spring 2009: section 3 – lecture 2 reading – chapter 3 chapter 10, pages 251 - 267
TRANSCRIPT
Spring 2009: Section 3 – lecture 2
Reading – Chapter 3
Chapter 10, pages 251 - 267
Cell Cycle and Cell Division
Cell CycleFour phases or stages
G1 – growth stage 1S - synthesis stage
G2 – growth stage 2M – cell division (mitosis or
meiosis)
G1 phase
production of components for DNA replication
S phase
DNA replication starts
G2 phase
DNA replication continues
production of the components for cell division
M phase
cell division, either mitosis or meiosis
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.
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
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 cdc2 subunit
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.
Cell cycle regulation by MPFearly 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
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
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
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.
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)
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
Interphase
- Chromatin not condensed
- Nuclear membrane present
- Nucleolus present
Prophase- Sister chromatids
start to condense- Spindle fibers start
to form- Nuclear membrane
and nucleolus start to disappear
Metaphase- Sister chromatids
condensed- Sister chromatids
align on the metaphase plate
- No nucleolus, no nuclear membrane
Anaphase- Sister chromatids
separate to form daughter chromosomes
- Daughter chromatids start to migrate to the poles, centromere first
Sister Chromatids Daughter Chromosomes
Metaphase Anaphase
Telophase- Daughter
chromosomes reach the the spindle poles
- Chromatin starts to relax
- Nuclear membrane starts to reform
- Nucleolus becomes visible
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
The two divisions are designated the reduction division and the equational division.
Reduction division – separation of homologous chromosomes. This is the division when the chromosome number is reduced.
Equational division
- separation of sister chromatids. This results in four cells having half the number or chromosomes as the original cell.
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
To describe everything that occurs in prophase I it is divided into five sub-stages:
- leptotene
- zygotene
- pachytene
- diplotene
- diakinesis
Leptotene
- chromosomes become visible
- telomeres are in contact with the nuclear membrane
- nucleolus present
Zygotene
- chromosomes continue to condense
- homologous chromosomes pair
- pairing is known as synapsis
- Nucleolus and nuclear membrane are still present
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.
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
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
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
Formation of ring bivalent with terminalization of chiasmata
Formation of rod bivalent with terminalization of chiasmata
Metaphase I
- bivalents align on metaphase plate
- presence of multiple chromosomes pairing or chromosomes not pairing are indicators of chromosome additions, deletions or modifications
Rod bivalents
Ring bivalents
Univalents