# 128 Cell division – Mitosis and Meiosis

Mitosis is a nuclear division giving rise to genetically identical cells in which the chromosome number is maintained by the exact duplication of chromosome.

Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid. 

Mitosis 

Mitosis is the way in which any cell (plant or animal) divides when an organism is:

• growing
• repairing a damaged part of its body
• replacing worn out cells

Growth means getting bigger. An individual cell can grow a certain amount, but not indefinitely. Once a cell gets to a certain size, it becomes difficult for all parts of the cell to obtain oxygen and nutrients by division. In order to grow any more, the cell divides to form two smaller cells, each of which can then grow and divide again.

Mitosis is also used in asexual reproduction. For example, sweet potato plant can reproduce by growing adventitious roots or runners which eventually produce new plants.

Process of mitosis

• During the process, all the chromosomes in the parent cell are copied. 
• Each copy remains attached to the original one --> each chromosome is made up of 2 identical threads joined together. 
• The parent cell (with 4 chromosomes) split to form 2 nuclei each with 2 chromosomes as the parent nucleus cell. 
• At the end of a mitotic cell division, the number of cells is doubled and the daughter cells produced are genetically identical to the parent. 

Mitosis 

Meiosis 

Meiosis is the way in which gametes (sex cells) are produced.  Gametes have only half the number of chromosome of a normal body cell. They have 1 set of chromosome instead of 2. When they fuse together, the zygote formed has 2 sets. 

• Human gametes are formed by the division of cells in the ovaries and testes 
• The gametes  produced are haploid, but they are formed from diploid cells, so meiosis involves halving the normal chromosome number - the pairs of chromosomes are separated. 
• During meiosis, the new cells get a mixture of homologous chromosomes from father and mother --> A sperm cell could contain a chromosome 1 from father and a chromosome 2 from mother.
• There are all sorts of combinations --> gametes are genetically different form the parent cells. Meiosis produces genetic variation. 
• When ova are formed in a woman, all the ova will carry an X chromosome. When sperm are formed in a man, half the sperm will carry an X chromosome, half will carry a Y chromosome. 

Meiosis

Sample question

Complete the following passage, using only words form the list below.

diploid   gametes   haploid   meiosis   mitosis   red blood cells

The transfer of inherited characteristics to new cells and new individuals depends on two types of cell division. During ___________, the chromosomes are duplicated exactly and _________ cells are produced.

However, during ______________, the chromosome sets are first duplicated and then halved, producing cells. These cells will become _________ .   [4 marks]

Student’s answer

During meiotosis û , the chromosomes are duplicated exactly and identical û cells are produced.

However, during meiosis ü, the chromosome sets are first duplicated and then halved, producing cells. These cells will become gametes. ü

Examiner’s comments

The first answer is not clear – it mixes up the terms ‘mitosis’ and ‘meiosis’. Sometimes candidates do this deliberately when they are not sure of the answer, hoping that the examiner will give them the benefit of the doubt. (We don’t!). This candidate has not followed the rubric (instructions) in the question for the second answer: the term ‘identical’ does not appear in the word list. The correct answers are ‘mitosis’ and ‘diploid’.

Try this

1) The nuclei of human liver cells contain 46 chromosomes. Complete the table below to show how many chromosomes would be present in the cells listed. [3 marks]

Type of cell	Number of chromosomes
Ciliated cell in windpipe
Red blood cell
Ovum

2) Describe 2 differences, other than the number of chromosomes, between nuclei produced by mitosis and those produced by meiosis. [2 marks].

Answer:

1) Ciliated cell: 46

Red blood cell: 0 (this cell has no nucleus)

Ovum: 23

2) Two differences from:

• chromosomes in daughter mitotic cells will be identical to parental chromosomes (or there is no variation).
• genes in daughter mitotic cells will be identical to parental genes.
• chromosomes in daughter mitotic cells will be in homologous pairs, but they will be single in meiotic nuclei. 

Video: Mitosis  

Video: Meiosis 

Video: Difference between mitosis and meiosis