LIFE CYCLE
The biological term life cycle refers to all growth and development milestones in a living organism up to the point of sexual maturity. Usually, life cycles are classified by the number of sets of chromosomes the cells within the organism have. Even though these organisms are classified as one type of life cycle, they may actually have both diploid and haploid cells in them at the same time.
The term haploid refers to a cell that has only one set of chromosomes and a diploid cell has two sets of chromosomes. While many common animals have a diploid life cycle, there are others who have haploid life cycles and some species that undergo alternation of generations and have both diploid and haploid stages in their life cycle.
A species that has a haploid life cycle would only have a diploid cell when it is a zygote directly after fertilization between a male and female gamete. The zygote would then undergo either meiosis or a haploid mitosis (a division of the cell without copying the DNA first) to form only haploid cells. Many types of fungi and protists have a haploid life cycle.
Humans are a species that have a diploid life cycle. However, humans do produce some haploid cells. The human gametes, or the sex cells used for sexual reproduction, are haploid cells. They are produced from diploid germ cells that undergo meiosis. At the end of the second division, called meiosis II, the result is a total of four haploid cells. It is important that the gametes have only half the number of chromosomes as a regular body cell. This is because when two haploid gametes fuse during fertilization, it ensures the zygote is diploid and has the correct number of chromosomes and not too many or too few. The wrong number of chromosomes could cause serious disorders or even death.
In an organism, like a plant, that has an alternation of generations life cycle, it will spend its time during the haploid stage as a gametophyte. Gametophytes, as the name implies, will produce gametes that are haploid that can fuse during sexual reproduction with other gametes to create offspring. Again, the offspring will be diploid and have the correct number of chromosomes because the gametes were haploid. The diploid stage results in sporophytes which can reproduce asexually by releasing a spore or other type of diploid cell that will undergo mitosis and produce offspring.