Question

answer to 1 please

1. Describe in detail the cell cycle control system that regulates the timing of DNA replication in eukaryotic cells 12 20 F3 F4 F6 F2 F5 F7

Answer 1

The eukaryotic cell cycle is a four-stage process in which the cell increases in size (gap 1, or G1 stage), copies its DNA (synthesis, or S stage), prepares to divide (gap 2, or eG2 stage), and divides (mitosis, or M stage).

The stages G1, S, and G2 make up interphase, which accounts for the span between cell divisions. On the basis of the stimulatory and inhibitory messages a cell receives, it decides whether or not it should enter the cell cycle and divide.

So , these processes of cell cycle are triggered by a cell-cycle control system.The objective of this control is to confer a  safely guide to  the cell through the events of the cell cycle .

There are several points in the cell cycle, called checkpoints, at which the cycle can be arrested if previous events have not been completed :

-G₁ checkpoint determines whether all conditions are favorable for cell division to proceed.At this point , the cell irreversibly commits to the cell division process. External influences, such as growth factors, play a large role in carrying the cell past the G₁ checkpoint.

The cell will only pass the checkpoint if it is an appropriate size and has adequate energy reserves. At this point, the cell also checks for DNA damage. A cell that does not meet all the requirements will not progress to the S phase. The cell can halt the cycle and attempt to remedy the damage, or the cell can advance into G₀ (inactive) phase and await further signals when conditions improve.

-Checkpoints at the end of G1 and at the beginning of G2 are designed to assess DNA for damage before and after S phase.

The most important role of the G₂ checkpoint is to ensure that all of the chromosomes have been accurately replicated without mistakes or damage. If the checkpoint mechanisms detect problems with the DNA, the cell cycle is halted and the cell attempts to either complete DNA replication or repair the damaged DNA. If the DNA has been correctly replicated, cyclin dependent kinases (CDKs) signal the beginning of mitotic cell division.

-Checkpoint during mitosis ensures that the cell’s spindle fibres are properly aligned in metaphase before the chromosomes are separated in anaphase.

If DNA damage or abnormalities in spindle formation are detected at these checkpoints, the cell is forced to undergo programmed cell death, or apoptosis.

The main role in the cell cycle control rest in a group of proteins called cyclins- dependant kinases.(CdKs) .

But the activity of these protein depends on another group of proteins called cyclins with which they form complexes  .

There are four classes of cyclins, each defined by the stage of the cell cycle at which they bind Cdks and function. Three of these classes are required in all eukaryotic cells:

1.G₁/S-cyclins bind Cdks at the end of G₁ and commit the cell to DNA replication.

2.S-cyclins bind Cdks during S phase and are required for the initiation of DNA replication.

3.M-cyclins promote the events of mitosis.

The activities of cyclin-Cdk complexes are influenced by several mechanisms, including phosphorylation of the Cdk subunit, the binding of special inhibitory proteins (CKIs), proteolysis of cyclins, and changes in the transcription of genes encoding Cdk regulators.