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16.2) Sexual reproduction

16.2) Sexual reproduction

 

Sexual reproduction: is a process involving the fusion of two gametes (sex cells) to form a zygote and the production of offspring that are genetically different from each other.

 

Fertilisation: is the fusion of gamete nuclei.

 

Gametes are made in the reproductive organs.

The process of cell division that produces the gametes is called meiosis.

In sexual reproduction, the male and female gametes come together and fuse, that is, their cytoplasm and nuclei join together to form a single cell called a zygote.

  • In flowering plants the male gametes are found in pollen grains and the female gametes, called egg cells are present in ovules.
  • In animals, male gametes are sperm and female gametes are eggs.
  • Male gamete is microscopic and mobile. The sperm swim to the ovum; the pollen cell moves down the pollen tube.
  • The female gametes are always larger than the male and are not mobile.

 

Chromosome numbers:

  • In normal body cells (somatic cells) the chromosomes are present in the nucleus in pairs.
  • Humans have 46 chromosomes; 23 pairs. This is known as diploid
  • When the gametes are formed, the number of chromosomes in the nucleus of each sex cell is halved. This is the haploid
  • During fertilisation, when the nuclei of the sex cells fuse, a zygote is formed.
  • It gains the chromosomes from both gametes, so it is a diploid cell.

 

The advantages and disadvantages of sexual reproduction:

 

Cross-breeding:

  • It is possible for biologists to use their knowledge of genetics to produce new varieties of plants and animals.
  • A long-term disadvantage of selective breeding is the loss of variability, by eliminating all the offspring who do not bear the desired characteristics, many genes are lost from the population.
  • At some future date, when new combinations of genes are sought, some of the potentially useful ones may no longer be available.
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