in new cells that are genetically identical to the parent cell. Mitosis is cell division that is used by the body for growth and repair. Meiosis, on the other hand, is a type of cell division that produces new cells with only half the chromosomal complement (a total of 23 chromosomes). These 23 chromosomes are half the set of the original cell. Meiosis only occurs in the germ line cells, i.e. the ova in women and the sperm in men. If fertilisation occurs, the resulting offspring will inherit 23 chromosomes from the mother and 23 chromosomes from the father, resulting in a full 46 chromosomal complement. Meiotic division prevents the doubling of chromosomal numbers from one generation to the next.
Mitosis
Mitosis occurs rapidly during growth and tissue repair. It is a well-controlled process and consists of two major steps – the division of the nucleus followed by the division of the cytoplasm. Although mitosis is a continuous process it can be described as a series of four stages followed by a resting period where there is no cellular division (Table 1.1).
Table 1.1 The stages of mitosis and interphase
| Stages | Events |
| Chromosomes get shorter and fatter by coiling themselves. They now become visible under a light microscope. Each chromosome has two strands (two copies of the original chromosome) that are held together by the centromere. Strands of protein called spindle fibres appear. | |
| Chromosomes line up together and the spindle fibres become attached to each side of the centromere. | |
| The spindle fibres contract, pulling the two copies of each chromosome to opposite areas within the nucleus. | |
| The two new sets of chromosomes form two new nuclei. The chromosomes revert to being long and thin. The cytoplasm then divides to form two new cells. | |
| Normal cellular function. The cells make copies of their chromosomes ready for the cycle to start again. |
With mitosis each daughter cell is an exact copy of the previous cell. All cells receive identical chromosomal material.
The cycle of events during mitosis usually lasts several hours. The mitotic division of the chromosomal material during prophase, metaphase, anaphase and telophase takes a relatively short period of time and the resting phase (interphase) takes up most of the time within the cell cycle (see Figure 1.15).
Figure 1.15 The cell cycle
The whole cell cycle takes approximately 24 hours, although this depends on which type of cell is involved. Mitosis usually only accounts for about an hour. Interphase is when no cellular division takes place. However, even during interphase, the cell needs to get ready for division so it increases in size. This stage is known as Gap 2 or G2. After division the cell needs to continue to grow so that it can achieve its optimum size; this is known as Gap 1 or G1.
Normally cells can undergo a total of 80 mitotic divisions before the cell dies, although this is dependent on the age of the individual.
Meiosis
Each cell contains two sets of chromosomes which exist in pairs. Meiosis results in cell division that produces new cells with only half the chromosomal complement. This is needed for the formation of germ cells (sperm in men, ova in women) so that two germ cells can fuse to form a full chromosomal complement.
Halving the full complement is achieved in two steps called meiosis I and meiosis II. Meiosis I is very similar to mitotic division in that two daughter cells are produced, both with 46 chromosomes. The main difference is that meiosis I takes much longer in comparison to mitosis and results in the ‘crossing over’ of chromosomal material (see Figure 1.16). Chromosomes ‘swap’ or exchange pieces of their structure with their partner chromosome before separating. This results in the daughter cells not having identical genetic material. This is the cause of genetic variability between individuals.
Meiosis II does not involve chromosomal replication but does involve the stages of prophase, metaphase, anaphase and telophase where chromosomes separate, new nuclei are formed and the cell splits into two. At the end of meiosis II the cells contain 23 individual chromosomes.
Figure 1.16 Crossing over
Differences between mitosis and meiosis
The differences between mitosis and meiosis are illustrated in Figure 1.17 and Table 1.2.
Figure 1.17 Mitosis and meiosis
Table 1.2 The differences between mitosis and meiosis
| Mitosis | Meiosis |
| one division | two divisions |
| results in 2 daughter cells | results in 4 daughter cells/gametes |
| genetically identical | genetically different |
| same chromosome number | chromosome number halved |
| occurs in all body cells | occurs only in germ cells |
| occurs throughout life | occurs only after sexual maturity |
| used for growth and repair | used for production of gametes |
| ACTIVITIES 1.2, 1.3 AND 1.4 |
1.2. Explain why there is significant genetic variation as a result of meiosis but not of mitosis.
1.3. Describe the phases of the cell cycle.
1.4. Explain the reason why germ cells have to undergo meiotic division.
GENETIC INFORMATION
Chromosomes are made up of long chains of DNA (Deoxyribonucleic Acid) and protein molecules. It is the DNA within the chromosomes that holds all genetic information. The total length of the DNA within each cell is over 2m (6 feet) and, in order to fit within the cell’s nucleus, it has to exist in a tightly packaged form. This is achieved by the DNA being coiled around protein structures called histones (see Figure 1.18). The DNA wraps around eight histones to form a structure called a nucleosome. Thousands of nucleosomes are formed, which gives the DNA molecule the appearance of a string of beads. Further coiling of these nucleosome beads results in a shortened structure called a chromatin fibre. It is these tightly packaged chromatin fibres that make up chromosomes.
Figure 1.18 Histones, nucleosomes
