1. CH Meiosis

2. Meiosis
Meiosis is cell division that produces gametes with half the number of chromosomes as a body cell

3. Meiosis Diploid cell: has 2 of each kind of chromosome
All body cells, except gametes, are diploid cells (such as liver or lung cells)
When they divide, this is called mitosis
Haploid cell: has 1 of each kind of chromosome
Gametes are haploid cells
When they divide, this is called meiosis

4. Why Meiosis?
Each type of organism has a specific number of chromosomes (humans have 46 chromosomes)
Because each parent passes along a chromosome, there has to be some way for offspring to have the same number of chromosomes as their parents and not double

5. Why Meiosis?
Gametes that are produced are sperm cells for males and egg cells for females
Sexual reproduction: when haploid egg and sperm cells join together to form a diploid zygote
In humans, each sperm and egg cell has 23 chromosomes (which is half of 46, the normal number of chromosomes) or Sperm (23) + Egg (23) = Zygote (46)

6. Why Meiosis?
For the most part, healthy human zygotes cannot have more than 46 chromosomes
Once formed, zygotes undergo mitosis to grow and develop (asexual reproduction)

7. Before Meiosis Homologous chromosomes are attached to each other
Homologous chromosomes: 2 of the same type of chromosome
Chromosomes copy themselves so that they can be passed along to the daughter cells

8. Before Meiosis
Tetrad: 2 homologous chromosomes attached together (= 4 sister chromatids)

9. Genetic Variation
Genetic variation: rearrangement of alleles that can produce completely new combinations (this is essential for evolution to occur)

10. Genetic Variation
Crossing Over- When the homologous chromosomes pair so tightly that a piece of the chromatid breaks off. The piece changes place with a piece from the other homologous tetrad. This scrambles the traits around = genetic variety.

11. Genetic Variation
Genetic Recombination: the reassortment of chromosomes and genetic material, either by crossing over or independent assortment, which increases genetic variety

12. Genetic Variation
Both crossing over and independent assortment are completely RANDOM…so this is why humans can sometimes look completely different from other family members

13. Meiosis I—2 new cells are formed
Prophase I:
*Tetrads condense (2 homologous chromosomes attached together)
*Crossing over occurs between chromosomes
*Nuclear membrane disappears
Centrosomes move to opposite ends of cell to form spindle fibers
Metaphase I: Tetrads line up in the middle of the cell

14. Meiosis I
Anaphase I: Tetrads split and each set of sister chromatids moves to opposite ends of the cell
Telophase I: Cytokinesis (division of the cytoplasm) results in 2 new cells, each with homologous chromosomes

15. Meiosis II (4 new cells are formed) *LIKE MITOSIS*
Prophase II: Centrosomes on the chromosomes form spindle fibers in preparation to be pulled apart
Metaphase II: Chromosomes (sister chromatids) line up randomly in the middle

16. Meiosis II
Anaphase II: Sister chromatids separate and move to opposite ends of the cell
Telophase II/Cytokinesis: : New nuclei are formed and four daughter cells are produced, each containing ONE chromosome from each homologous pair

18. Mistakes in Meiosis
Nondisjunction: when homologous chromosomes don’t separate from each other during meiosis
So homologous chromosomes move together into a new gamete giving that gamete too many chromosomes, while the other new gamete is missing the chromosome

19. Mistakes in Meiosis Organisms with extra chromosomes can survive
Organisms with too few chromosomes usually do not survive

20. Mistakes in Meiosis
Polyploidy: when organisms have more than the normal number of chromosome sets
instead of 46 chromosomes, they might have double or triple that number
Can be lethal for animals but not plants (plants just usually get bigger)

21. Conditions that Result from Extra Chromosomes
Examples of Nondisjunction
Genotype (X=Female sex chromosome,
Y= male sex chromosome)
Turner’s Syndrome XO
Trisomy X (metafemales)
XXX
Klinefelter’s syndrome
XXY
Jacob’s syndrome
XYY
Down syndrome
TRISOMY 21 (3 copies)
Patau’s syndrome
TRISOMY 13
Lethal condition in males OY

22. Comparing Mitosis and Meiosis
DNA replicates in interphase
ONE division of chromosomes
Separate sister chromatids
TWO divisions of chromosomes
Separate homologous chromosomes (meiosis I)
Separate sister chromatids (meiosis II)
Makes 2 identical DIPLOID cells
Makes 4 HAPLOID cells
Chromosome # is identical to parent cell
Chromosome # is HALF of the parent cell
Used for replacing old cells, growth
Used to make gametes