Question

If the number of dyads at each pole during Anaphase-I is 25 the gametes will contain how many chromosomes?
A. 25
B. 50
C. 100
D. None of the above

Answer 1

Hint: Anaphase-I begins when two chromosomes of bivalent tetrad separate and move to opposite poles of the cell thanks to spindle action. During anaphase-I sister chromatids remain attached at the centromere and move together towards opposite end poles.

Complete answer: A dyad consists of a pair of homologous chromosomes or sister chromatids. It is often found during anaphase I of Meiosis at a process called a disjunction.
1. In anaphase I, the aim of the spindle apparatus is clear. This phase can only happen after a positive spindle checkpoint result at the preceding phase, i.e. the metaphase.
2. Most kinetochores are attached to one microtubule – usually, a bundle – the breakdown of single sections of microtubule doesn't cause the detachment of a chromosome from the spindle, yet this attachment must still be strong.
3. The connecting fibers of the microtubule spindle shorten through the ending of small sections, while kinetochores lead the chromosomes up- or downwards; the spindle network elongates, separating the poles even further.
4. Overlapping sections of microtubules – plus ends originating from both poles – slide apart. This mechanism is very regulated. The cell stretches and elongates, whereby the poles become further apart.
5. As the poles have 25 dyads, so each gamete will have 25 chromosomes.

So, the correct answer is option A. 25.

Note: In anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. This is often confused with anaphase of mitosis. The major differentiating factor between the two stages is that the sister chromatids remain joined after metaphase in the stage of meiosis I, while they separate from each other in mitosis.