Cell- A structural and functional unit
All organisms are made up of cells. All the organisms start their life from a single cell. The cell then grows by dividing into two, wherein each parent cell forms two daughter cells which mature and further divide to form new cells. The process of cell division is vital to all living organisms.
It is a process by which one single cell gives rise to two or more cells. Cell division is of two types, namely, mitosis and meiosis. During the process of cell division, cell growth takes place, followed by DNA replication, nuclear division and finally cytoplasmic division. The process of cell division must take place in a coordinated manner so that each cell divides correctly and the genomes in the progeny cells remain intact.
The series of events by which a cell duplicates its genome, synthesises its constituents and eventually divides to form two daughter cells is known as the cell cycle.
Although the process of cell growth is a continuous one, DNA synthesis takes place only during one particular stage in the cell cycle.
After DNA synthesis, the replicated chromosomes or the DNA are distributed to the daughter nuclei through a complex series of genetically controlled events during cell division. The duration of a cell cycle varies from one organism to another. A typical eukaryotic cell, like those in human beings, divides once in about every twenty four hours, whereas in yeast, division takes place in about ninety minutes.
Phases of cell cycle
Also, the cell cycle is divided into two basic phases – the interphase and the M phase or the mitosis phase. In living organisms, the cell cycle is a series of events that leads to cell duplication and division, resulting in the growth of organisms.
a) The interphase is the resting phase during which the cell prepares to divide by undergoing cell growth and DNA replication in an orderly manner.The interphase is sub-divided into three phases – the G1 phase or Gap 1, S phase or synthesis phase and G2 phase or Gap 2.
- During the G1 phase or the first growth phase, normal cell functions occur. The cell continues to grow and remains metabolically active but the DNA doesn’t replicate.
- In the S phase or synthesis phase, the DNA synthesises or replicates, and the amount of DNA in the cell doubles without any increase in the number of chromosomes. Also, the centriole begins to duplicate in the cytoplasm.
- Finally, in the G2 phase or Gap 2, the cell prepares for mitosis and cell division by synthesising proteins while the cell continues to grow.
b) The next phase of the cell cycle is the M phase, where actual cell division or mitosis takes place. The M phase begins with nuclear division, which is the division of daughter chromosomes, known as mitosis or karyokinesis. It ends with the division of the cytoplasm, which is known as cytokinesis.
- In a 24-hour average cell cycle of a human cell, proper cell division lasts for only about an hour, while more than 95 percent of the cell cycle duration is the interphase.
- In animal cells, mitotic cell division occurs only in diploid somatic cells.
- In plants, it is seen in both haploid and diploid cells.
- In adult animals, for example in humans, cells in the heart do not exhibit cell division. Such cells that do not display further cell division exit the G1 phase and enter an inactive stage called the Go phase or the quiescent stage.
- Animals also have many cells that divide occasionally, only to replace cells lost due to cell death or injury.
- Cells in Go stage remain metabolically active but proliferate only when needed.