Question

What is the Eukaryotic Cell Cycle?

Answer 1

Eukaryotes are organisms whose cells contain a nucleus enclosed within a nuclear envelope. They belong to the domain called Eukaryota or Eukarya. The cell cycle, or the cell-division cycle, is the series of events that take place in a cell which cause it to divide into two daughter cells and lead to the formation of a new organism.

Complete answer:
In eukaryotic cells, the cell cycle is divided into two main stages: interphase and mitotic (M) phase (which includes mitosis and cytokinesis). During interphase, the cell grows and accumulates nutrients for mitosis and replicates its DNA along with some of its organelles. During the mitotic phase, the replicated chromosomes, organelles and cytoplasm separates into two new daughter cells. To ensure the proper replication of cellular components, there are certain control mechanisms known as cell cycle checkpoints. These checkpoints exist after each key step of the cell cycle and it determines if the cell can progress to the next phase. For example, during G1, the cell passes through a checkpoint that ensures environmental conditions (including signals from other cells) are favourable for replication. Another important checkpoint exists just before a cell moves from G2 to mitosis. Here, a number of proteins scrutinize the cell's DNA to make sure it is structurally intact and properly replicated. Yet another critical cell cycle checkpoint exists mid-mitosis which determines whether the chromosomes in the cell have properly attached to the spindle. This step decreases the possibility that the resulting daughter cells will have unbalanced numbers of chromosomes which is a condition called aneuploidy.

State	Phase	Abbreviation	Description
Resting	Gap 0	G0	The cell leaves the cycle and stops dividing.
Interphase	Gap 1	G1	Cells increase in size, the G1 checkpoint ensures everything is in place for DNA synthesis.
Synthesis	S	DNA replication occurs.
Gap 2	G2	Cell continues to grow, G2 ensures everything is ready to enter mitosis.
Cell division	Mitosis	M	Cell stops growing, and cellular energy is focussed on division. Checkpoint in the middle of mitosis ensures the cell is ready to complete cell division.

Note:
A regulation error in the cell cycle components may lead to the formation of tumors. When some genes like the cell cycle inhibitors, RB, p53 etc. mutate, they may cause the cell to multiply uncontrollably, leading to the formation of a tumour. The duration of cell cycle in tumour cells is equal to or longer than that of a normal cell cycle but the proportion of cells in active cell division in tumours is much higher than that in normal tissue. Thus, there is a net increase in the number of cells.