CBSE Class 10 Science 2022 · Paper 31/2/3 · Question 11

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. Body cells normally carry their chromosomes in pairs, one member of each pair from each parent (46, or 23 pairs, in human beings). Stability of DNA: the germ cells (gametes) are formed by a special division, meiosis, in which each gamete receives only one chromosome of each pair, that is half the chromosome number and half the DNA of a body cell (23 in human beings). At fertilisation a male gamete and a female gamete fuse, and the zygote receives one set from each parent, which restores the normal number ($23+23=46$). Because the number is halved in the gametes and restored at fertilisation, each generation has the same number of chromosomes and the same amount of DNA as its parents, so the DNA of the species stays stable.