Cell Growth and Division

• Growth and reproduction are characteristics of cells.
• All organisms start life as a single cell.
• Cells reproduce by dividing into two.
• Parental cells give rise to two daughter cells each time they divide.
• Daughter cells grow, divide, and form new cell populations.
• Cycles of growth and division allow a single cell to form a structure consisting of millions of cells.

Cell Cycle

• Cell division is important in living organisms.
• Cell division, DNA replication, and cell growth take place during the division of a cell.
• These processes are coordinated to ensure correct division and formation of progeny cells containing intact genomes.
• The cell cycle is the sequence of events by which a cell duplicates its genome, synthesizes its constituents, and divides into two daughter cells.
• Cell growth (cytoplasmic increase) is a continuous process.
• DNA synthesis occurs during one specific stage in the cell cycle.
• Replicated chromosomes (DNA) are distributed to daughter nuclei by a series of events during cell division.
• These events are under genetic control.

Phases of Cell Cycle

• A typical eukaryotic cell cycle takes approximately 24 hours in human cells in culture.
• The duration of the cell cycle varies from organism to organism and also depends on cell type, for example, yeast can do a cycle in only about 90 minutes.
• The cell cycle is divided into interphase and M phase.

Interphase

• The M phase represents the actual cell division or mitosis, interphase represents the phase between two successive M phases.
• Interphase is significant as it is during this time the cell prepares for division.
• In a 24-hour human cell, cell division takes about an hour.
• Interphase accounts for 95% of the duration of the cell cycle.
• The M Phase starts with the separation of daughter chromosomes, along with the division of cytoplasm (cytokinesis).
• Interphase is the resting phase where the cell gets ready for division by undergoing both cell growth and DNA replication.
• Interphase is divided into G1 phase, S phase, and G2 phase.

G1 Phase (Gap 1)

• G1 phase corresponds to the interval between mitosis and initiation of DNA replication.
• During G1 phase, the cell grows continuously but DNA does not replicate.

S Phase (Synthesis)

• S phase marks the period during which DNA synthesis or replication occurs.
• 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, the chromosome number remains the same, i.e., 2n.

G2 Phase (Gap 2)

• During G2 phase, proteins are synthesized in preparation for mitosis.
• In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
• The cell continues to grow in preparation for cell division.