Concept explainers
(a)
To write:
The phases of meiosis in which crossing-over occur.
Introduction:
Meiosis is a reductional cell division occurs in germ cells producing four cells of half chromosome number from that of the parent cell. In meiosis, single interphase followed by two nuclear divisions − Meiosis I and Meiosis II. Meiosis I involves the separation of the homologous chromosome (reductional division) while in meiosis II, separation of chromatids (equational division) occur.
Explanation of Solution
Crossing over occurs in Prophase I. During this phase, interchange of fragments between paternal and maternal sets of chromosome occurs through crossing over and this process is considered as a basis of genetic variability in offspring. Because of crossing over and interchange of segments between homologous chromosomes, chiasmata or cross-shaped structure formed.
Hence, crossing over takes place in prophase I and chromosomes become aligned in pairs at the equator during anaphase I of meiosis I.
(b)
To write:
The phases of meiosis in which chromatids separate at their centromere and migrate to opposite poles.
Introduction:
Meiosis is a reductional cell division occurs in germ cells producing four cells of half chromosome number from that of the parent cell. In meiosis, single interphase followed by two nuclear divisions − Meiosis I and Meiosis II. Meiosis I involves the separation of the homologous chromosome (reductional division) while in meiosis II, separation of chromatids (equational division) occur.
Explanation of Solution
Chromatids separate at their centromeres and migrate to opposite poles: During anaphase II, typical mitotic separation of centromeres and chromatids of each chromosome and migration towards their opposite poles take place.
The chromatids separate at their centromeres during anaphase II of meiosis II.
(c)
To write:
The phases of meiosis in which chromosomes become aligned in pairs at the equator events occur.
Introduction:
Meiosis is a reductional cell division occurs in germ cells producing four cells of half chromosome number from that of the parent cell. In meiosis, single interphase followed by two nuclear divisions − Meiosis I and Meiosis II. Meiosis I involves the separation of the homologous chromosome (reductional division) while in meiosis II, separation of chromatids (equational division) occur.
Explanation of Solution
Chromosomes become aligned in pairs at the equator: Metaphase I phase is characterized by the alignment of bivalent chromosomes at the equator of the cell. The chromosomes point towards the opposite direction of the poles and chiasmata lie at the equator.
The chromatids separate at their centromeres and migrate to opposite poles during anaphase II of meiosis II.
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Stern's Introductory Plant Biology
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