What are Cell Cycle Checkpoints

In a multicellular organism cell division is required for normal growth development and maintenance. But how many times a cell will divide and when it will divide all this varies with the type of cell.
• In the human body cells that divide continuously. For example stem cells, that give rise to blood cells and skin cells. These are the examples where cell division takes place to replace the dead cells.
• Another category is of the cells that do not normally divide but they may begin to divide under certain conditions. For example liver cells and lymphocytes.
  If there is a wound or damage in liver the remaining liver cells divide to repair it.
  When lymphocytes come in contact of a foreign antigen they begin to proliferate and induce an immune response.
• Also there are cells that do not divide at all. For example mature nerve and muscle cells.

With such a variation in the frequency of cell division there must be some kind of control system to maintain and check any uncontrolled cell division.

Cell Cycle Regulation

  Cell cycle is regulated by a combination of external signals received by cell on its internal environment. we will study this cell cycle control system in the next video lecture today we will look at the control points in the cell cycle where this control system works what are these control points and why they are important is our main point of discussion let's begin

  Every cell that divides proceeds from one phase of the cell cycle to the next in an order. Each phase of the cell cycle has an important function and therefore it is essential that each phase should be properly completed before proceeding to the next phase.
  But if a cell does not follow the sequence of phases of the cell cycle errors will take place.

Errors of Cell Cycle :

Incomplete DNA replication
   A cell begins mitosis without completing the S phase. That means all chromosomes of a cell have not been completely copied.
Unequal separation of chromosome
  Before moving toward the spindle poles during mitosis, the chromosomes are properly attached to the spindle so that each daughter cell receives an equal number of chromosomes. But if chromosome move towards the spindle poles before their proper attachment, the distribution of chromosomes will be not equal.
  One cell may get less and another may get more chromosomes.

Cell Cycle Checkpoints
Cell Cycle Checkpoints

  Obviously the cell does not want such errors to take place. Therefore the cell cycle control system does necessary checks at certain Control Points in the cell cycle to make sure that cell has completed each phase correctly, before moving to the next phase.
   At these control points it is decided whether to stop or go ahead. These control put in the cell cycle are known as the cell cycle checkpoints.

There are three cell cycle checkpoints and these checkpoints are at the three key transition points in the cell cycle.
• The first checkpoint is at the G1 to S transition and it is known as the G1 checkpoint.
• The second checkpoint is at the G2 to M transition and it is known as the G2 checkpoint.
• The third checkpoint is at the metaphase to anaphase transition and it was known as M checkpoint.

G1 Checkpoint

• The G1 phase varies among cell types. Cells that have stopped dividing are arrested during the G1 phase. This arrested state is known as G0 state.
• Most cells in our body are in G0 state. cells such as mature nerve and muscle cells never leave G0 state.
• But cells such as liver cells can leave this G0 state when required. For example to repair the damaged liver cells.
• The first checkpoint occurs during late G1 at this checkpoint a cell has two options :
- First is to continue through the cell cycle
- Second to exit the cell cycle and enter the G0.
• A cell will continue through the cell cycle only if it has prepared itself for the next phase.
• So at this checkpoint following factors are checked for accuracy
- The cell must attain a proper size.
- The cell must receive necessary signals.
- DNA should be undamaged.
If these factors are not met, cell cycle stops until these conditions are fulfilled.
• Otherwise cell undergoes programmed cell death or apoptosis.
• If a cell successfully passes G1 checkpoint, the cell is committed to the next phase, that is the S phase.

G2 Checkpoint

• In the S phase of the cell cycle chromosomes are duplicated.
• The G2 checkpoint occurs after the S phase at the boundary between the G2 and M phase.
• If chromosomes are not replicated correctly or DNA is damaged cells remain in the G2 phase. The cell is allowed to repair DNA and complete replication.
• If DNA is successfully replicated the cell passes this checkpoint and now cell is committed to mitosis.

M checkpoint

• In anaphase the chromosomes are separated. This is very crucial because this separation will distribute chromosomes equally in the two daughter cells.
• The accuracy of this separation depends on the proper attachment of all the chromosomes to the spindle during metaphase.
• Therefore the third checkpoint occurs between metaphase and anaphase. This checkpoint makes sure that chromosomes are properly attached to the spindle apparatus.

  All three key checkpoints prevent the division of cells that have errors or damages. If errors are detected the process of the cell cycle is halted. The cell cycle is regulated at these checkpoints by both internal and external signals. These internal and external signals together form the cell cycle control system.

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