Mitosis

Year 11: Biology

5. Maintenance of organisms requires growth and repair

Identify mitosis as a process of nuclear division and explain its role

Mitosis

The process by which cells reproduce for growth or tissue repair in multicellular is called cell division or mitosis

Mitosis and Meiosis

Before mitosis and meiosis can be explained, some background information is needed. Inside the cell there are many organelles, or small organs.

One is the nucleus. Inside the nucleus is the DNA that tells the cell what to do. The DNA is found in the form of 46 chromosomes that are contained in the nucleus. All 46 are blueprints for some sort of body part or feature.

They look like this:

The other background info needed is the knowledge of the cell cycle. Mitosis and meiosis are part of the cell cycle. There are 4 parts of the cycle. The first 3 are known as interphase.

The first part occurs after the cell is born. It is called Growth 1 (G1). During this phase the cell completes its function as a cell. As it metabolizes and grows, it performs the process it was meant to do. If it is a lung cell, it helps use oxygen, if its a liver cell, it helps the liver function, etc.

The next phase is the S phase, or Synthesis phase. During this time the DNA inside the cell replicates, making a total of 92 chromosomes inside the nucleus.

After replication, the cell enters G2, or Growth phase 2. During this phase the cell continues to grow and metabolizes. Also, all of the organelles are doubled, save for the nucleus. That comes next

Mitosis is the final step in the cell cycle. Basically, it is the division of the nucleus and the chromosomes within it. There are 4 steps in mitosis and a fifth one that is or isn't part of it, depending on what source you get it from. First, prophase occurs

In prophase, the loosely strung chromatin in the nucleus starts to get really tightly wound. It becomes so dense and closely coiled that is visible under a microscope (chromatin isn't). Also, the nucleus starts to deteriorate and the centrioles (dormant or non-existent during interphase) start sending out spindle fibers.

During metaphase, the spindle fibers attach to the chromosomes and align them along the middle (equator) of the cell (by this time the nucleus is completely gone).

Anaphase comes next and the chromosomes are pulled apart by the spindle fibers. The separated sister chromosomes (chromosomes that are identical to one another; created during DNA replication) are pulled to the poles of the cells in preparation for telophase, the final step of mitosis.

In telophase, the chromosomes arrive at the poles, the spindle fibers break down, and two new nuclei begin to form around the two new groups of chromosomes.

Also, a cleavage furrow begins to form along the outside of the middle of the cell. The disputed step is next and is called cytokinesis.

Cytokinesis is simply the division of the cytoplasm (i.e. the other organelles besides the nucleus). The cleavage furrow gets bigger and bigger until the cell just splits into two new cells. The cell has become two new, identical (to each other and to the parent cell) cells that can enter the cell cycle. Mitosis is how body cells reproduce themselves