Question

The interzonal fibers occur in
(A) Prophase
(B) Early metaphase
(C) Late prophase
(D) Anaphase

Answer 1

Hint: The fibres that are formed in between the centromeres of the homologous chromosomes are interzonal fibres. This is usually seen in metaphase I stage.

Complete answer:
Anaphase is the shortest duration. It degenerates proteins binding the two chromatids in the region of the centromere. As a result, the centromere of each chromosome divides. This converts the two chromatids into daughter chromosomes each being attached to the spindle pole of its side by independent chromosomal fiber. The chromosomes move towards the spindle poles with the centromeres projecting towards the poles and the limbs trailing behind. Anaphasic chromosomes appear differently depending upon the position of their centromeres- V-shaped (metacentric), L-shaped (submetacentric), J- shaped (acrocentric), and I-shaped(telocentric). The two pole-ward moving chromosomes of each type remain attached to each other by interzonal fibres.
In the early prophase, the chromatin fibres condense through spiralization to form elongated chromosomes. In mid prophase, chromosomes shorten and become distinct with each having two chromatids attached to a narrow point called the centromere. In late prophase, the centrosomes reach the poles, form asters, and begin to develop spindle fibres.
In metaphase the complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell. By this stage, condensation of chromosomes Is completed. At this stage, the metaphase chromosome is made up of two sister chromatids which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres to the chromosomes that are moved into position at the Centre of the cell.

So, the correct answer is option D i.e Anaphase


Note: Late prophase begins with the disruption of the nuclear envelope, which is broken down into small membrane vesicles that closely resemble the endoplasmic reticulum and tend to remain visible around the mitotic spindle.