One of the properties of life is the ability to reproduce to create offspring that can carry on the genetics of the parent or parents to the following generations. Living organisms can accomplish this by reproducing in one of two ways. Some species use asexual reproduction to make offspring, while others reproduce using sexual reproduction. While each mechanism has its pros and its cons, whether or not a parent needs a partner to reproduce or it can make offspring on its own are both valid ways to carry on the species.
Different kinds of eukaryotic organisms that undergo sexual reproduction have different types of sexual life cycles. These life cycles determine how the organism will not only make its offspring but also how the cells within the multicellular organism will reproduce themselves. The sexual life cycle determines how many sets of chromosomes each cell in the organism will have.
Diplontic Life Cycle
A diploid cell is a type of eukaryotic cell that has 2 sets of chromosomes. Usually, these sets are a genetic mixture of both the male and female parent. One set of the chromosomes comes from the mother and one set comes from the father. This allows a nice mixture of the genetics of both parents and increases diversity of traits in the gene pool for natural selection to work on.
In a diplontic life cycle, the majority of the organism's life is spent with most of the cells in the body being diploid. The only cells that have half the number of chromosomes, or are haploid, are the gametes (sex cells). Most organisms that have a diplontic life cycle start from the fusion of two haploid gametes. One of the gametes comes from a female and the other from the male. This coming together of the sex cells creates a diploid cell called a zygote.
Since the diplontic life cycle keeps most of the body cells as diploid, mitosis can happen to split the zygote and continue splitting future generations of cells. Before mitosis can happen, the cell's DNA is duplicated to make sure the daughter cells have two full sets of chromosomes that are identical to each other.
The only haploid cells that happen during a diplontic life cycle are gametes. Therefore, mitosis cannot be used to make the gametes. Instead, the process of meiosis is what creates the haploid gametes from the diploid cells in the body. This ensures that the gametes will have only one set of chromosomes, so when they fuse again during sexual reproduction, the resulting zygote will have the two sets of chromosomes of a normal diploid cell.
Most animals, including humans, have a diplontic sexual life cycle.
Haplontic Life Cycle
Cells that spend the majority of their lives in a haploid phase are considered to have a haplontic sexual life cycle. In fact, organisms that have a haplontic life cycle are only composed of a diploid cell when they are zygotes. Just like in the diplontic life cycle, a haploid gamete from a female and a haploid gamete from a male will fuse to make a diploid zygote. However, that is the only diploid cell in the entire haplontic life cycle.
The zygote undergoes meiosis at its first division to create daughter cells that have half the number of chromosomes compared to the zygote. After that division, all of the now haploid cells in the organism undergo mitosis in future cell divisions to create more haploid cells. This continues on for the organism's entire life cycle. When it is time to sexually reproduce, the gametes are already haploid and can just fuse with another organism's haploid gamete to form the zygote of the offspring.
Examples of organisms that live a haplontic sexual life cycle include fungi, some protists, and some plants.
Alternation of Generations
The final type of sexual life cycle is a kind of mix of the two previous types. Called alternation of generations, the organism spends about half of its life in a haplontic life cycle and the other half of its life in a diplontic life cycle. Like the haplontic and diplontic life cycles, organisms that have an alternation of generations sexual life cycle begin life as a diploid zygote formed from the fusion of haploid gametes from a male and a female.
The zygote can then either undergo mitosis and enter its diploid phase, or perform meiosis and become haploid cells. The resulting diploid cells are called sporophytes and the haploid cells are called gametophytes. The cells will continue to do mitosis and split in whichever phase they enter and create more cells for growth and repair. Gametophytes can then once again fuse to become a diploid zygote of the offspring.
Most plants live the alternation of generations sexual life cycle.