Miosis Lecture

1. Meiosis
• What does this
picture tell us
about meiosis?
• How is the
purpose of
meiosis different
from the purpose
of mitosis?

2. Why is meiosis necessary?
Why can’t mitosis be the cell division process that
runs sexual reproduction?
What would happen to the chromosome number
from one generation to the next?
46
92
184
368
736
1472

3. In sexual reproduction, meiosis and
fertilization are complementary processes;
Meiosis produces the haploid chromosome
number (n) and…
Fertilization restores the diploid (2n)
chromosome number in the zygote.
Meiosis is often called ‘Reduction Division’
since the process reduces the number of
chromosomes in half (2n  n)
(Mitosis would be 2n  2n or n  n)

4. Meiosis involves two divisions
Each division includes prophase,
metaphase, anaphase, and telophase.
A Roman Numeral is added to each phase.
Prophase I  Metaphase I  Anaphase I
 Telophase I …
Interphase (no DNA replication) …
Prophase II  Metaphase II 
Anaphase II  Telophase II

5. Stage Descriptions/Characteristics
ProphaseI
Synapsis occurs –
Homologous
chromosomes pair up
Each group is made up
of 4 chromatids.
Each group of 4
chromatids is called
a…
TETRAD.

6. Crossing Over
• When parts of non-sister
chromatids change
places…
• (increases genetic variety)

7. Metaphase I
The tetrads are aligned at the equator.
[note the crossing over…]

8. Anaphase I
Tetrads split and chromosomes (in replicated form) move
to opposite poles.

9. Telophase I
Two cells form – each cell is haploid but
each chromosome is in replicated form.

10. Interphase
(INterkinesis)
DNA is in chromatin form.
No DNA replication occurs!
Why?
 because each cell contains
chromosomes already in
replicated form – no need to
copy…

11. Prophase II
Each cell is haploid with chromosomes in
replicated form.
 note the
Crossing over…

12. Metaphase II
Chromosomes align at the equator in both
cells.

13. Anaphase II
Chromosomes separate – Chromatids move
towards opposite poles.
(Centromeres split.)

14. Telophase II
4 cells result – each cell is haploid with
unreplicated chromosomes.
Each cell will become a gamete (sperm/egg)

18. What phase of meiosis?

19. Telophase I or Telophase II?

21. Meiosis I or II just like Mitosis?