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20.Chromosomes And DNA eLearn.Punjab

Chromosomes are thread like structures that appear inside the nucleus at the time of cell division.
They were first observed by the German embryologist Walther Fleming in 1882, when he was
examining the rapidly dividing cells of salamander larvae. Since their discovery, chromosomes have
been found in the cells of all eukaryotes. Their number however varies from species to species.
Pencillium, a fungus, has only one pair of chromosomes, while some ferns have more than 500 pairs.
A mosquito has 6, honeybee 32, corn 20, sugarcane 80, frog 26 and a mouse has 40 chromosomes.
Human cells have 46 chromosomes, consisting of 23 pairs (Fig 20.1). Each of these 46 chromosomes
contains hundreds or thousands of genes that play important roles in determining how a person’s
body develops and functions. The possession of all these chromosomes is therefore, essential
for survival. Missing of a part or whole of chromosome leads to serious consequences, and death
occurs.

Fig. 20.1 Human chromosomes

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