C HAPTER . Cell Cycle . M Phase . Significance of Mitosis .
Meiosis . Significance of Meiosis . . Phases of Cell Cycle A typical eukaryotic cell cycle is illustrated by human cells in culture.
These cells divide once in approximately every hours (Figure . ). 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 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 successive M phases. It is significant to note that in the hour average duration of cell cycle of a human cell, cell division proper lasts for only about an hour. The interphase lasts more than % of the duration of cell cycle. 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 ) S phase (Synthesis) G phase (Gap ) G phase corresponds to the interval between mitosis and initiation of DNA replication. During G 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 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G , even after S phase the number of chromosomes remains the same, i.e., 2n. In animal cells, during the S