The stages of mitosis and cell division frim prophase to telophase

General Biology grade 12

1. G Phase  T he phase of the cell cycle during which a cell is neither dividing nor preparing to divide . Some cells enter G temporarily Mature cardiac and nerve cells permanently remain in G Control of the Cell cycle Few hours to an average of 2-5 days and to an entire human lifetime

2. Regulation of the Cell Cycle by External Events Both initiation and inhibition of cell division are triggered by events external to the cell. Examples: 1.Death of a nearby cell 2. Sweeping (release of growth promoting hormones (HGH)- Human Growth Hormone - Lack of HGH- Dwarfism Too much HGH – Gigatism 3. Crowding of cells can inhibit cell division 4. Size of the cell can initiate division

3. Regulation at Internal Checkpoint Normal cells move through the cell cycle in a regulated way . It is essential that the daughter cells produced be exact duplicates of the parent cell. Mistakes in the duplication or distribution of chromosomes lead to mutation that may be passed on to every new cell produced from an abnormal cell. To prevent a compromised cell from continuing to divide, there are 3 main cell cycle checkpoints .

4. What are checkpoints? A  checkpoint  is a stage in the eukaryotic cell cycle at which the cell examines internal and external cues and "decides" whether or not to move forward with division.

5. There are a number of checkpoints, but the three most important ones are: 1.The G 1 checkpoint 2.The G 2 checkpoint 3. The spindle checkpoint, at the transition from metaphase to anaphase.

6. G 1 Checkpoint This checkpoint determines whether all conditions are favorable for cell division to proceed. A cell that does not meet all the requirements will be allowed to progress into the S phase. Once the cell passes the G1 checkpoint and enters S phase, it becomes irreversibly committed to division. The cell can halt the cycle and attempt to remedy the problematic condition or the cell can advance into the G phase and await further signals when conditions improve.

7. G 2 checkpoint This check point bars entry into the mitotic phase if certain conditions are not met. Most important role is to ensure that all the chromosomes have been replicated and that the replicated DNA is not damaged. If the checkpoint mechanisms detect problems with the DNA, the cell attempts to either complete DNA replication or repair the damaged DNA.

8. M Checkpoint It occurs near the end of the metaphase stage of karyokinesis. Also known as the spindle checkpoint, because it determines whether all the sister chromatids are correctly attached to the spindle microtubules. The cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to at least two spindle fibers arising from opposite poles of the cell

9. MITOSIS Karyokinesis, (division of nucleus) also known as mitosis . Divided into series of phases: 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase

10. PROPHASE The “ first phase ”, the nuclear envelope starts to disassociate into small vesicles. The nucleolus disappears. The centrosomes begin to move to opposite poles of the cell. Microtubules that will form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly with the aid of condensing proteins and become visible under a light microscope.

11. PROPHASE The “ first phase ”, the nuclear envelope starts to disassociate into small vesicles. The nucleolus disappears. The centrosomes begin to move to opposite poles of the cell. Microtubules that will form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly with the aid of condensing proteins and become visible under a light microscope.

12. PROMETAPHASE The “first change phase” The remnant of the nuclear envelope fragment. Mitotic spindle continues to develop. Chromosomes become more condensed and discrete. Kinetochores appear at the centromeres. Mitotic spindle microtubules attach to kinetochores. Centrosomes move toward opposite poles.

13. Kinetochores are  large protein assemblies that connect chromosomes to microtubules of the mitotic and meiotic spindles in order to distribute the replicated genome from a mother cell to its daughters

14. METAPHASE Mitotic spindle is fully developed; centrosomes are at opposite poles of the cell. Chromosomes are lined up at the metaphase plate. Each sister chromatid is attached to a spindle fiber originating from opposite poles.

15. ANAPHASE Cohesion protein binding the sister chromatids together break down. Sister chromatids are pulled toward opposite poles. Non-kinetochores spindle fibers lengthen, elongating the cell.

16. TELOPHASE The “distance phase” The chromosomes reach the opposite poles and begin to decondense The mitotic spindles are depolymerized into tub used in monomers that will be used to assemble They wil be used for each daughter cell.