Introduction to Meiosis

1. Meiosis:
The Formation of Gametes

2. Goal of Meiosis?
• To form gametes, or reproductive
cells, of organisms that reproduce
through sexual reproduction.
• Sexual reproduction =
combination of
two reproductive cells
to form offspring

3. Recall…
• In mitosis, it was somatic cells that
divided.
• One cell reproduced asexually to
produce identical cells (offspring)
• The two resulting daughter cells are
identical, clones of the parent cell…
• Same number of chromosomes in parent
and daughter cells (offspring)

4. Let’s think about
humans…
• Human somatic cells have 46
chromosomes each…
• …in 23 pairs.
• Homologous chromosomes = pairs
• (Diploid = in sets of 2 = 2n)
• 22 pairs = autosomes
• 1 pair = sex chromosomes

5. Human example,
continued…
• When somatic cells reproduce,
they always produce cells with
the same number of
chromosomes.
• Ex: skin cells have 46
chromosomes, and through
mitosis, always produce new skin
cells with 46 chromosomes each.

6. Human example,
continued…
• Why do skin cells always have to
produce more skin cells that are
identical (in other words, always
having 46 chromosomes)?
• Because 46 chromosomes is the
complete set of instructions (DNA) for
the cell.
• Anything other than 46 chromosomes for
human somatic cells would be abnormal (ie,
NOT good!)

7. Production of
new skin cells =
ASEXUAL reproduction

8. But, what about…
• …when humans make new humans?
• …in other words, produce the next
generation…
• What has to happen for a new human?
+ = Zygote
(fertilized egg)
(single cell)

9. Sexual Reproduction:
a new organism is produced
when the sex cells from two
parents combine.
Sex cells = gametes

10. Think about this…
• …What if the egg and the sperm
each have the same number of
chromosomes as somatic
cells…
• …46 chromosomes each.
• What would happen when they
combined to make a new
human?

11. It would be:
46 46+ = 92
Should a human have 92 chromosomes?!?!

12. Human Fertilization
has to be:
23
Chromosomes
23
Chromosomes
46
Chromosomes

13. Human somatic cells
have
46 chromosomes
But…
gametes have to be 23
(half)
2n = 46 n = 23

14. The process called
MEIOSIS produces

15. Meiosis = process of
reduction division
Number of chromosomes per cell is cut in half.
The homologous chromosomes are separated.
Haploid = half a set of chromosomes
= hap = half = (n)

16. Do: Homologous chromosomes model
Go to: Meiosis Flip Book

17. Meiosis
There are two divisions in Meiosis:
•Meiosis I
•Meiosis II

18. Meiosis I
Meiosis I : Ends with 2 cells.
The chromosomes in the cells are still
double stranded.

19. Meiosis I
• DNA replicates prior to Meiosis I
• Each chromosome pairs with its
corresponding homologous
chromosome to form a tetrad.
• During Prophase I, an exchange of
genes occurs (crossing over)

20. Crossing Over

21. Meiosis II
Meiosis II: Ends with 4 cells.
Each of the 4 cells is a gamete. Each gamete has
half the chromosomes of the original cell.

22. Meiosis II
• The cells produced during
Meiosis I have different sets of
chromosomes and genes due to
“crossing over”
• It is like they have been
shuffled.

23. Meiosis II, cont.
• Each chromosome contains
2 chromatids
• They line up at the center of
each cell
• Paired chromatids separate
• 4 daughter cells are produced
(haploid)