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Haploid-Diploid Life Cycle

A haploid is a set of chromosomes containing one member of each chromosome pair. It can also be defined as the number of chromosomes in a gamete (a cell with only one pair of chromosomes) of an organism. It is symbolized by "n" or "1n." A diploid is a set of chromosomes containing two sets of chromosomes (one male and one female). The symbol for a diploid is "2n" because it is double the haploid. The haploid-diploid life cycle is also called an alternation of generations.

  1. The Facts

    • A life cycle that includes both the 1n state and the 2n stage is referred to as the haploid-diploid life cycle. These two stages are separated by the processes of meiosis and syngamy (fertilization of gametes). Sexually reproducing organisms undergo meiosis (reductional cell division in which the number of chromosomes per cell is halved). Meiosis leads to four haploid gametes each of which contains one of every pair of maternal and paternal chromosomes.

    Features

    • Mitosis is a process by which a cell divides chromosomes into two identical sets. Once the cells undergo mitosis, the organism can do one of two things. It can produce gametes through mitosis or it can start asexual reproduction.

    Function

    • The particular course of action depends on the type of organism that undergoes mitosis. If gametes are produced then they are fertilized (or sygamized). Following fertilization, the fused gametes become a zygote and the process begins again.

    Haploid Life Cycle

    • The haploid life cycle is common in fungi and algae and is sometimes referred to as the gametophyte phase. Prior to meiosis, the organism is a diploid zygote (new organism formed as a result of fertilization). After the zygote undergoes meiosis, it develops into a haploid spore. Meiosis creates four cells from each zygote which in turn can become spores or other structures, depending on the organism. Each of the four cells goes through mitosis and becomes an organism.

    Diploid Life Cycle

    • Unlike the haploid life cycle, the diploid life cycle has a sporophyte (and not the gametophyte). In this phase, meiosis produces four gametes which then develop into mature gametes through the process of mitosis. When these gametes mature they fuse and develop into a zygote. Through mitosis, the zygote develops into an organism which then produces a sporocyte (or a meiocyte), depending on the type of organism. This organism then undergoes meiosis and again produces four gametes so that the cycle can repeat. Depending on the organism, it may also undergo asexual reproduction.

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