If the nucleus of a cell contains one complete chromosomal set, it is called a haploid cell. The number of chromosomes in such cells is indicated by n, where 'n' stands for the number of chromosomes. The haploid number is different in different organisms.
This is a cell which contains only one set of chromosomes. These, in humans and other mammals, are mostly the sex cells (egg and sperms) produced by a process called meiosis. When the egg is fertilized by a sperm, it gives rise to a diploid cell.
A cell divides into two halves, and gives rise to two daughter cells. This process is seen only in eukaryotic cells, and this is the only part in the life cycle of a cell that involves sexual reproduction. It helps in the combination of genes of the parents, without increasing the number of chromosomes. In the reproductive organs of higher animals like humans, the gonads, that is the testes and ovaries divide to form sex cells by meiosis.
The nucleus of the cell divides twice during meiosis, and produces four daughter cells. In diploid organisms, meiosis occurs by the formation of gametes (sperm and egg). These gametes are haploid cells, containing only one set of chromosomes. These gametes unite during fertilization, which helps in restoring the diploid number.
In case of plants, meiosis occurs before spore formation, and the spores have haploid number of chromosomes. When the haploid spore unites with the ovary, it gives rise to progeny by sexual reproduction. Meiosis is very important, as it helps in gene mixing and variation, to give rise to a better progeny. The cells combine to form a diploid cell with different genes from the parent. Thus, the progeny is more genetically advanced than that of the parent.
The number of haploid cells in humans is 1 set of 23 chromosomes, of which there are 22 sets of autosomal or non-sex chromosomes, and 1 set of sex chromosomes. After fertilization, it gives rise to diploid cells with 46 chromosomes. These 46 chromosomes are formed by the combination of 2 sets of 23 chromosomes from the parent cells.
Haploid cells are usually the gamete cells of any organism. The entire plant body contains haploid cells that gives it a better genetic advantage. These plants usually have pure genetic lines. In all animals, including humans, the egg and sperm are the only haploid cells.