• Cell division, DNA replication and cell growth are closely related and coordinated processes.
• The cell cycle refers to the sequence of events by which a cell duplicates its components and divides into
two daughter cells.
• Although cell growth is continuous, DNA synthesis only occurs at a specific stage in the cell cycle.
• The replicated DNA is distributed to the daughter cells during cell division through a complex series of
events that are under genetic control.
• A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately
every 24 hours.
• However, the duration of the cell cycle can vary from organism to organism and from cell type to cell type.
• Yeast cells for example can progress through the cell cycle in only about 90 minutes.
• The cell cycle is divided into two phases: interphase and M phase (mitosis phase).
• The M phase is when the cell actually divides through mitosis.
• The interphase occurs between two successive M phases.
• In humans,Cell division through mitosis only lasts for about 1 hour out of the total 24 hour cell cycle.
• The interphase lasts for more than 95% of the cell cycle duration.
• The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes and
usually ends with division of cytoplasm.
•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 phases: G1 phase, S phase and G2 phase.
• G1 phase is the gap between mitosis and the start of DNA replication. During this phase, the cell is metabolically
active and the cell grows but does not replicate its DNA.
• S or synthesis phase is when DNA replication takes place. The amount of DNA doubles from 2C to 4C.
• The chromosome number remains the same even after DNA replication. If the cell had 2n chromosomes in G1
phase, it will still have 2n chromosomes after S phase.
• During S phase in the cell cycle, DNA replication begins in the nucleus and centrioles duplicate in the cytoplasm.
• During G2 phase, proteins are synthesized to prepare for mitosis while cell growth continues.
• Some cells like heart cells do not divide and many other cells divide only when needed to replace damaged or
dead cells.
• These cells exit G1 phase and enter a quiescent stage called G0 where they remain metabolically active but do not
proliferate unless required.
a) Phases of cell cycle
CELL CYCLE
