Actively dividing eukaryote cells pass through a series of stages known as cell cycle. The cell cycle is to accurately duplicates chromosomal DNA and then segregate its copies into two genetically identical daughter cells.
|Learning Objective: Cell cycle and the role of the cell cycle plays carrying out the essential functions in the cell.|
This is what you will learn:
Phases of Cell Cycle
The time from the end of one mitosis to the start of the next mitosis is called interphase. Interphase is the longest period in cell division. It consists of following:
- G1 phase
- S (Synthesis) phase
- G2 phase
In this phase, mitosis partitions the genetic material and cell divides (Figure 1).
The basic organization of the cycle and its control system are essentially the same in all eukaryotic cells.
This course covers the stages involved in a cell cycle and its various checkpoints.
During interphase, the cell is engaged in metabolic activity and preparing for the mitosis.
Chromosomes are not clear in the nucleus, although a dark spot called nucleolus and centrioles may become visible.
Interphase comprises G1, G2, and S phases of the cell cycle.
The period between the end of telophase and before the entry S phase is called G1 phase.
Biochemically, it is a very active phase and most variable phase. Cell mass increases in this phase.
It constitutes 25-50% of the total interphase duration, whereas, in some cells G1 may be very short or absent.
In cells preparing for cell division there is a marked synthesis of mRNA, tRNA and proteins during G1.
The enzymes and substrates necessary for DNA synthesis during S phase, are also synthesized during this phase.
Commitment to chromosome or DNA replication in S phase occurs in G1 phase. The commitment point is very clear in yeast cell, where it is called start.
In animal cell, it is called the restriction point, where a cell is committed to the cell division.
Non-cycling state is called G0 state or resting state. Some cells do not divide at all and are called the resting cells.
These cells seem to be withdrawn from the cell cycle into another state. they resemble G1 phase, however, it is distinct from G1 phase as cells are not able to go to S phase.
The absence of nutrients or growth factors cause cells to enter a resting state. For example, in absence of nutrients yeast cells and in deprivation of growth factors, mammalian cells get arrested early in G1 in the stage G0.
DNA synthesis occurs after the G1 phase. Each chromosome forms two sister chromatids, which are firmly attached to the centromeric region.
The centrosome is duplicated during the S phase. These two centrosomes will give rise to the mitotic spindle, the apparatus that regulates the movement of chromosomes during mitosis.
Centrioles present in centrosomes help organize cell division. Centrioles are not found in plants and most fungi.
In the G2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome cell division.
Cell gets prepared metabolically and physically before it enters first stage of mitosis.
A nuclear division (mitosis) followed by a cell division (cytokinesis).
Mitosis is the process of cellular division that produces two identical daughter cells from one parent cell.
Chromosomes replicated during the S phase are divided in such a way that each daughter cell receives a copy of a chromosome.
In actively dividing animal cells, the whole process takes about one hour.
Importance of the Cell Cycle
- The most important function of the cell cycle is to duplicate DNA in the chromosomes and then segregate the copies precisely into two genetically identical daughter cells.
- Cell cycle stops or prevent cell division when it is not needed.
- In multicellular organisms, cell division (a part of cell cycle) allows individuals to grow and change by expanding the number of total cells.
Things to Remember
- All multicellular organisms use cell division for growth and the maintenance.
- The cell cycle has two major phases: interphase and the mitotic phase.
- Interphase is divided into G1, G2, and S phase.
- Mitosis is the process of cellular division that produces two identical daughter cells from one parent cell.
- The most important function of the cell cycle is to duplicate DNA and segregate the copies precisely into two genetically identical daughter cells.