In the last topic, structure and organization of chromosomes within a nucleus have been discussed. From life cycles of plants and animals, we have learnt that the only link between parents and offspring are sex cells or gametes. Therefore, it is natural to expect that if there is any physical basis of inheritance, then structures responsible for inheritance should pass through these gametes. We also know that hereditary characters are located in macromolecules called DNA (deoxyribonucleic acid) located in chromosomes. Therefore, it is the behaviour of chromosomes during cell division, which should govern the patterns of inheritance. Therefore in this section cell division will be discussed in some detail. Reproductive cycles in some plants and animals will also be described, so that principles of genetics utilizing different organisms to be discussed in further topics will be easily understood.
Since a cell also has to maintain continuity from one generation to another and since the hereditary material has to copy itself most faithfully, a cell has to divide in growing tissues and elsewhere in such a manner that the two daughter cells are similar to each other and resemble the parent cell from which these were produced. However, in cell divisions taking place in sex cells, the daughter cells may differ from one another and also from parent cell, but they would still have most of the essential features in common. Primarily there are two kinds of cell divisions : mitosis
which is meant for multiplication of cell number and meiosis
which helps in alternation of generations. Irrespective of whether or not, products of meiosis (spores) directly take part in fertilization (syngamy), meiotic division always (unless it is abnormal meiosis) reduces the chromosome number to half, which is restored to normal diploid number at the time of zygote formation.