1. CELL DIVISION MITOSIS & MEIOSIS

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

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 When do each of these occur? Chromatin
Homologous chromosomes
Chromatid
Double-chromatid chromosomes
Single-chromatid chromosomes
When do each of these occur?

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

6. Interphase Nuclear envelop intact
Mitosis
Interphase
Onion root tip
Whitefish blastula
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 Double-chromatid chromosomes evident
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. Mitosis
Metaphase
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 Spindle fibers pull chromatids apart
Mitosis
Anaphase
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 Groups of single-chromatid chromosomes reach poles of cell
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 cell plate
Cytokinesis begins with appearance of cleavage furrow
Nucleoli reform

12. Returning to Interphase
Mitosis
Returning to Interphase
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 What does “diploid” mean?
We have __ total chromosomes.
In the term 2n, what does “n” stand for in us? In a gypsy moth?
Why does mitosis occur? Major functions?
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. SOMATIC CELLS What are typical body cells called? DNA Mitosis
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).
Egg and sperm both have exactly half the number of chromosomes of somatic cells
At fertilization, n + n = 23; = 46!

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
One pair of homologues to start
DNA is replicated
Meiosis 1 = 2n ---> n
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 Meiosis 1 DNA replication takes place
A parent cell produces two daughter cells each with one member of each original pair of homologous chromosomes (to create haploid daughter cells)
Crossing over may occur

36. Meiosis Summary Meiosis 2 There is no more DNA replication
The chromatids of each chromosome separate and each daughter cell divides
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
The cells produced in meiosis are _____ (haploid or diploid?)
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?