9 - Cell cycle + mitosis Flashcards
Why do cells need to divide ?
- New organisms
- Growth
- Cell replacement
- New organisms
Unicellular organisms e.g. bacteria
- Growth
Multicellular organisms grow by adding more cells
Cell replacement
Wear and tear (skin exfoliation / gut)
Programmed cell death
What are the basic requirements for a cell to divide effectively?
- The DNA of the parent cell must be duplicated
- The chromosomes containing the replicated DNA must then be physically segregated into the two daughter cells
- The daughter cells must physically divide from each other.
Purpose of cell division
to produce 2 genetically identical daughter cells from the original parent cell.
Binary Fission
Relatively simple process by which bacteria duplicate their DNA and segregate their chromosome into 2 daughter cells
Much less complicated than the mechanisms employed by eukaryotic cells
Binary fission stages
- Chromosome replications begins: 1 copy of the origin moves rapidly towards other end of cell
- Replication continues: 1 copy of the origin is at each end of cell
- Replication finishes: plasma membrane grows inward and new cell wall is deposited
- 2 daughter cells result
Eukaryotic cell division
More complicated than prokaryotes:
Nuclear membrane
Multiple chromosomes
Organelles
The cell cycle
G1 ‘gap’: Cell grows, prepares to replicate DNA
S ‘synthesis’: Cell grows, synthesis of duplicate DNA ready for mitosis
G2 ‘gap’ : cell grows, prepares for mitosis
M ‘mitotic phase’: chromosomal segregation (mitosis) and cell separation (cytokinesis)
Mitosis
‘The physical process of segregating chromosomes into daughter cells’
- 23 pairs of chromosomes
• Body cells are diploid (one copy of each chromosome from each parent)
Mitosis – segregating chromosomes
At the start of mitosis the chromosome consists of two sister chromatids linked at the centromere.
By the end the pair has been segregated into the new daughter cells
Interphase
Cell grows, replicates its DNA and gets ready for mitosis
G2 of interphase
- Intact nuclear envelope
- Chromosomes replicated – remain indistinct as loosely packed chromatin (DNA+protein) fibres
- Centrosome replicated (MTOCs)
- Microtubules extend radially forming asters
Prophase
- Chromatin fibres condense – forming discrete chromosomes
- Nucleoli disappear
- Centrosomes move away from each other
- Mitotic spindle begins to form
Prometaphase
- Breakdown of nuclear envelope
- Some microtubules attach to chromosomes at their kinetochores
- Other microtubules interact with those from opposite poles
Metaphase
- Centrosomes at opposite poles
- Chromosomes align on metaphase plate
- Sister kinetochores attached to microtubules coming from opposites poles
anaphase
- Begins with separation of centromeres
- Sister chromatids move towards opposite poles of the cell
- Each chromatid becomes a new chromosome
- Poles move further apart
Telophase
- Elongation of cell by polar microtubules
- Daughter nucleoli begin to form at poles of cell
- Nuclear envelopes form
- Chromatin begins to decondense
what does the mitotic spindle consist of
- Tubules
- astral
- kinetochore
- non-kinetochore/polar
- centrosomes
- chromatid pairs - arranged on the metaphase plate
Functions of the mitotic spindle
To organise chromatids along the metaphase plate and then to pull sister chromatids apart
How does the mitotic spindle physically pull chromatid pairs apart?
Microtubule motors:
- Astral motors
- Kinetochore motors
- Non-kinetochore / polar motors
Astral motors (dynein)
- pull astral microtubules towards poles during prophase
- microtubules de-polymerise and shorten
- hold astral microtubules in place during metaphase and later
Kinetochore motors
- attach chromosomes to microtubules
- pull on microtubules during anaphase - chromosomes move towards centrosomes
- microtubules de-polymerise and get shorter
Non-kinetochore/polar motors
- motors are attached to a microtubule from either side where the polar microtubules overlap
- motors push the microtubules away in opposite directions during metaphase and anaphase
- microtubules polymerise and get longer
- cell elongates
• Pulling (dynein)
– kinetochore motors pull chromosomes towards the centrosome/pole.
– astral motors pull centrosomes toward inner face of the plasma membrane
– Both shorten depolymerise microtubules
Pushing (kinesin)
– non-kinetochore/polar motors add subunits (polymerises) microtubules to drive the poles of the spindle apart.
This elongates the cell to aid telophase/cytokinesis
When does the physical separation of chromosome pairs by the mitotic spindle occur
anaphase
How does the mitotic spindle physically separate chromosome pairs
- Proteins holding sister chromatids together are inactivated
- Chromatids separate
- Kinetochore microtubules have motor proteins (dynein) which ‘walk’ a chromosome to the nearest pole
- Microtubules shorten by depolymerisation at their kinetochore ends
- Non-kinetochore microtubules elongate whole cell during anaphase
- Motor proteins (kinesin) attach to microtubules and lengthen them by addition of subunits
Cytokinesis in animal cells
- Microfilaments form a ring at the furrow
- Ring contracts - owing to interaction between actin and myosin filaments
- furrow deepens until cell is pinched in two
Cytokinesis in plant cells
- Cell plate forms at equatorial plane of the cell
2. Cell wall forms - from plate contents
