The Cell Cycle: Interphase, Mitosis and Cytokinesis

You are not the same you

The skin you have today is not the skin you had a few weeks ago. The cells on the surface wear away and are replaced from below. A scraped knee heals. A child grows into an adult. All of this depends on cells making more cells. But a cell can't just split in half like a cookie: each new cell needs a complete copy of the instructions in the nucleus. The cell cycle is the careful process that makes sure that happens.

Chromosomes: the instruction books

The DNA in a nucleus is not one long tangle. It is divided into separate pieces called chromosomes. Each human body cell has 46 chromosomes (23 pairs: one set from each parent). Most of the time the chromosomes are long, thin and spread out, so you can't see them individually under a light microscope. This loose form is called chromatin.

Before a cell divides, it copies every chromosome. The two copies, called sister chromatids, stay joined at a region called the centromere. When the cell divides, the sister chromatids are pulled apart, so each new cell gets one complete copy of every chromosome.

The cell cycle

The cell cycle has two big parts:

1. Interphase (most of the cycle). The cell is not resting; it is very busy.

• G1 (gap 1): the cell grows, makes proteins and organelles, and does its normal job (a liver cell makes bile, for example).
• S (synthesis): the cell replicates (copies) its DNA. Every chromosome becomes two sister chromatids.
• G2 (gap 2): the cell keeps growing and makes the proteins it needs for division.

2. M phase (division), which has two steps:

• Mitosis: the nucleus divides into two nuclei with identical chromosomes.
• Cytokinesis: the cytoplasm divides, forming two daughter cells.

The cell has checkpoints where it "checks" things before moving on: Is the cell big enough? Was the DNA copied correctly? Are all the chromosomes attached to spindle fibres? If something is wrong, the cycle pauses for repairs, or the cell may destroy itself. You'll see in the cancer lesson what happens when these checkpoints fail.

Mitosis, step by step: PMAT

Mitosis is a continuous process, but biologists divide it into four stages. A memory trick: Please Make Another Two (Prophase, Metaphase, Anaphase, Telophase).

1. Prophase. The chromosomes condense (coil up tightly), becoming short and thick enough to see. Each one is a pair of sister chromatids. The nuclear membrane breaks down. Spindle fibres begin to form. In animal cells they grow from a pair of structures called centrioles, which move to opposite ends (poles) of the cell. Plant cells have no centrioles but still build a spindle.
2. Metaphase. Spindle fibres attach to each centromere and line the chromosomes up across the middle of the cell (the equator). M for middle.
3. Anaphase. The centromeres split and the sister chromatids are pulled apart toward opposite poles. Each chromatid is now a separate chromosome. A for apart.
4. Telophase. A full set of chromosomes arrives at each pole. A new nuclear membrane forms around each set, and the chromosomes unwind back into chromatin. The spindle breaks down. T for two nuclei.

Then comes cytokinesis, which usually starts during late anaphase or telophase.

Cytokinesis: plants and animals do it differently

• Animal cells: the cell membrane pinches inward around the middle, making a groove called the cleavage furrow, until the cell is pinched in two (like tightening a drawstring).
• Plant cells: a rigid cell wall can't pinch. Instead, vesicles gather in the middle and join to form a cell plate, which grows outward until it meets the old wall. The cell plate becomes the new cell wall between the two daughter cells.

Why mitosis matters

Mitosis makes daughter cells that are genetically identical to the parent cell. That is exactly what the body needs for:

• Growth: you grew from one fertilized cell to roughly tens of trillions of cells.
• Repair: when you cut your skin or break a bone, nearby cells divide to fill the gap.
• Replacement: cells that wear out (skin, the lining of the intestine, blood cells) are replaced constantly.
• Asexual reproduction: some organisms, such as many single-celled organisms and plants grown from cuttings, reproduce this way.

There is another kind of division, meiosis, that makes sperm and egg cells with half the chromosomes. You will study it in Grade 11 Biology.