This is an equational division in which a cell divides into two equal halves, each containing all the essential parts. In such a division, first the chromosome number gets doubled i.e. each chromosome gets divided into two, thereby doubling the total number of chromosomes in the cell. Later on, each half receives half the chromosomes. For example, each human cell has 46 chromosomes, which become 92 in number in mitosis. Each new mitotic cell gets half of it, i.e. 46. Thus the chromosome number remains constant in the species in each cell.
This type of division continues in the body cells (except sex cells) all the time and is the major source of repair, growth and development.
Meiosis is a special cell division found only in sex cells (sperms and eggs). In this, a reduction division takes place first i.e. after duplication of each chromosome, the 92 chromosomes thus produced get divided into four parts, each having 23 chromosomes. Thus each daughter cell gets 23 chromosomes. That is why this is called a reduction division. Each of the four sperms containing 23 chromosomes combines with an egg having 23 chromosomes, thus giving rise to a zygote having 46 chromosomes in all. Thus the number of chromosomes again remains the same in the species.
When two offspring are born at the same time, they are referred to as twins. Depending upon the mode of formation, they are of two types.
Such twins develop from a single fertilized egg, which divides into exactly two halves some time after fertilization. The twins born from such a division are called identical twins and they share virtually identical physical and psychological features.
Fraternal twins are the result of fertilization of two eggs with two separate sperms. The twins produced as a result of such separate fertilization are just like ordinary siblings.