1. Meiosis Notes

2. Meiosis
Meiosis occurs in sexual reproduction when a diploid germ cell produces four haploid daughter cells that can mature to become gametes (sperm or egg).

3. Meiosis Goal: reduce genetic material by half
Why? n (mom) + n (dad) = 2n (offspring)
from mom
from dad
child
too
much!
meiosis reduces
genetic content
Just right!

4. Fertilization

5. Chromosomes

6. Meiosis: cell division in two parts
Sister
chromatids
separate
Haploid
Diploid
Meiosis I
(reduction
division)
Meiosis II
Chromatids
separate
(equational
division) 2n n n
Result: one copy of each chromosome in a gamete.

7. Meiosis I : the reduction division
Prophase I
(early)
(diploid)
(late)
Metaphase I
Anaphase I
Telophase I
Nucleus
Spindle
fibers
Nuclear
envelope

8. Prophase I Chomatids pair and condense. Crossing over occurs
Spindle forms.
Nuclear envelope
fragments.

9. Metaphase I
Chromatid pairs align along the equator of the cell.

10. Anaphase I Chromosomes separate and move to opposite poles.
Sister chromatids remain attached at their centromeres.

11. Telophase I Nuclear envelopes reassemble. Spindle disappears.
Cytokinesis divides cell into two.

12. Meiosis II Gene X Sister chromatids carry identical genetic
Meiosis II produces gametes with
one copy of each chromosome and thus one copy of each gene.
Sister chromatids carry
identical genetic
information.

13. Meiosis II : the equational division
Prophase II
(haploid)
Metaphase II
Anaphase II
Telophase II
Four
Non-identical
haploid
daughter cells

14. Prophase II
Nuclear envelope fragments.
Spindle forms.

15. Metaphase II
Chromosomes align along equator of cell.

16. Anaphase II
Sister chromatids separate and move to opposite poles.

17. Telophase II Nuclear envelope assembles. Chromosomes unravel.
Spindle disappears.
Cytokinesis
divides cell into
two.

18. Results of meiosis
Four haploid cells
One copy of each
chromosome

19. Meiosis allows the creation of unique individuals
through sexual reproduction.
Meiosis keeps the number of chromosomes the same for each generation.

20. Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 4
Genetically identical?
Yes No
Chromosome #
Same as parent
Half of parent
Where
Somatic cells
Sex cells
When
Throughout life
At sexual maturity
Role
Growth and repair
Sexual reproduction

21. MITOSIS MEIOSIS Occurs in somatic (body) cells
Occurs only in gonads (sex organs: ovary/testes)
Produces cells for repair,
maintenance, growth, asexual reproduction
Only produces gametes (sex cells: egg/sperm)
Results in identical diploid (2n) daughter cells
Reduction division results in haploid (n) cells

22. Mistakes in Meiosis
Nondisjunction = The failure of homologous chromosomes to separate properly during meiosis

23. Trisomy: When a gamete with extra chromosomes is fertilized by a normal gamete, the zygote will have an extra chromosome
Ex. In human, if a gamete with an extra chromosome number 21 is fertilized by a normal gamete, the resulting zygote has 47 chromosomes instead of 46. This will develop into a baby with Down syndrome.

24. Monosomy = When a gamete that is missing a chromosome fuses with a normal gamete during fertilization, the resulting zygote lacks a chromosome.
Most often this is lethal. One non-lethal example is Turner syndrome, in which human females have only a single X chromosome instead of two.

25. Polyploids = organisms with more than the usual number of chromosomes sets
Often results in death of the zygote. However, polyploids often occur in plants