Cell Division – SS2 Biology Lesson Note (Second Term)

Cell division is the process in which a cell divides to form daughter cells. There are two types, mitosis and meiosis. In mitosis, a parent cell produces two daughter cells with the same number of chromosomes and characteristics as itself. Mitosis brings about growth, repair of damaged cells and asexual reproduction.

Behavioural objectives

By the end of the lesson, students should be able to:

• name the two types of cell division;
• define mitosis and say where it takes place;
• describe the five stages of mitosis;
• state the importance of mitosis;
• list life processes that involve mitosis.

What happens to a cell after it divides?

Life begins as a single fertilised cell, which divides into two daughter cells, then into four, and so on. After cell division, the daughter cells increase in mass and size (enlargement). Eventually, each cell develops into a special type of cell (specialisation) by changing its shape and structure to carry out a particular function. Most specialised cells, at maturity, lose their ability to divide.

What are the types of cell division?

There are two types of cell division:

1. Mitosis
2. Meiosis

What is mitosis?

Mitosis is a cell division that produces two daughter cells with the same number of chromosomes and characteristics as those of the parent cell. This cell division takes place only in somatic cells (i.e. body cells that are not involved in the production of gametes) such as skin, bone marrow and meristematic tissues in plants.

What are the stages of mitosis?

Mitotic cell division involves five main stages:

1. Interphase: This is referred to as the resting stage, where the chromosomes become elongated and form a network of fine threads called chromatin.
2. Prophase: In early prophase, chromosomes become visible, the nucleolus shrinks, the centrioles start moving away from each other in opposite directions and the formation of spindle fibres begins. During late prophase, chromosomes become shorter, thicker and visible. Each chromosome now forms two distinct chromatids joined by a centromere. The nucleolus and nuclear membrane disappear entirely.
3. Metaphase: Paired chromatids arrange themselves along the equator of the spindle and are attached to the spindle at the centromere.
4. Anaphase: Sister chromatids separate, start migrating to the opposite poles of the cell and eventually reach the poles.
5. Telophase: The cell starts dividing into two by constricting at the equator. The nucleolus and nuclear membrane are reformed in each daughter cell, the spindle fibres degenerate and the chromosomes eventually regain their threadlike form.

Why is mitosis important?

1. It brings about growth, development and specialisation, especially in multicellular organisms.
2. Asexual reproduction by binary and multiple fission in protozoa, and by budding in yeasts, results from mitosis.
3. It ensures that the diploid condition of the cells is retained from generation to generation.
4. It aids in the repair of damaged cells.

Which life processes involve mitosis?

1. Formation of new cells in the Malpighian layer of the skin
2. Production of red and white blood cells in the bone marrow
3. Healing of wounds
4. Growth in the meristem
5. Binary fission

Evaluation

1. Name the two types of cell division.
2. Define mitosis and give two examples of places where it takes place.
3. State what happens to the chromosomes during anaphase.
4. List four importances of mitosis.

Assignment

1. Describe what happens in prophase and telophase of mitosis.
2. List three life processes that involve mitosis.