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Q: Write a note on Alternation of Generation . Do mention the different life cycles in plants .
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Answers
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Alternation of generations is a type of life cycle found in terrestrial plants and some algae in which subsequent generations of individuals alternate between haploid and diploid organisms. This can be contrasted to sexual reproduction in animals, in which both haploid and diploid cells are found in every generation. Alternation of generations has several distinct features, and these features can be slightly modified between species. In general, the generations alternate between the sporophytes capable of creating spores and the gametophytes, capable of creating gametes.
●●Sporophyte
To form a sporophyte, two haploid gametes come together to form a diploid zygote. Typically, haploid organisms are defined by having an “n” number of chromosomes. When two gametes of the same species come together, they each have n chromosomes. Therefore, the diploid zygote which forms is considered to have 2n worth of genetic material, or exactly twice as much. Not only is there twice as much DNA, but it represents codes for the same proteins in the same organism. The sporophyte is a multicellular organism formed from multiple rounds of mitosis on the zygote. Thus, the sporophyte individual remains a 2n organism.
●●Gametophyte
This represents the next generation in the alternation of generations, as the haploid spore is created. The spore is technically a new organism, and has only half the DNA as the parent organism. This spore will undergo successive rounds of mitosis to form a new multicellular individual, the gametophyte. Where the sporophyte generation creates spores, the gametophyte generation creates gametes. Gametes are produced by special organs on the gametophyte, the gametangia. These gametes are then broadcast into the environment, or transferred between plants.
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