Answer:
D. Crossing over
Explanation:
During prophase-I of meiosis-I, a process of genetic recombination occurs. During this process, two non-sister chromatids of the paired homologous chromosomes exchange a part of the genetic segment. This is called crossing over. Crossing over results in the production of recombinant chromatids that carry new combinations of alleles that were otherwise not present in the parental chromosomes. Therefore, crossing over generates some extra combinations of alleles.
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B- they are denser than water
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Explanation:
The phase of Prophase I of meiosis is very long and divided into 5 subphases: Leptotene, Zygote, Pachytene, Diplotene, and Diakinesis. During a phase of the Diplotene, the degree of condensation is high, which allows individualizing the sister- chromatids that remain attached by the cohesins. The synaptonemal complex disintegrates, and from the centromeres begins a repulsion between homologous chromosomes, which remain associated only with the places where they occur as permutations.
These sites are called chiasmas (Greek, crossed) because they show the crossover of homologous chromatids. Chiasmas represent the cytological finding of the occurrence of permutation. The presence of at least one bivalent chiasm is essential to ensure the correct segregation of the homologous chromosomes in anaphase I.