a life cycle in which organisms switch back and forth between diploid and haploid stages
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The life cycle you are referring to is called alternation of generations. It is commonly seen in plants and some protists. In this life cycle, organisms undergo two distinct stages: a diploid (2n) stage known as the sporophyte, and a haploid (n) stage known as the gametophyte.
To understand how this life cycle works, let's take the example of a flowering plant:
1. Sporophyte Stage (2n): The life cycle begins with a diploid individual, known as the sporophyte. This is the dominant stage of the life cycle. The sporophyte produces special structures called sporangia, which are located in the flowers or cones of the plant.
2. Spore Formation: Within the sporangia, the sporophyte undergoes meiosis. Meiosis is a process that reduces the chromosome number by half, resulting in the formation of haploid spores. Each spore contains only one set of chromosomes.
3. Dispersal and Germination: The haploid spores are then released from the sporangia and dispersed by various means, such as wind or water. When conditions are favorable, the spores germinate and develop into a new generation known as gametophytes.
4. Gametophyte Stage (n): The gametophytes are haploid individuals and are typically small and inconspicuous compared to the dominant sporophyte stage. They develop reproductive structures called gametangia, which produce gametes through mitosis.
5. Gamete Formation: Within the gametangia, mitosis occurs to produce gametes. In plants, the male gametes are called sperm and are produced in structures called antheridia, whereas the female gametes, called eggs, are produced in structures called archegonia.
6. Fertilization: The sperm from the male gametophyte needs to reach the female gametophyte for fertilization to occur. This can happen through various means like wind, water, or pollinators. Once fertilization occurs, a diploid zygote is formed.
7. Sporophyte Formation: The zygote develops into a new sporophyte, continuing the cycle. This new sporophyte will eventually produce sporangia, and the cycle repeats.
In this way, the life cycle alternates between diploid (sporophyte) and haploid (gametophyte) stages, with each stage giving rise to the other. Alternation of generations ensures genetic diversity and allows for adaptation to different environmental conditions.