Cell division

Cells multiply by the process of mitosis. in which each chromosome divides to form two identical daughter chromosomes: these separate into two clusters and a cell wall forms between them. In this way each of the millions of cells forming the body of a plant exactly the same set of genes, barring r accidents.

a population i. This picture s the tolly ot trying to define a species from a single plant.

As a prelude to the development of reproductive cells (gametes), the chromosomes undergo a special type of division, meiosis. First they form up in pairs, each chromosome of one set of 11 pairing up with its partner in the other set. At this stage an exchange of genetic material takes place, so that when the chromosomes separate, those derived from parent A have some genes from parent B, and those derived from parent B have corresponding genes from parent A. There followsa reduction division in which only half of the overall (diploid) number of chromosomes passes into each pollen grain or egg cell. These are haploid (that is, containing the halved or basic number of chromosomes). Fertilization restores the original number of 22 in a cell, and a new plant develops from the seed with the same overall number as both parents but with uniquely shuffled arrangement. It is a feature of sexual reproduction that, by the laws of chance, no two individuals will be identical (5.1).

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