Question

A giant chromosome with a number of chromonemata is
A. Lampbrush chromosome
B. Heterochromosome
C. Supernumerary chromosome
D. Polytene chromosome

Answer 1

Hint: Giant chromosomes are chromosomes that are very large as compared with normal sized chromosomes. They have thousands of DNA strands. Chromonemata are spirally coiled filaments along with its chromosomes are aligned. Chromomere is formed resulting from local coiling of a continuous DNA thread.

Complete answer:
A giant chromosome is formed by a large number of chromonemata. These giant cells are found in some animal cells. These are formed by continuous replication where division of cells is absent.

> Option A - Lampbrush chromosome: These kinds of chromosomes are found in the oocytes of amphibians and some insects. These kinds of chromosomes consist of axes from which axis paired loops extend in opposite directions, giving the appearance of lampbrush. It doesn’t contain chromonemata but contains large extended loops.

> Option B - Heterochromosome: It is a sex chromosome that is rich in heterochromatin. It mainly contains heterochromatin and hence doesn’t match the requirement of our statement in the question. So we will not consider this option.

> Option C - Supernumerary chromosome: It is a structurally abnormal chromosome not a giant chromosome. It is an additional chromosome basically smaller than others. It is not a giant chromosome.

> Option D - Polytene chromosome: This chromosome begins as a normal chromosome like all other chromosomes. Later on, due to repeated DNA replication without any kind of division or cell division these chromosomes become large. Poly in the name of polytene chromosome is due to occurrence of many chromonemata given by Kollar.

Hence, the correct answer is option (D).

Note: Lampbrush chromosome and polytene chromosome both are giant cells. The main difference between both of these is that polytene chromosome is present in the salivary glands of insects and lampbrush chromosomes occur in oocytes of vertebrae except mammals and some invertebrates. Also, polytene chromosome has many long strands called chromonemata while in the other one that is lampbrush chromosome laterally extending large loops are present.