Meiosis and mitosis

# 1.2 Sexual reproduction

Sexual reproduction includes two distinctive processes:

1. The production of haploid gametes, such as sperm and ova, which involves the specialized nuclear division called meiosis.

2. The fusion of gametes at fertilisation, which results in the restoration of the diploid number of chromosomes.

The relationship between these two processes, and the changes in chromosome number that each process brings about, is shown in outline in Figure 3. This figure represents a hypothetical organism with only four chromosomes, that is, two pairs of homologous chromosomes — one long pair and one short pair — as shown for each of the parents in the first row in the figure. The chromosomes of the female parent (maternal chromosomes) are shown in red and those of the male parent (paternal chromosomes) are shown in blue. As a result of meiosis, the chromosome number is halved in the gametes, each of which contains two chromosomes, as shown in the second row in Figure 3. Notice that the set of chromosomes in the gametes is not a random collection; it is made up of one member of each homologous pair — one long chromosome and one short. The bottom part of the figure shows that fertilisation restores the original number of four chromosomes in the zygote, which contains a pair of the long homologous chromosomes and a pair of the short homologous chromosomes. Note that in the zygote, half the chromosomes are shown in red and half in blue, since one member of each homologous pair comes from the female parent and its partner comes from the male parent.

Figure 3 How the number of chromosomes changes at gamete production (meiosis) and at fertilisation in a hypothetical organism with only two pairs of homologous chromosomes. For simplicity, each chromosome is shown as a single strand, rather than as a pair of chromatids (Figures 1 and 2).
Figure 3

## Question 3

What would happen to the chromosome number in future generations if gamete production did not involve the halving of the diploid chromosome number?

## Answer

If the chromosome number were not reduced by half during gamete formation, to produce the gametes containing the haploid number, a zygote would have twice the diploid number; the chromosome number would double in each subsequent generation.

The distribution of one member of each homologous pair of chromosomes to each gamete is a consequence of the precision of the process of meiosis. An understanding of the distribution of chromosomes, both during the production of gametes and at fertilisation, is important for exploring the inheritance of genes.

## Activity 1

Insert the missing term in each of the following sentences.

1. ____________chromosomes are pairs of chromosomes which are present in diploid cells and have the same appearance and function.

2. The number, size and shape of all the chromosomes in a cell is called the ____________and is characteristic for a species.

3. The process of____________is involved in the production of gametes, which have the haploid number of chromosomes.

4. The process of combining two gametes to produce a zygote is called____________.

## Answer

The missing terms are: (1) homologous; (2) karyotype; (3) meiosis; (4) fertilization.

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