Cell growth is accomplished through the synthesis of new molecules of proteins, nucleic acids, carbohydrates and lipids. As the accumulation of these molecules causes the volume of a cell to increase, the plasma membrane grows to prevent the cell from bursting. But cells cannot continue to enlarge indefinitely — as a cell grows larger, there is an accompanying decrease in its surface area/volume ratio and hence in its capacity for effective exchange with the environment. Therefore, cell growth is generally accompanied by cell division, whereby one cell gives rise to two new daughter cells (The term daughter is used by convention and does not indicate that cells have gender.)

For single-celled organisms, division increases the total number of individuals in a population. In multi-cellular organisms, cell division either increases the number of cells, leading to growth of the organism, or replaces ones that have died. In an adult human, for example, about two million stem cells in bone marrow divide every second to maintain a constant number of red blood cells in the body.

A cell passes through a series of discrete stages, collectively known as the cell cycle. It begins when two new cells are formed by the division of a single parental cell and ends when one of these cells divides again into two cells. To early cell biologists studying eukaryotic cells with the microscope, the most dramatic events in the life of a cell were those associated with the point in the cycle when the cell actually divides. This division process, called the M phase, involves two overlapping events in which the nucleus divides first and the cytoplasm second. Nuclear division is called mitosis, and the division of the cytoplasm to produce two daughter cells is termed cytokinesis.

While visually striking, the events of the mitotic phase account for a relatively small portion of the total cell cycle; for a typical mammalian cell, the mitotic phase usually lasts less than an hour.

Cells spend the majority of their time in the growth phase between divisions called interphase. Most cellular contents are synthesised continuously during interphase, so cell mass gradually increases as the cell approaches division. During interphase the amount of nuclear DNA doubles, and experiments using radioactive DNA precursors have shown that the new DNA is synthesised during a part-icular portion of interphase named the S phase (S for synthesis). A time gap called Gl phase separates the S phase from the preceding M phase, and a second gap, the G2 phase, separates the end of S phase from the beginning of the next M phase.

Although the cells of a multi-cellular organism divide at varying rates, most studies of the cell cycle involve them growing in culture, where the length of the cycle tends to be similar for different types. We can easily determine the overall length of the cell cycle – the generation time – for cultured cells by counting the cells under a microscope and determining how long it takes for the population to double. In cultured mammalian cells, for example, the total cycle usually takes about 18-24 hours. Once we know the total length of the cycle, it is possible to determine the length of specific phases. For mammalian cells in culture, S phase is about six to eight hours in length. On the other hand, the M phase lasts less than an hour (usually 30-45 minutes).

In contrast to the S and M phases, whose lengths tend to be quite similar for different mammalian cells, the length of G1 is quite variable, depending on the cell type. G2 is shorter than G1 and is more uniform in duration among different cell types, usually lasting four to six hours.

The writer is Associate Professor, Head, Department of Botany, Ananda Mohan College, Kolkata and also Fellow, Botanical Society of Bengal.