Mitosis
What are the different stages of the cell cycle?
How are these stages distinguished?
What is the outcome of mitosis?
The cell cycle is the series of growth and reproduction of a cell. The cell cycle is consisting of four different parts which are:
- G1 - is the growth and preparation of the chromosomes for reproduction
- S - is doing the synthesis of the DNA and it also duplicates the centrosome
- G2 - is preparing the the cell cycle
- M - is the mitosis
Proteins that are in the cytoplasm which are controlling the passage of the cell, there are three main competitors which are:
- Cyclins - in the cell their level will rise or fall with the different stages of the cell cycle.
- G1 cyclins (D cyclins)
- S-phase cyclins (cyclins E and A)
- Mitotic cyclins (B cyclins)
- Cyclin-dependent kinases (CDKS) - their levels in the cell will be stable, but they have to bind to appropriate cyclins in order to be activated.
- G1 CDK (CDK 4)
- S-phase CDK (CDK 2)
- M-phase CDK (CDK 1)
- Anaphase-promoting complex (APC) - also called "Cyclosome" is a complex that is designed to be a APC/C.
- it is triggering events that are causing the cohesin to destruct, which is allowing the sister chromatids to separate.
- it is degrading the mitotic B cyclins
In the cell cycle are different steps involved.
All of these events are taking the cell to the metaphase of the mitosis.
This is activating the anaphase-promoting complex (APC/C) which is allowing the the sister chromatids to separate at the metaphase plate and move towards the poles which is equal toanaphase. Through that the mitosis is being completed.
- rising level of G1-cyclins are binding to their CDKS and are sending signals to the cell for preparing the chromosomes to replicate
- rising level of the S-phase promoting factor (SPF), this is including the cyclins of A and are bounding to CDK 2. While it enteres the nucleus it is preparing the cell to duplicate its DNA and also its chromosomes. While the DNA is being replicated the E-cyclin is getting destroyed and the mitotic-cyclin level begins to rise
- the M-phase promoting factor (which is the complex of the mitotic B-cyclin with the M-phase CDK [CDK 1]) is translocating into the nucleus initiates:
- the assembly of mitotic spindle
- the breakdown of the nuclear envelope
- also the cessation of all the gene transcriptions
- and condensation of the chromosomes
All of these events are taking the cell to the metaphase of the mitosis.
This is activating the anaphase-promoting complex (APC/C) which is allowing the the sister chromatids to separate at the metaphase plate and move towards the poles which is equal toanaphase. Through that the mitosis is being completed.
In mitosis are different stages which are a total of five.
- Interphase - the nucleus is well defined and is surrounded by a nucleus envelope which is containing one or more nucleoli. Only the outside part of the nucleus is containing two centrosomes, which has being formed through the early interphase by the replication of a single centrosome. Microtubules are being extended from the centrosomes which are called "asters" when they are in a radial array. So the centrosomes are already being duplicated but are still packed loosely in a form of chromatin fibers.
- Prophase - during this phase the nucleoli is disappearing in the nucleus. The chromatin fibers are getting tightly winded and combine into chromosomes. These chromosomes are lined up in pairs and join in the centromere. While in the cytoplasm the spindle fibers are forming and which are made of microtubules. The centrosomes are moving away from each other because they are propelled by the mircotubules, spindle fibers or what is between these two.
- Metaphase - now the centrosomes are at the opposite side of the cell poles. All the chromosomes are lines up at the "metaphase plate". Each centrosome has one chromosome that they paired with. Also each chromatid is being attached to a microtubules to form a spindle.
- Anaphase - in this phase the chromosomes are being separated and are moving along the microtubules towards the other side of the cell pole. Meanwhile the cell poles are drifting farther and farther away. When the anaphase is being completed, both of the cell poles are going to have a complete set of the chromosomes.
- Telophase - now the microtubules are getting even longer and the daughter nuclei is going to form at the two cell poles. While nuclear envelopes are being formed to reappear the nucleoli. This affects the chromatin of chromosomes to wind. Now the mitosis is being completed. One of the nucleus has been divided into two genetically identical nuclei. Cytokinesis forces the cell to transform the one cell into two cells.