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1.1.4: Life Cycles

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    Learning Objectives
    • Describe the role of ploidy in the haplodiplontic (alternation of generations) life cycle of plants.
    • Compare and contrast gametophyte and sporophyte generations.

    Organisms or cells that contain two copies of each chromosome are considered diploid, or 2n (they contain homologous chromosomes in pairs that were acquired from their parents). Organisms or cells that contain one set of chromosomes are considered haploid, or n, meaning they only contain one of each chromosome, either material or paternal. Your body cells are diploid and contain pairs of chromosomes. Your gametes (egg and sperm cell) are haploid and contain unpaired chromosomes. For more on ploidy, see Biology 2e 11.1 Meiosis.

    Plants have a haplodiplontic (alternations of generations) life cycle. In plants, there are two distinct generations that alternate: the gametophyte and the sporophyte generation. The gametophyte begins as a single-celled spore that germinate into a multicellular life stage. This mature gametophyte life stage is haploid and by mitosis (division of cells with identical genetic information) produces gametes. These gametes are either egg or sperm cells. Often cross fertilization occurs where the sperm cell of one plant individual fertilizes the egg cell of another plant individual. The results is a fertilized egg, or zygote, which is diploid. This begins the sporophyte generation in plants. The sporophyte then divides into a multicellular life stage (division occurs by mitosis). This mature sporophyte will now produce spores by meiosis, a type of cell division that reduces chromosome numbers to half. The sporophyte is diploid, but the spores are haploid. The spores disperse and germinate into new gametophytes, repeating the process (see Figure \(\PageIndex{1}\)).

    A generalized plant life cycle.
    Figure \(\PageIndex{1}\): The plant life cycle is haplodiplontic (alternations of generations). Fern is shown as an example. The haploid (n) multicellular life stage is called the gametophyte as it produces gametes (egg and sperm cells) that fuse during fertilization to form a diploid (2n) zygote. This zygote develops into a mature diploid sporophyte by mitosis, carrying both genetic information from the fused gametes. The mature sporophyte produces spores by meiosis, which are haploid. The spores germinate by mitosis to develop into a mature multicellular gametophytes, which is still haploid. The life cycle continues as gametes are produced (OpenStax licensed CC-BY).

    Note that both the gametophyte and sporophyte are mature plants. However, they reproduce in different ways and have different chromosome counts. The gametophyte generation produces gametes by mitosis that are fertilized, undergoing sexual reproduction. The sporophyte generation produces spores by meiosis that are dispersed. Both gametophyte and sporophytes grow by mitosis.

    Contributors and Attributions

    Curated and authored by Kammy Algiers using 11.2 Sexual Reproduction from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.

     

    This page titled 1.1.4: Life Cycles is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers (ASCCC Open Educational Resources Initiative) .