Explain what are the checkpoints and its regulation?

The cell cycle is a precisely controlled process that ensures cells grow, replicate their DNA and divide accurately. To maintain genetic integrity, cells have built-in checkpoints that function as surveillance systems, monitoring progress and detecting abnormalities at critical stages. These checkpoints play an essential role in preventing errors that could lead to uncontrolled cell growth, mutations, or diseases like cancer.

Checkpoints ensure that each stage of the cell cycle is completed correctly before the cell progresses to the next phase. If a problem is detected, the checkpoint pauses the cycle, allowing time for repairs. If the damage is irreparable, the checkpoint can activate apoptosis (programmed cell death) to prevent the spread of defective cells.
Checkpoints ensure that each stage of the cell cycle is completed correctly before the cell progresses to the next phase. If a problem is detected, the checkpoint pauses the cycle, allowing time for repairs. If the damage is irreparable, the checkpoint can activate apoptosis (programmed cell death) to prevent the spread of defective cells.

Types of Cell Cycle Checkpoints

There are three major checkpoints in the cell cycle:
  1. G1 Checkpoint (Restriction Point) – Occurs at the end of G1 phase and ensures conditions are favorable for DNA replication.
  2. G2 Checkpoint – Occurs at the end of G2 phase and verifies the accuracy of DNA replication before mitosis.
  3. M Checkpoint (Spindle Assembly Checkpoint, SAC) – Occurs during metaphase and ensures chromosomes are correctly aligned before separation.
The regulation of each checkpoints is primarily controlled by cyclins, cyclin-dependent kinases (CDKs), tumor suppressor proteins (such as p53) and checkpoint kinases (CHK1 and CHK2) that work together to ensure the proper progression of the cell cycle. If these regulatory mechanisms fail, cells may divide uncontrollably, leading to cancer and other genetic disorders.

1. G1 Checkpoint (Restriction Point)

The G1 checkpoint also known as the restriction point, is the first critical checkpoint in the cell cycle. It occurs at the end of G1 phase, ensuring that the cell is ready to enter the S phase, where DNA replication occurs.

The G1 checkpoint determines whether the cell has grown sufficiently, possesses the necessary nutrients and has received external signals to proceed with division. It also ensures that the DNA is undamaged before replication begins. If these conditions are not met, the cell either pauses the cycle, enters a resting phase (G0) or undergoes apoptosis if severe DNA damage is detected.

Role of the G1 Checkpoint

The main functions of the G1 checkpoint include:
  • Assessing cell size: Ensuring the cell has grown enough to support division.
  • Checking nutrient availability: Making sure the cell has sufficient resources for DNA replication.
  • Verifying presence of growth factors: Growth signals from the environment must be present for cell division to proceed.
  • Detecting DNA damage: Ensuring that the genetic material is intact before replication.
If all conditions are met, the cell progresses to the S phase. If problems are detected, the cell can:
  1. Pause the cycle until conditions improve.
  2. Enter a resting state (G0 phase), where it remains without dividing.
  3. Undergo apoptosis if DNA damage is too severe.

Regulation of the G1 Checkpoint

The G1 checkpoint is controlled by key proteins:

1. Cyclin D-CDK4/6 Complex:

  • This complex activates genes required for DNA replication.
2. Retinoblastoma Protein (Rb):
  • Rb prevents premature DNA replication by inhibiting transcription factors like E2F. When phosphorylated by Cyclin D-CDK4/6, Rb releases E2F, enabling progression to the S phase.
3. p53 Tumor Suppressor Protein
  • p53 detects DNA damage. If damage is found, it activates p21, which inhibits CDKs and halts the cycle for repairs. If damage is severe, p53 triggers apoptosis.

Consequences of G1 Checkpoint Failure

If the G1 checkpoint fails, cells with DNA damage may enter the S phase and replicate their faulty DNA. This can lead to:
  • Uncontrolled cell division, as seen in cancers where p53 is defective.
  • Accumulation of mutations, increasing the risk of genetic diseases.
A mutation in the p53 gene is found in over 50% of human cancers, leading to unchecked cell division and tumor formation.

2. G2 Checkpoint

The G2 checkpoint occurs at the end of the G2 phase, just before mitosis begins. It ensures that DNA replication was successful and error-free before the cell enters mitosis.

This checkpoint is essential because errors in DNA replication can lead to mutations, chromosomal abnormalities and cancer. If DNA damage is detected, the checkpoint halts the cycle, allowing time for repair. If the damage is severe, apoptosis is triggered to prevent defective cell division.

Role of the G2 Checkpoint

The primary functions of the G2 checkpoint include:
  • Verifying complete DNA replication: Ensuring no missing or incomplete genetic material.
  • Detecting DNA damage: Preventing mutations from being passed on to daughter cells.
  • Ensuring cell readiness for mitosis: Confirming that all necessary cellular components are prepared for division.
If everything is correct, the cell enters mitosis. If errors are detected, the checkpoint pauses the cycle to allow repairs. If damage is irreparable, the cell undergoes apoptosis.

Regulation of the G2 Checkpoint

The G2 checkpoint is controlled by:

1. Cyclin B-CDK1 Complex (MPF):

  • Cyclin B-CDK1 complex, also known as the Maturation Promoting Factor (MPF). Cyclin B binds to CDK1, forming the Cyclin B-CDK1 complex, which promotes mitotic entry. The activity of CDK1 is controlled by phosphorylation and dephosphorylation events.
2. Wee1 Kinase and Cdc25 Phosphatase:
  • Wee1 kinase inhibits CDK1, keeping the cell in G2 phase if DNA is damaged. If DNA is undamaged, Cdc25 phosphatase removes inhibitory modifications on CDK1, allowing mitosis to begin.
3. Checkpoint Kinases (CHK1 and CHK2):
  • Detect DNA damage and prevent mitotic entry by inhibiting Cdc25, keeping CDK1 inactive.

Consequences of G2 Checkpoint Failure

If the G2 checkpoint fails, cells with damaged or incomplete DNA can enter mitosis, leading to:
  • Chromosomal abnormalities such as deletions or duplications.
  • Increased mutation rates, contributing to cancer progression.

3. M Checkpoint (Spindle Assembly Checkpoint)

The M checkpoint or Spindle Assembly Checkpoint (SAC), occurs during metaphase of mitosis. It ensures that chromosomes are correctly attached to the spindle fibers before separation.

Proper chromosome segregation is crucial because errors can result in aneuploidy, a condition where cells have an abnormal number of chromosomes, leading to genetic disorders such as Down syndrome or cancer.

Role of the M Checkpoint

The M checkpoint ensures:
  • All chromosomes are properly attached to spindle fibers.
  • Chromosomes are aligned correctly at the metaphase plate.
  • Equal chromosome distribution occurs during cell division.
If misaligned chromosomes are detected, the checkpoint delays anaphase until the issue is resolved.

Regulation of the M Checkpoint

The M checkpoint is controlled by:

1. Mad and Bub Proteins

  • Detect incorrect chromosome attachment and If a chromosome is not properly attached, Mad1, Mad2, Bub1 and BubR1 inhibit the Anaphase-Promoting Complex (APC/C) to delay division.
2. Anaphase-Promoting Complex (APC/C)
  • Once all chromosomes are correctly attached, APC/C is activated and degrades securin, which releases separase. Separase breaks down cohesin, allowing sister chromatids to separate and be pulled to opposite sides of the cell.

Consequences of M Checkpoint Failure

A defective M checkpoint can result in:
  • Aneuploidy, where cells have an incorrect chromosome number, causing genetic disorders.
  • Cancer, due to improper chromosome segregation leading to genomic instability.






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