Cell Division – SS2 Biology Lesson Note (Second Term)

Cell division is the process in which a parent cell divides to form daughter cells. The two types are mitosis and meiosis.

Behavioural objectives

1. Name the two types of cell division.
2. Define mitosis and say where it takes place.
3. Describe the five stages of mitosis.
4. State the importance of mitosis.
5. List life processes that involve mitosis.
6. State the differences between mitosis and meiosis.

What are the key terms in cell division?

These are the words a student must know before studying mitosis and meiosis.

• Daughter cell: a new cell formed when a parent cell divides.
• Chromosome: a thread-like structure in the nucleus that carries genes.
• Chromatin: the network of fine, long threads formed by chromosomes when the cell is not dividing.
• Chromatid: one of the two identical halves of a chromosome that has copied itself.
• Centromere: the point where the two chromatids of a chromosome are joined.
• Centriole: a small body in animal cells that moves to the poles and helps form the spindle.
• Spindle fibres: thread-like structures that guide and pull the chromatids apart.
• Somatic cell: a body cell that does not form gametes, such as a skin cell.
• Gamete: a sex cell, such as a sperm cell or an egg cell.
• Diploid (2n): having two full sets of chromosomes in the cell.
• Haploid (n): having one set of chromosomes, which is half the diploid number.

What are the types of cell division?

There are two types of cell division: mitosis and meiosis.

• Mitosis: one parent cell gives two daughter cells with the same chromosome number as the parent.
• Meiosis: one parent cell gives four daughter cells with half the chromosome number of the parent.

What happens to a cell after it divides?

After division, the daughter cells grow, specialise and, in most cases, stop dividing.

• Start of life: life begins as a single fertilised cell, which divides into two cells, then four, and so on.
• Enlargement: the daughter cells increase in mass and size.
• Specialisation: each cell changes its shape and structure to carry out a particular function.
• Loss of division: most specialised cells lose the ability to divide when they mature.

What is mitosis?

Mitosis is a cell division that produces two daughter cells with the same number of chromosomes as the parent cell.

• Where it happens: in somatic cells, such as skin cells, bone marrow cells and the meristematic tissues of plants.
• Result: the daughter cells are identical to the parent cell and to each other.

What are the stages of mitosis?

Mitosis is described in five stages: interphase, prophase, metaphase, anaphase and telophase.

1. Interphase: the stage between divisions, often called the resting stage, though the cell is busy growing and copying its DNA.
   • The chromosomes are long and thin.
   • They form a network of fine threads called chromatin.
2. Prophase: the chromosomes become visible.
   • Early prophase: chromosomes become visible and the nucleolus shrinks.
   • Early prophase: the centrioles move to opposite sides of the cell, and spindle fibres begin to form.
   • Late prophase: chromosomes become shorter and thicker.
   • Late prophase: each chromosome shows two chromatids joined by a centromere.
   • Late prophase: the nucleolus and nuclear membrane disappear completely.
3. Metaphase: the paired chromatids line up along the equator of the spindle.
   • Each is attached to a spindle fibre at the centromere.
4. Anaphase: the sister chromatids separate.
   • The spindle fibres pull them to opposite poles of the cell.
   • They finally reach the poles.
5. Telophase: the cell divides into two.
   • The cell constricts at the equator (in plant cells a cell plate forms instead).
   • The nucleolus and nuclear membrane form again in each daughter cell.
   • The spindle fibres break down.
   • The chromosomes return to their thread-like form.

Why is mitosis important?

Mitosis is important because it gives growth, repair and asexual reproduction while keeping the chromosome number constant.

1. Growth: it brings about growth, development and specialisation, especially in many-celled organisms.
2. Asexual reproduction: binary and multiple fission in protozoa, and budding in yeast, result from mitosis.
3. Constant chromosome number: it keeps the diploid condition of cells the same from generation to generation.
4. Repair: it replaces damaged and worn-out cells.

Which life processes involve mitosis?

Mitosis takes place in growth, repair, replacement of cells and asexual reproduction.

1. Skin renewal: new cells form in the Malpighian layer of the skin.
2. Blood formation: red and white blood cells are produced in the bone marrow.
3. Wound healing: new cells replace the damaged ones.
4. Plant growth: cells divide in the meristem.
5. Binary fission: a single-celled organism, such as Amoeba, divides into two.

What is meiosis?

Meiosis is a cell division that produces four daughter cells, each with half the chromosome number of the parent cell.

• Where it happens: in the reproductive organs, such as the testes and ovaries in animals and the anthers and ovules in flowers.
• Product: gametes (sex cells), which are haploid.
• Importance: when two gametes fuse at fertilisation, the diploid number is restored.

How does meiosis take place?

Meiosis happens in two successive divisions, called meiosis I and meiosis II.

1. Before meiosis: the cell copies its chromosomes in interphase.
2. Meiosis I: pairs of similar chromosomes separate, giving two cells with half the chromosome number.
3. Meiosis II: the chromatids of each chromosome separate, as in mitosis.
4. End result: four haploid daughter cells.

How does mitosis differ from meiosis?

Mitosis makes two identical body cells for growth and repair. Meiosis makes four different haploid gametes for sexual reproduction.

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.
5. State two differences between mitosis and meiosis.

Assignment

Describe what happens in prophase and telophase of mitosis, then list three life processes that involve mitosis.