70. Mitosis and the Cell Cycle

The cell cycle is the series of stages that a cell goes through to divide and produce new cells. This process is essential for growth, development, and the repair of tissues in multicellular organisms.

1. The Three Main Stages of the Cell Cycle

A cell's life cycle generally consists of three functional phases:

• Growth: The cell increases in size and produces more sub-cellular structures, such as mitochondria and ribosomes.

• DNA Replication: The DNA is duplicated so that each of the two new cells will have a complete set of genetic material.

• Mitosis and Cytokinesis: The actual division of the genetic material and the cell body into two identical "daughter" cells.

2. Chromosomes and DNA Replication

In eukaryotic cells, DNA is organized into chromosomes—tightly coiled packets of DNA located in the nucleus.

• Pairs: Humans have 23 pairs of chromosomes (46 in total), with one set inherited from each parent.

• Duplication: Before division, the cell duplicates each chromosome. The copy stays attached to the original, forming a characteristic X-shape. Each half of the "X" is an identical arm containing the same DNA.

3. The Process of Mitosis

Once the DNA has been replicated and the cell has grown, mitosis begins:

1. Alignment: All 46 duplicated (X-shaped) chromosomes line up along the center of the cell.

2. Separation: Cell fibers attach to each half of the chromosomes and pull the identical arms to opposite poles (sides) of the cell.

3. Division: This ensures that each side of the cell has an identical set of 46 chromosomes.

4. Cytokinesis

The final stage of the cell cycle is cytokinesis:

• The cell membrane and cytoplasm pull apart and divide.

• This results in two separate daughter cells.

• Outcome: These daughter cells are genetically identical to each other and to the original parent cell.

Summary Table: Cell Cycle Stages