Cell Cycle and Cell Division

• The cell cycle is the life of a cell from its formation from a dividing parent cell until its division into two daughter cells.
• The cell cycle consists of two stages: interphase and cell division.
• Cell Cycle = Interphase + Cell division.
• Cell division (Mitotic or M-Phase) = Mitosis + cytokinesis.
• Mitosis is nuclear division.
• Cytokinesis is the division of cytoplasm.
• Multinuclear cells result if mitosis isn't followed by cytokinesis.

The Cell Division

• Cell division, the reproduction of cells, is the basis for the continuity of life.
• Key roles of cell division:
  • Reproduction
  • Growth and development
  • Tissue renewal and repair
• Most cell divisions produce genetically identical daughter cells, with the exception of meiosis (sperms and eggs).

Cellular Organization of the Genetic Material

• The cell's DNA content, its genetic information, is called its genome.
• DNA molecules are packaged into chromosomes, common in eukaryotic cells.
• Chromatin is the DNA and proteins that make up chromosomes.
• Each eukaryotic species has a characteristic number of chromosomes in each cell nucleus.
• Human somatic cells have 46 chromosomes (2 sets of 23).
• Germ cells (Sperm & Ovum) have 23 chromosomes (one set of 23).
• The number of chromosomes in somatic cells varies among species.

Interphase

• Interphase is the stage between two successive cell divisions.
• Interphase represents about 90% of the cell cycle, taking about 23 hours of a 24-hour cycle.
• M-Phase is the shortest phase of the cell cycle.
• Interphase subphases: G1-phase (1st gap), S-phase (synthesis), and G2-phase (2nd gap).
• The cell grows in size in all three phases, producing proteins and cytoplasmic organelles like mitochondria and ER.

G1-phase (1st gap)

• The cell grows in size.
• Formation of enzymes required for DNA synthesis occurs.
• The centrosome is duplicated.
• Non-cycling cells enter G0.

S-phase (synthesis)

• DNA replication and duplication of chromosomes occur.
• Synthesis of chromosomal proteins takes place.

G2-phase (2nd gap)

• Protein synthesis and growth continue to prepare the cell for M-phase.

Generation time

• Generation time is the time it takes to complete one cell cycle (24 hours in a human cell).
• Nerve cells, skeletal muscle cells, and RBCs stop dividing and enter G0.