1. Meiosis

2. Meiosis
a specialized type of cell division that results in four haploid daughter cells, each with half the number of chromosomes as the parent cell and all genetically distinct from the parent cell and each other.

3. Meiosis
In sexually reproducing organisms, gametes (sperm and egg) must be produced
The gametes must have only HALF the normal number of chromosomes in order to be reproductive cells

4. Meiosis
Advantage:
Meiosis is crucial in ensuring genetic variation (biological diversity) among populations which helps a population survive changes in the environment.
Disadvantages:
Requires more energy than asexual reproduction because gametes must be produced then have the opportunity to meet
Embryo must be protected and nurtured

5. Understanding Ploidy
Diploid (2n): 2 sets of chromosomes in somatic cells (body cells)
mitosis is one diploid cell to two diploid cells
Haploid (n): 1 set of chromosomes in gametes (sperm and egg cells)
meiosis is one diploid cell to four haploid cells

6. Stages of Meiosis
For meiosis to occur, the DNA must replicate once then divide twice
Homologous chromosomes: a pair of chromosomes with same size, shape, and carrying the same alleles for the same traits

7. Interphase The diploid cell (2N) is in a neutral state
cell is growing and DNA replicates

8. Prophase I Replicated chromosomes condense becoming visible
nuclear membrane breaks down
spindles appear develop, centrioles move to poles of cell;
2 more things occur:
Pairing of homologous chromosomes: the 2 pairs of homologous chromosomes move together and align end-to-end to form a tetrad
Crossing over: segments of tetrads break and reattach to the other chromosome

10. Metaphase I Nucleus has completely disintegrated
Tetrads align at the equatorial plane
Spindle fibres attach the centriole to the centromere of each chromosomes in the homologous pair

11. Anaphase I
The spindle fibres begin to reel in the homologous chromosomes to the poles
The sister chromatids are not separated in the first division, each chromosome within the homologous pair goes to opposite poles of the cell
Disjunction: the normal separation of the homologous chromosomes

12. Telophase I
Chromosomes have reached the poles of the cell and the chromosomes uncoil back into chromatin
Nuclear membrane begins to form around each group of divided chromosomes
Cytokinesis occurs to split the cell into two daughter cells (cell plate in plates and cleavage furrow in animal cells)

13. Prophase II
Duplicated chromosomes condense and become visible under the microscope
Nuclear membrane breaks down
Centrioles migrate to poles of the cell and begin to develop spindle fibres

14. Metaphase II Nucleus has completely disintegrated
Duplicated chromosomes align at the equatorial plane
Spindle fibres attach the centriole to the centromere of each chromosome in the duplicated pair

15. Anaphase II
The spindle fibres begin to reel in the chromosomes to the poles
The sister chromatids are separated in this second division, and each sister chromatid is reeled to the opposite pole of the cell

16. Telophase II
Chromosomes have reached the poles of the cell and the chromosomes uncoil back into chromatin
Nuclear membrane begins to form around each group of divided chromosomes
Cytokinesis occurs to split each cell into two daughter cells (cell plate in plates and cleavage furrow in animal cells)