Meiosis: Chromosome Reduction & Genetic Variation — NEET Biology
📚 Practice ConceptMeiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.
Which of the following statements correctly explain the origins of genetic variability during meiosis? S1. The primary cause of genetic variation in meiosis is the random assortment of homologous chromosomes during Anaphase I. S2. Crossing over, which involves the exchange of segments between non-sister chromatids of homologous chromosomes, directly results in new combinations of alleles on a chromosome. S3. Synapsis, the intimate pairing of homologous chromosomes, is a prerequisite for crossing over to occur effectively, thereby indirectly contributing to genetic recombination. S4. Meiosis II contributes to genetic variation by segregating sister chromatids, which may no longer be identical due to preceding crossing over in Meiosis I. S5. The formation of the synaptonemal complex during zygotene ensures the physical proximity required for successful recombination events.
View solution & NCERT explanation
Correct answer: B — S2, S3, S4, and S5 are correct
Let's evaluate each statement: S1. Incorrect. While the random assortment (independent assortment) of homologous chromosomes during Anaphase I is a significant source of genetic variation, it is not the *sole primary* cause. Crossing over is equally crucial. NCERT mentions both as contributing factors without ranking one as 'primary'. S2. Correct. Crossing over is explicitly described as the exchange of genetic material between non-sister chromatids of homologous chromosomes, leading to recombination and new combinations of genetic material. S3. Correct. Synapsis, the process of homologous chromosomes pairing, forms the bivalent structure and the synaptonemal complex, which are essential for crossing over to take place. Thus, synapsis is an indirect but fundamental contributor to genetic recombination. S4. Correct. If crossing over occurs in Meiosis I, sister chromatids are no longer identical. Their subsequent separation in Meiosis II ensures that these newly formed recombinant chromatids are distributed into different gametes, thereby enhancing genetic diversity. S5. Correct. The synaptonemal complex formed during zygotene functions to align and hold homologous chromosomes together, creating the necessary physical association that enables crossing over between non-sister chromatids in pachytene. Therefore, statements S2, S3, S4, and S5 are correct.
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