Cell Cycle and Cell Division

C HAPTER TEN CELL CYCLE AND CELL DIVISION

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Are you aware that all organisms, even the largest, start their life from a single cell? You may wonder how a single cell then goes on to form such large organisms. Growth and reproduction are characteristics of cells, indeed of all living organisms. All cells reproduce by dividing into two, with each parental cell giving rise to two daughter cells each time they divide. These newly formed daughter cells can themselves grow and divide, giving rise to a new cell population that is formed by the growth and division of a single parental cell and its progeny. In other words, such cycles of growth and division allow a single cell to form a structure consisting of millions of cells.

Ten point one CELL CYCLE CELEBR Cell division is a very important process in all living organisms. During the division of a cell, DNA replication and cell growth also take place. All these processes, i.e., cell division, DNA replication, and cell growth, hence, have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes. The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle. The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control.

CELL CYCLE AND CELL DIVISION

Ten point one point one Phases of Cell Cycle

A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every twenty-four hours. However, this duration of cell cycle can vary from organism to organism and also from cell type to cell type. Yeast for example, can progress through the cell cycle in only about ninety minutes.

The cell cycle is divided into two basic phases:

· Interphase

· M Phase (Mitosis phase)

The M Phase represents the phase when the actual cell division or mitosis occurs and the interphase represents the phase between two in the twenty-four hour average duration of cell cycle of a human cell, cell division proper lasts for only about an hour. The interphase lasts more than ninety-five percent of the duration of cell cycle.

successive M phases. It is significant to note that Figure Ten point one A diagrammatic view of cell cycle indicating formation of two cells

The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis). The interphase, though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. The interphase is divided into three further phases:

· G, phase (Gap one)

· S phase (Synthesis)

· G two phase (Gap two)

G one phase corresponds to the interval between mitosis and initiation of DNA replication. During G one phase, the cell is metabolically active and continuously grows but does not replicate its DNA. S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as two C then it increases to four C. However, there is no increase in the chromosome number; if the cell had diploid or two n number of chromosomes at G one, even after S phase the number of chromosomes remains the same, i.e., two n.

In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. During the G two phase, proteins are synthesised in preparation for mitosis while cell growth continues.

How do plants and animals continue to grow all their lives? Do all cells in a plant divide all the time? Do you think all cells continue to divide in all plants and animals? Can you tell the name and the location of tissues having cells that divide all their life in higher plants? Do animals have similar meristematic tissues?

You have studied mitosis in onion root tip cells. It has sixteen chromosomes in each cell. Can you tell how many chromosomes will the cell have at G one phase, after S phase, and after M phase. Also, what will be the DNA content of the cells at G one, after S and at G two, if the content after M phase is two C?