The Cell Cycle

Cells are Alive!

All cells are alive – they ‘breathe’, they take in food, they throw out waste, they create more of their own kind, and they die. They are chemical engines that continuously use energy from their environment to maintain their internal conditions for survival and reproduction.

One key function of all cells is to create more cells of their own kind. Living organisms are constantly making new cells in order to grow and to replace dead cells. They do this through a process called Cell Division. Cell division is occurring all the time in living organisms. Trillions of cell divisions are taking place in the average human body every day!

The growth and development that a cell undergoes from its formation to its reproduction is known as its Cell Cycle.

How Cells Divide

New cells are formed by division – each cell divides to form two cells, so there are two cells where earlier there was only one. Cells divide in two main ways –

1.Mitosis

Most cells are produced by Mitosis. In this process, asingle cell divides into two new, exactly identical cells. Each daughter cell then grows in size and becomes capable of dividing again, so now there are four cells. And so on. Most one-celled organisms as well as many multicellular plants and animal cells grow this way.

When the cell is not dividing, it is undergoing growth and carrying out its normal activities. At some point of time the chromosomes in the cell duplicate to form two chromosomes. At this stage, the cell is ready to divide, giving one chromosome of each pair to the two new daughter cells. In this manner, the genetic code and characteristics of the parent cell are passed on to each of the two daughter cells.

2. Meiosis

In human beings and many other living organisms, two cells are required for cell reproduction to take place. This process is known as Meiosis and it occurs through special sex cells – a male sex cell known as Spermand a femalesex cell known as Egg. A sperm and an egg, each from two different living things (a male and a female), must unite in order for cell reproduction to occur.

New sex cells are at first produced through the regular process of mitosis. This is followed by a special kind of cell division, called meiosis, in which the sperm cell and the egg cell undergo separate divisions of their own so as to create four sex cells.Each new sex cell now has only half the number of chromosomes as the parent. The male sex cell and the female sex cellthen combine in a process called Fertilization to create a single fertilized eggwhich contains one chromosome from each of the parent sex cells. In this manner, the newly created living being has exactly the same set of chromosomes as its two parents, with one chromosome coming from each parent to form pairs.

Check Point

1. Cell cycle includes steps that a cell undergoes from its ______ to its ______.
2. Cell division takes place in two main ways – ______ and ______.
3. When asingle cell divides into two new, exactly identical cells, the process is called ______.
4. Chromosomes pass on the ______ of the parent cell to each of the daughter cells.
5. The special sex cells, male and female, are known as ______ and ______.

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