1. Meiosis
& Sexual Reproduction

2. Remember that 1 chromosome is made when 2 chromatids join at the centromere.
These 2 chromatids are EXACT copies of each other
Every human cell has 23 pairs of chromosomes
Total of _____
1 set from Mom, 1 set from Dad (_______ from each!)
They each give us the same chromosomes.
Same size, shape….
They are only different in their GENES! (DNA that codes for a protein)
Different versions of a gene are called ALLELES
Think flower color or hair color 46 23

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. Chromatids vs. Homologous Chromosomes

5. Combining the 1 set from each parent gives us 23 pairs (46 total) – DIPLOID
Diploid just means 2 of each chromosome
All human cells except sex cells!
HAPLOID means 1 of each chromosome
In humans, only sex cells!

6. HAPLOID
HAPLOID
DIPLOID

7. Meiosis – Making the Sperm and Egg
Sperm and Egg are haploid
Meiosis has 2 phases, Meiosis I and Meiosis II
Meiosis I = Homologous chromosomes separate
Meiosis II = Sister chromatids separate

8. Meiosis I : (the reduction division) -Homologous Chromosomes Separate
Prophase I
(early)
(diploid)
(late)
Metaphase I
Anaphase I
Telophase I
Nucleus
Spindle
fibers
Nuclear
envelope

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

10. Occurs during Prophase I when parts of homologous chromosomes overlap
Crossing Over
Occurs during Prophase I when parts of homologous chromosomes overlap
Letters A/a and B/b represent alleles

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

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

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

14. Meiosis II : (the equational division) -Sister Chromatids Separate
Meiosis II : (the equational division) -Sister Chromatids Separate *****very similar to Mitosis*****
Prophase II
(haploid)
Metaphase II
Anaphase II
Telophase II
Four
Non-identical
haploid
daughter cells

15. Prophase II
Nuclear envelope fragments.
Spindle forms.

16. Metaphase II
Chromosomes align along equator of cell.

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

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

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

20. Animations

21. Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 4
Results
identical diploid (2n) daughter cells
Reduction division results in haploid (n) cells
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

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

24. 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.

25. 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.

26. Practice Questions What do Haploid and Diploid mean?
How are homologous chromosomes and sister chromatids different?
Describe crossing over in your own words.
Which phase of Meiosis (I or II) is most like Mitosis? Why?

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

28. Hermaphrodites (yes, they DO exist in the human population)
-Have both male and female genitals

29. Parthogenesis spontaneous reproduction where the egg develops into an adult without ever being fertilized by a male.