1. Cell Division:
Meiosis

2. MEIOSIS gametes are formed through meiosis
gametes - specialized reproductive cell involved in sexual reproduction (sperm or egg cells)
meiosis occurs in sexual reproduction when a diploid cell produces 4 haploid daughter cells that can mature to become gametes (sperm or egg)

3. MEIOSIS
homologous – description of chromosomes that occur in pairs; having a corresponding structure
homologous - term used to refer to chromosomes that each have a corresponding chromosome from the opposite-sex parent
homologous chromosomes are similar in shape, position & information, but not identical to each other.

4. MEIOSIS
diploid (“two sets”) – description of a cell that contains a double set of chromosomes, one from each parent
haploid - containing a single set of chromosomes

5. Chromosomes Human’s have 46 chromosomes
23 from your mother’s egg
23 from your father’s sperm
“n” refers to the # of SETS of chromosomes
A normal human cell is a diploid cell (2N) = 46
A sex cell or gamete (egg or sperm) is a haploid cell (N) = 23
Sperm (23) + Egg (23) = Zygote (46)

6. “Putting It All Together” - Fertilization

7. MEIOSIS
meiosis – process of reduction division in which the # of chromosomes per cell is cut in half & the homologous chromosomes that exist in a diploid cell are separated; process that produces haploid gametes from diploid cells

8. Meiosis produces 4 genetically different haploid cells.
Meiosis consists of 2 phases:
Meiosis I
Meiosis II
To begin, you have one diploid cell
In the end, you have four haploid cells

11. 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!

12. MEIOSIS: CELL DIVISION IN 2 PARTS
Sister
chromatids
separate
Haploid
Diploid
Meiosis I
(reduction
division)
Meiosis II
Chromatids
separate
(equational
division) 2n n n
Result: 1 copy of each chromosome in a gamete

13. MEIOSIS I : REDUCTION DIVISION
Prophase I
(early)
(diploid)
(late)
Metaphase I
Anaphase I
Telophase I
Nucleus
Spindle
fibers
Nuclear
envelope

14. nuclear envelope fragments
PROPHASE I
early prophase
chromatids pair
crossing over occurs
late prophase
chromosomes condense
spindle forms
nuclear envelope fragments

15. RECOMBINATION
During prophase I, each
chromosome pairs with its
homologous chromosome to
form a tetrad
tetrad - structure containing 4 chromatids that forms during meiosis

16. RECOMBINATION crossing-over - process by which homologous chromosomes
exchange portions of their
chromatids during meiosis
crossing-over
physical exchange of chromosomal (genetic) material between chromatids of homologous (sister) chromosomes
Result: Generation of new combinations of genes (alleles).

17. RECOMBINATION occurs in prophase of meiosis I generates diversity
Creates chromosomes with new combinations of alleles for genes A to F.

18. RECOMBINATION letters denote genes case denotes alleles A a a A B b bF F f f

19. RECOMBINATION A a A a
alleles have crossed over to produce new gene combinations B b B b C C c c D D d d E E e e F F f f

21. METAPHASE I
chromatid pairs align along equator of the cell

22. ANAPHASE I chromosomes separate & move to opposite poles
sister chromatids remain
attached at their centromeres

23. TELOPHASE I & CYTOKINESIS I
nuclear envelopes reassemble
spindle disappears
cytokinesis divides cell into 2

24. MEIOSIS II Gene X sister chromatids carry identical genetic
Meiosis II produces gametes with 1 copy of each chromosome & thus 1 copy of each gene
neither cell replicates its chromosomes
each cell splits (similar to mitosis)
produces 4 daughter cells
sister chromatids carry
identical genetic
information

25. MEIOSIS II : EQUATIONAL DIVISION
Prophase II
(haploid)
Metaphase II
Anaphase II
Telophase II
Four
Non-identical
haploid
daughter cells

26. PROPHASE II
nuclear envelope fragments
spindle forms

27. METAPHASE II
Chromosomes align along equator of cell

28. sister chromatids separate & move to opposite poles
ANAPHASE II
sister chromatids separate
& move to opposite poles

29. TELOPHASE II & CYTOKINESIS II
nuclear envelope assembles
chromosomes unravel
spindle disappears
cytokinesis divides cell into 2

30. 1 copy of each chromosome
RESULTS OF MEIOSIS
4 haploid cells
1 copy of each chromosome

31. Comparing Mitosis and Meiosis
Similar processes with very different & distinct outcomes
Mitosis results in production of 2 genetically identical diploid cells (2N), whereas meiosis produces 4 genetically different haploid cells (N).

32. Mitosis vs. meiosis
Meiosis KM

33. genetically identical? yes no
MITOSIS
MEIOSIS
# of divisions 1 2
# of daughter cells 4
genetically identical?
yes no
chromosome #
same as parent
results in identical diploid (2n) daughter cells
half of parent
reduction division results in haploid (n) cells
where
somatic (body) cells
only in cells in gonads (sex organs: ovary/testes)
when
throughout life
at sexual maturity
role
growth and repair
asexual reproduction
produces gametes (sex cells: egg/sperm)
sexual reproduction

34. 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

35. Meiosis KM