Chromosomes are threads composed of chromatin-a combination
of DNA and protein-that sit in the cell’s nucleus and hold the vast majority of
its genetic information.
They are usually depicted as sticks that are pinched in the
middle, but they actually take on this form during cell division. For most of
the time, the chromosomes are long, loose strings, like fabric necklaces, and
the genes are like patches of colour woven into the design.
(left diagram. human chromosome)
The number of chromosomes differs from organism to organism,
and they almost invariably come in pairs: individuals inherit one copy from
their mother and one from their father. Only in reproductive cells called
gametes(in animals, the eggs and sperms) is just a single set present.
Ordinary paired chromosomes are known as autosomes, of which
humans have 22 pairs, and most animals also have sex chromosomes that can
differ between males and females. In humans, people who inherit two X
chromosomes are female, while those who have one X and one Y chromosome are
male.
(right diagram. structure of human chromosome)
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