1. CELL DIVISION
MITOSIS & MEIOSIS

2. Cell Cycle 2 distinct phases
S
Interphase
Mitotic
What's the most important
event of interphase?
Chromosome duplication
G2

3. What is significant about DNA in
the S and G2 phases?
All chromosomes are duplicated
That means: two copies of each chromosome
What are the copies called?
Sister chromatids
Do they contain identical genes?
Yes, but…
What about meiosis?

4. Some terminology
• Chromatin
• Homologous chromosomes
• Chromatid
• Double-chromatid chromosomes
• Single-chromatid chromosomes
When do each of these occur?

5. THE STEPS OF MITOSIS
• Interphase
(actually, this is not part of mitosis itself)
• Prophase
• Metaphase
• Anaphase
• Telophase

6. Interphase
Onion root tip Whitefish blastula
Mitosis
• Nuclear envelop intact
• Chromosomes duplicated but not apparent
• DNA loose, uncondensed, called chromatin
• 2 nucleoli visible
in onion root tip
• Nucleoli not visible
in whitefish
blastula

7. Prophase Mitosis
Onion root tip Whitefish blastula
• Double-chromatid chromosomes evident
• Chromatin becomes super-coiled & compact
• Nuclear envelop breaks down
• Centrioles migrate toward poles of cell
forming the spindle
• Nucleoli disappear
in onion root tip

8. Double-chromatid chromosomes

9. Metaphase Mitosis
Onion root tip Whitefish blastula
• Double-chromatid chromosomes line up
on equatorial plate of cell
• The spindle fibers attach to the
chromosomes at the centromeres

10. Anaphase Mitosis
Onion root tip Whitefish blastula
• Spindle fibers pull chromatids apart
• Centromeres are pulled apart
• Groups of single-chromatid chromosomes
move opposite poles of the cell

11. Telophase Mitosis
Onion root tip Whitefish blastula
• Groups of single-chromatid
chromosomes reach poles of cell
• Nuclear envelop begins to reform
• 2 new daughter cells formed
• Cytokinesis begins
with appearance of
cleavage furrow
• Cytokinesis begins with
appearance of cell plate
• Nucleoli reform

12. Returning to Interphase Mitosis
Onion root tip Whitefish blastula
• Cytokinesis completes
• Chromosomes ‘disappear’ as
interphase chromatin reforms
• 2 new daughter cells enter G1 phase of
the cell cycle

13. A SUMMARY OF
MITOSIS

14. Mitosis Questions
1. What does “diploid” mean?
2. We have __ total chromosomes.
3. In the term 2n, what does “n” stand for in
us? In a gypsy moth?
4. Why does mitosis occur? Major functions?
5. In what cells (general term) does mitosis
occur?

15. Mitosis Questions
6. What are chromatids?
7. What happens in Anaphase to result in
each new cell receiving duplicate
parental DNA?
8. In a species whose diploid number is
224, what would its sperm/eggs contain?

16. Why is this duplication of
parental DNA necessary?
Something to do with passing on genetic information?

17. Why undergo mitosis at
all?
Something to do with cells getting damaged, old, lost?
Something to do with the organism growing, infant to adult?

18. What are typical body cells called?
SOMATIC CELLS
These cells divide continuously
The new cells receive an exact
copy of all the parent cell’s:
DNA
What is the process of somatic cell
duplication called?
Mitosis

19. What are these somatic
cells?
Diploid or 2n
What does this mean?
They contain the full
number of chromosomes
in pairs
How many
in humans?
46
23 pairs

20. Mitosis occurs only in
somatic cells
What about sex cells?
Called gametes
Eggs and sperm
Produced in ovaries or testes
• Gametes are not diploid (2n)
• Instead, they are haploid (n)

21. Our haploid (n) number is 23
So our eggs and sperm have how many
chromosomes?
Half the number
Why?

22. Fertilization is the union of
an egg and a sperm
If the egg and sperm were
both diploid, what would the
fertilized egg (zygote) be?

23. It would be a genetic
mess!
Instead, gametes are haploid (n).
At fertilization, n + n = 23; 23 + 23 = 46!
Egg and sperm both
have exactly half the
number of
chromosomes of
somatic cells

24. Note how mitosis and
meiosis differ:
• Number of divisions?
• Number of chromosomes?
• Number of products?

25. Meiosis !
Somehow somatic cells (2n) in our
ovaries or testes must produce
gametes (n)
Meiosis is the cell division process that enables
the transformation from 2n to n

26. How is meiosis more
complicated than mitosis?
Gametes must contain precisely half the diploid
number of chromosomes
They must contain one of each homologous pair of
chromosomes
Remember karyotypes?
Each chromosome has a partner
They come in pairs
One from mom
One from dad

27. Human Karyotype

28. Which pair of chromosomes in us
in not homologous?
It’s the 23rd pair in males, the XY pair
Remember what homologous
means?
All our other pairs of
chromosomes are
homologous

29. How many divisions does meiosis have?
What are the divisions called?

30. 1 2 3 4
1. One pair of homologues to start
2. DNA is replicated
3. Meiosis 1 = 2n ---> n
4. Meiosis 2 = chromatids separate into 4 products

32. In males, how many progeny
are produced?
Typically 4 viable sperm are produced
following each Meiosis 2

33. In females, how many progeny
are produced?
Just one viable ovum (egg) is produced,
plus 3 small polar bodies

34. Crossing over
When does it occur?
Only during Prophase of Meiosis 1
Homologous chromosomes get
together in temporary tetrads
Overlap (cross over) and trade their DNA
Why is this a good thing to do,
generally?

35. Meiosis Summary
1. Meiosis 1
a. DNA replication takes place
b. A parent cell produces two daughter cells
each with one member of each original
pair of homologous chromosomes (to
create haploid daughter cells)
c. Crossing over may occur

36. Meiosis Summary
2. Meiosis 2
a. There is no more DNA replication
b. The chromatids of each chromosome separate
and each daughter cell divides
c. At the end of Meiosis 2, there are 4 daughter
cells from each parent cell. Each daughter cell
has half the number of chromosomes as the
parent cell

37. Meiosis Questions
1. The cells produced in meiosis are _____
(haploid or diploid?)
2. Sex cells are called ________.
_____ are produced by males, ____ by
females.
3. What’s crossing over and why is it
important?

38. Meiosis Questions
4. In what meiotic stage does crossing
over occur?
5. Why are sex cells n, not 2n?
6. If a species’ diploid number is 50, what
is n?