1. Spring 2009: Section 3 – lecture 2
Reading – Chapter 3
Chapter 10, pages

2. Cell Cycle and Cell Division
Four phases or stages
G1 – growth stage 1
S - synthesis stage
G2 – growth stage 2
M – cell division (mitosis or meiosis)

4. G1 phase
production of components for DNA replication
S phase
DNA replication starts

5. G2 phase
DNA replication continues
production of the components for cell division
M phase
cell division, either mitosis or meiosis

6. Regulation of the cell cycle
To insure that DNA synthesis and component production has occurred before mitosis or meiosis starts, it is necessary to have some form of cell cycle regulation.
Some system is needed to signal the start of mitosis and the end of mitosis.

7. One such system involves the synthesis and degradation of specific proteins in the cell.
Primary proteins involved are:
- cyclin
- cdc2 (cell division cycle) and other cdc proteins
- MPF – M phase promoting factor

8. MPF – M phase promoting factor
- combination of cyclin and cdc2
- can be present in two forms, inactive and active
- activation occurs through phosphorylation of cyclin subunit and dephosphorylation of the cdc subunit

9. The presence of MPF triggers mitosis, the breakdown of the nuclear membrane, and cyclin degradation.
The absence of cyclin allows mitosis to end.
As mitosis ends cyclin and MPF are low.

10. Cell cycle regulation by MPF
early interphase
- low cyclin concentration
- low MPF concentration
intermediate interphase
- synthesis of cyclin
- combination of cyclin and cdc2 to produce inactive MPF
late interphase
- continued increase in MPF
- activation of MPF

11. Cell cycle regulation by MPF
early mitosis
- presence of active MPF starts breakdown of the nuclear membrane and signals start of mitosis
- degradation of cyclin component starts
mid- mitosis
- cyclin degradation causes decrease in both MPF and cyclin
late mitosis
- reduction in cyclin and MPF signals end of mitosis and allows for reformation of the nuclear membrane

12. The level of phosphorylation of cdc2 may influence both the start of the S phase as well as the start of the M phase.
- phosphorylated cdc2 needed to start S phase
- dephosphorylated cdc2 needed to start M phase

13. Interference with production of cyclin or the phosphorylation or dephosphorylation of components of MPF can lead to disruption of the cell cycle.
This has been demonstrated using mutants for the various components.

14. Condensing of the chromosomes at the start of the M phase is facilitated by two proteins, condensin and cohesin, that are SMC (structural maintenance of chromosome) proteins Condensin – aids in the organization of the highly condensed chromosomes Cohesin – holds the sister chromatids until the first stage of the M division (prophase)

15. Mitosis – Cell division that results in daughter cells having the same amount of DNA as the original parent cell.
Stages of mitosis
- prophase
- metaphase
- anaphase
- telophase

16. Interphase
Chromatin not condensed
Nuclear membrane present
Nucleolus present

17. Prophase
Sister chromatids start to condense
Spindle fibers start to form
Nuclear membrane and nucleolus start to disappear

19. Metaphase
Sister chromatids condensed
Sister chromatids align on the metaphase plate
No nucleolus, no nuclear membrane

20. Anaphase
Sister chromatids separate to form daughter chromosomes
Daughter chromatids start to migrate to the poles, centromere first

21. Sister Chromatids Daughter Chromosomes
Metaphase Anaphase

22. Telophase
Daughter chromosomes reach the the spindle poles
Chromatin starts to relax
Nuclear membrane starts to reform
Nucleolus becomes visible

23. Meiosis – Cell division where the chromosome number is reduced by half.
This is accomplished by having one cycle of chromosome replication followed by two divisions
By reducing the number of chromosomes the cells go from a somatic number of 2N to a gametic number of N

24. The two divisions are designated the reduction division and the equational division.
– separation of homologous chromosomes. This is the division when the chromosome number is reduced.

25. Equational division
- separation of sister chromatids. This results in four cells having half the number or chromosomes as the original cell.

26. Reductional Division
In this division the chromosome number is reduced and recombination between homologous chromosomes can occur.
There are four stages:
- prophase I
- metaphase I
- anaphase I
- telophase I

27. To describe everything that occurs in prophase I it is divided into five sub-stages:
- leptotene
- zygotene
- pachytene
- diplotene
- diakinesis

28. Leptotene
chromosomes become visible
telomeres are in contact with the nuclear membrane
nucleolus present

29. Zygotene
chromosomes continue to condense
homologous chromosomes pair
pairing is known as synapsis
Nucleolus and nuclear membrane are still present

30. The homologous chromosomes are held together by the synaptonemal complex.
The synaptonemal complex is a tripartite ribbon made of two lateral protein bands surrounding a medial protein complex.
The synaptonemal complex makes it possible for recombination to occur.

32. Pachytene
paired chromosomes continue to condense and shorten
exchange between non-sister chromatids can occur
exchange appears to be protein mediated
nucleolus and nuclear membrane still present

33. Diplotene
- synaptonemal complex starts to break down
homologous chromosome pairs start to separate
areas of exchange stay together longer, called chiasma
Nucleolus and nuclear membrane start to breakdown

35. Diakinesis
- chromosomes continue to condense and chiasmata terminalize
if chiasmata in both arms get a ring bivalent, if one arm a rod bivalent
nucleolus and nuclear membrane start to disappear

36. Formation of ring bivalent with terminalization of chiasmata
Formation of rod bivalent with terminalization of chiasmata

37. Metaphase I
bivalents align on metaphase plate
presence of multiple chromosomes pairing or chromosomes not pairing are indicators of chromosome additions, deletions or modifications

38. Rod bivalents
Ring bivalents
Univalents