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Cell Cycle and Regulation-Vinayak Jhunjhunwala
Cell Cycle
Introduction to Cell CycleThe organism develops from a single cell zygote to a complete organism because of cell division and cell growth.When cell cycles proceed inappropriately (e.g., cells divide uncontrollably), pathological conditions like cancer can result.
There are 2 major phase: Interphase and M phase
Interphase includes:G1 Phase: In this part of interphase, the cell grows in size and
synthesizes mRNA and proteins in preparation for subsequent steps leading to mitosis
S Phase: S-phase (synthesis phase) is the part of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase.
G2 Phase: After DNA replication, the cell leaves S phase and enters G2, when the cell prepares for mitosis or meiosis.
M Phase: Mitosis or meiosis takes place in this part. The DNA is divided into two cells.
Mitosis v/s MeiosisMitosis Meiosis
Occurs in all somatic cells Occurs only in reproductive (sex) cells
Chromosome number remains same, i.e., diploid (2n), hence it is equational division
Chromosome number reduces to half, i.e., haploid (n), hence it is reductional division
Two daughter cells are produced Four daughter cells are produced
One cell division involves four phasesConsists of two sub-divisions: Meiosis - I and Meosis - II each involving four phases. Prophase - I is again subdivided into five sub-stages:-leptotene, zygotene, pachytene, diplotene and diakinesis
No crossing over Crossing over takes place during prophase1 of meiosis
Daughter cells have identical chromosomes to parent cell, i.e., genetic material remains constant
Daughter cells have chromosomes with combined genetic material from both parents i.e., genetic variability occurs
Cell Regulation and checkpointsA checkpoint is a stage in the eukaryotic cell cycle at which at which the cell examines internal and external cues and "decides" whether or not to move forward with division.
There are a number of checkpoints, but the three most important ones are:
G1 checkpoint at G1/S transition
G2 checkpoint at G2/M transition
In M phase, from Metaphase to Anaphase
First Checkpoint: The G1 checkpointThe G1 checkpoint is the main decision point for a cell – that is, the primary point at which it must choose whether or not to divide. Once the cell passes the G1checkpoint and enters S phase, it becomes irreversibly committed to division. That is, barring unexpected problems, such as DNA damage or replication errors, a cell that passes the G1 checkpoint will continue the rest of the way through the cell cycle and produce two daughter cells.
G1 checkpoint checks for:● Cell Size● Nutrient● Growth Factors● DNA Damage
If a cell doesn’t get the go-ahead cues it needs at the G1 checkpoint, it may leave the cell cycle and enter a resting state called G 0. Some cells stay permanently in G0 while others resume dividing if conditions improve.
Second checkpoint: The G2 CheckpointTo make sure that cell division goes smoothly (produces healthy daughter cells with complete, undamaged DNA), the cell has an additional checkpoint before M phase, called the G2.At this stage, the cell will check:
DNA integrity & DNA replication.
If errors or damage are detected, the cell will pause at the G2 checkpoint to allow for repairs. 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 damage is irreparable, the cell may undergo apoptosis, or programmed cell death This self-destruction mechanism ensures that damaged DNA is not passed on to daughter cells and is important in preventing cancer.
Third Checkpoint:The spindle checkpointThe M checkpoint is also known as the spindle checkpoint: here, the cell examines whether all the sister chromatids are correctly attached to the spindle microtubules. Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until all the chromosomes are firmly attached to at least two spindle fibers from opposite poles of the cell.
How does this checkpoint work?
It seems that cells don't actually scan the metaphase plate to confirm that all of the chromosomes are there. Instead, they look for "straggler" chromosomes that are in the wrong place (e.g., floating around in the cytoplasm) If a chromosome is misplaced, the cell will pause mitosis, allowing time for the spindle to capture the stray chromosome
Overview of checkpoints in Cell cycle
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presentation’s main message or argument.
“The mers were also designed to reproduce only at long intervals, in
order to maintain the natural balance of the environment in which they were
placed ”- Joan D. Vinge
