CBSE Class 10 Science 2017 · Paper 31/1/1/C · Question 17

Chromosomes are thread-like structures in the nucleus of a cell, made of DNA (with proteins), that carry the genes, the information for the inheritance of features from parents to offspring. In the body cells they occur in pairs, one member of each pair from each parent (46, or 23 pairs, in human beings). How the number is maintained: the germ cells (gametes) are formed by a special cell division in which each germ cell receives only one chromosome of each pair, so it has half the number of chromosomes (23 in human beings). At fertilisation a male and a female germ cell fuse, and the zygote gets one set from each parent, which restores the normal number ($23+23=46$). So in every generation the progeny have the same number of chromosomes as their parents, and the DNA of the species stays stable.