IB Biology Sub-topic D2.1 Notes

Cell division

In this topic, you are expected to know about cell division to produce daughter cells. The division of the nucleus for this occurs one of two ways:

1. Mitosis - the division of the nucleus into two genetically identical daughter nuclei. This occurs in autosomal cells to produce more autosomal cells.

2. Meiosis -  the division of the nucleus into four genetically distinct daughter nuclei. This occurs to sex cells to produce gametes.

Mitosis

Beginning with mitosis, it is composed of four key stages: prophase, metaphase, anaphase, and telophase. You need to describe, in detail, the events that occur in each stage.

1. During prophase:
   1. Chromatin condenses into chromosomes by supercoiling around histones.
   2. The nuclear membrane breaks down.
   3. At either pole, centrioles grow a spindle fiber network composed of microtubules.
2. During metaphase:
   1. Spindle fibers attach on both sides to the center of chromosomes, called centromeres.
   2. The chromosomes are moved to the equator of the cell by microtubule motors and are lined up.
   3. Proper spindle fiber attachment to each sister chromatid is checked by the cell.
3. During anaphase:
   1. Microtubule motors pull sister chromatids to opposite poles of the cell, separating the cell’s replicated DNA.
   2. From this point forward, sister chromatids are now chromosomes themselves.
4. During telophase:
   1. Chromosomes reach the poles of the cell and uncoil back into chromatin.
   2. A nuclear membrane reforms around each chromatin pool, forming the two identical daughter nuclei.

It is very important to stress that mitosis only produces two identical daughter nuclei, whereas cytokinesis divides the cell to form two identical daughter cells. This is a common misconception that will lose you marks if you get it wrong!

The IB additionally expects you to be able to identify the stages of mitosis on a micrograph. In the micrograph below, we can clearly identify each stage:

From these micrographs, you are additionally expected to calculate a cell’s mitotic index, defined as the percentage of cells undergoing mitosis. In any micrograph, the formula for this is:

$\textrm{mitotic index} = \frac{\textrm{number of dividing cells}}{\textrm{total number of cells}}x100\%$