Principles of Inheritance and Variation Question 84
84. Radioactive thymidine when added to the medium surrounding living mammalian cells gets incorporated into the newly synthesized DNA. Which of the following types of chromatin is expected to become radioactive if cells are exposed to radioactive thymidine as soon as they enter the S-phase?
[1998]
(a) Heterochromatin
(b) Euchromatin
(c) Both heterochromatin and euchromatin
(d) Neither heterochromatin nor euchromatin but only the nucleolus
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Answer : b
Hints & Solution
(b) Heterochromatin is a specialised chromosome material which remains tightly coiled and stains darkly in interphase. Euchromatin is that portion of the chromosome that stains with low intensity and uncoils during interphase. In the beginning of S-sphase, DNA replication occurs. DNA replication can occur in diffuse/ less tightly coiled euchromatin.
