Question

Describe briefly the cell division. Describe the three major cell cycle checkpoints

Answer 1

Cell cycle:

• The cell cycle is the series of events which takes place in a cell that cause it to divide into two daughter cells.
• These events include the duplication of its DNA  or DNA replication and some of its organelles and subsequently following the partitioning of its cytoplasm and other components into two daughter cells by a process called  Cell division.
• The cell cycle is divided into two main stages - Interphase and the  Mitotic phase (M) (which includes  mitosis and cytokinesis).
• During interphase, the cell grows, accumulating nutrients needed for mitosis, and replicates its DNA and some of its organelles.
• Interphase proceeds in three stages, G₁, S, and G₂.

• During mitotic phase, the sequence of events is divided into phases, corresponding to the completion of one set of activities and the start of the next. These phases are sequentially known as: prophase, prometaphase, metaphase, anaphase, and telophase.

• Two key classes of regulatory molecules, cyclins and cyclin dependent kinases (CDKs), determines the cell's progress through the cell cycle.

• Cyclins form the regulatory subunits and CDKs are the catalytic subunits of an activated  heterodimer.
• Cyclins have no catalytic activity and CDKs are inactive in the absence of a partner cyclin. 
•  When activated by a bound cyclin, CDKs perform a common biochemical reaction called  Phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle.

Checkpoints:

• Cell cycle checkpoints are used by the cell to monitor and regulate the progress of the cell cycle.
• Checkpoints prevent cell cycle progression at specific points, allowing verification of necessary phase processes and repair of  DNA damage. The cell cannot proceed to the next phase until checkpoint requirements have been met.
• Checkpoints typically consist of a network of regulatory proteins that monitor and dictate the progression of the cell through the different stages of the cell cycle.
•  Three main checkpoints exist which include the G₁/S checkpoint, the G₂/M checkpoint and the metaphase (mitotic) checkpoint.
• Another checkpoint is the Go checkpoint, in which the cells are checked for maturity. 
• If the cells fail to pass this checkpoint by not being ready yet, they will be discarded from dividing.

• G₁/S check point checks whether it has enough raw materials to fully replicate its DNA (nucleotide bases, DNA synthase, chromatin, etc.).

• An unhealthy or malnourished cell will get stuck at this checkpoint.

• The G₂/M checkpoint is where the cell ensures that it has enough cytoplasm and phospholipids for two daughter cells.

• But sometimes more importantly, it checks to see if it is the right time to replicate.

• There are some situations where many cells need to all replicate simultaneously (for example, a growing embryo should have a symmetric cell distribution until it reaches the mid-blastula transition).

• This is done by controlling the G₂/M checkpoint.

•  

  The metaphase checkpoint is a fairly minor checkpoint, in that once a cell is in metaphase, it has committed to undergoing mitosis.

• In this checkpoint, the cell checks to ensure that the spindle has formed and that all of the chromosomes are aligned at the spindle equator before anaphase begins.

• Checkpoint regulation plays an important role in the organism development.

• DNA damage checkpoints were defined as regulatory pathways which control the ability of cells to arrest the cell cycle in response to DNA damage, perhaps allowing time for repair or other cellular functions.
• The proteins that accumulate at the damage site typically activates the checkpoint and halts the cell growth at the G1/S or G2/M boundaries.

 

Checkpoints:

• Cell cycle checkpoints are used by the cell to monitor and regulate the progress of the cell cycle.