1. Meiosis

2. What is Meiosis?
Meiosis: A special type of cell division necessary for sexual reproduction
Produces cells called gametes
Gamete: a mature reproductive cell
Egg (AKA “ova”)
Sperm
Ova being fertilized by sperm cell (human)

3. Chromosome Made of 2 chromatids (singular = chromatid)
“sister chromatids”
Bound by a centromere
Centromere: where the spindle fibers attach to pull the sister chromatids apart

4. Chromosomes cont. Haploid: (n) a single set of unpaired chromosomes
Gamete cells
Humans have 23 chromosomes in a gamete
Diploid: (2n) a pair of homologous chromosomes
Humans have 46 chromsomes (23 pairs)
Polyploid: having more than a pair of homologous chromosomes
Examples: triploid (3), tetraploid(4)

5. Human karyotype
Male

6. Before Meiosis Starts …
Interphase: Genetic material is replicated
each chromosome duplicates
the 46 chromosomes becomes a complex of two identical sister chromatids.
the cell is diploid (2n) because it still contains the same number of centromeres.

7. Meiosis I Separate the homologues

8. Prophase 1 exchange of information
Start with 1 cell (parent cell)
Homologous chromosomes condense
Matching chromosomes pair up
Matching chromosomes exchange information during crossing over.
Crossing over: matching regions on matching chromosomes break and then reconnect to the other matching chromosome.

9. Crossing over

10. Metaphase I alignment & attachment
Homologous chromosomes align at the cell equator
Spindle fibers attach to centromeres & centrioles
Spindle fibers
Centrioles

11. Anaphase I pulling apart
Spindle fibers shorten to pull homologous chromosomes apart
2 haploid sets
each chromosome has 2 sister chromatids
Cell elongates

12. Telophase I prepare to divide
Centrioles and spindle fibers vanish
Nuclear membrane surrounds each set of haploid sister chromatids
Chromatids uncoil into chromatin
Cytokinesis occurs
2 daughter cells result
Cytokinesis: pinching of cell in half

13. Meiosis II Separate the sister chromatids

14. Prophase II separation preparation
Begin with 2 cells
Nuclear envelope vanishes
Sister chromatids condense
Spindle fibers and centrioles form
NO DNA REPLICATION between Telophase I and Prophase II

15. Metaphase II alignment & attachment
Homologous chromosomes align at the cell equator
Spindle fibers attach to centromeres and centrioles
Spindle fibers
Centrioles

16. Anaphase II pulling apart
Sister chromatids are pulled apart
Sister chromatids are now known as “sister chromosomes”
Sister chromosomes move toward opposite ends of the cell

17. Telophase II prepare to divide
Centrioles and spindle fibers vanish
Nuclear membrane surrounds each set of haploid sister chromosomes
Chromatids uncoil into chromatin
Cytokinesis occurs
4 daughter cells result (23 chromosomes each)
Cytokinesis: pinching of cell in half