cell-division for IGCSE Grade 9 and 10.ppt

cell-division for IGCSE Grade 9 and 10.ppt

1. Cell Division

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

3. All chromosomes are duplicated
Do they contain identical genes?
What is significant about DNA in
the S and G
2
phases?
That means: two copies of each chromosome
What are the copies called?
Sister chromatids
Yes, but…
What about meiosis?

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

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

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

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

8. Double-chromatid chromosomes

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

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

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

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

13. A SUMMARY OF MITOSIS

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

15. 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

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

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

18. 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?

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

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

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

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

23. Human Karyotype

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

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

26. 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

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

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

30. 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?

31. 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

32. 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