomes (Centrioles) - two cylindrical cellular structures that form at the poles of a cell during meiosis.

14. Metaphase II
Chromosomes line up in the middle of the cell

15. Anaphase II Centromeres divide
Chromatids move to opposite poles of the cell
* Remember centromeres are what hold the two chromatids together to make a chromosome.

16. Telophase II Nuclear membrane forms around each set of chromosomes
Cytokinesis begins
Left with four haploid cells.

17. Sexual vs. Asexual Reproduction
Male and Female reproduce offspring (USES MEIOSIS)
Advantages
Genetic variability – offspring are genetically different than parents; Species can rapidly adapt to new environments
Disadvantages
More energy; Slower generation time
Only 1 parent (NO MEIOSIS)
Ex: (1) Binary Fission (bacteria) and (2)Budding (yeast)
Advantages
Less Energy; Faster Reproduction Time
Disadvantages
Offspring are clones (identical) of parent; no genetic variability

18. Karyotype
Ordered display if an individuals chromosomes arranged in pairs, starting with the longest.

19. Trisomy 21
Three number 21 chromosomes, making 47 total. Causes down syndrome.

20. Errors
Nondisjunction – a member of a chromosome pair fail to separate.
Deletion – a fragment of a chromosome is lost.
Duplication – a fragment of one chromosome joins to a sister chromatid.
Inversion – a fragment reattaches to the original chromosome, but in the reverse direction.
Translocation – a fragment attaches to a nonhomologous chromosome.