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  Section: Genetics » Physical Basis of Heredity » Cell Division (Mitosis and Meiosis)
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Reproductive cycles

Physical Basis of Heredity 2.  Cell Division (Mitosis and Meiosis)
Significance of meiosis
Reproductive cycles
Stages of meiosis
Synaptonemal complex
Recombination nodules
Comparison of meiosis and mitosis

In Chlamydomonas, as shown in Fig. 7.5, the haploid generation is rather long and the diploid generation involves formation of zygote. The zygote undergoes reduction division resulting in formation of haploid spores. Such is the situation s in many animal types also.

In Bryophytes, the plants are haploid and bear male and female sex organs. Sex cells are formed without reduction division and fuse to form diploid zygote, which develops in a dependent sporophyte. The reduction division takes place in this sporophyte giving rise to haploid spores. These spores develop into the gametophyte which is the main plant and is a haploid (Fig. 7.6).

Neurospora which is an important fungus from ascomycetes, has a haploid plant body, which develops sex organs. Fusion of sex nuclei takes place and the resulting diploid generation is of a short duration. Reduction division takes place in ascus during ascospore formation (Fig. 7.7).

In higher plants like maize or wheat, the plants are diploid and reduction division takes place at the time of sex cell formation. Sex cells are haploid and soon fuse to restore the diploid condition (Fig. 7.8). In case of higher animals, situation is much like that in higher plants (Fig. 7.9).

The above discussion indicates that an alternation between haploid and diploid generations is almost a rule in all kinds of living organisms where sexual reproduction takes place. Variation is, however, found in respect to the stage in the life cycle where reduction division takes place.
SexuaI reproduction cycle in Neurospora
Fig. 7.7. SexuaI reproduction cycle in Neurospora

Life cycle of an Angiosperm.
Fig. 7.8. Life cycle of an Angiosperm.
Life cycle of a higher animal.
Fig. 7.9. Life cycle of a higher animal.

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