Molecular Basis of Inheritance
Nucleic acids are long polymers of nucleotides. While DNA stores genetic information, RNA mostly helps in transfer and expression of information. DNA, being chemically and structurally more stable, is a better genetic material. However, RNA is thought to have evolved first. DNA replicates semiconservatively. During transcription, one strand of DNA acts as template to direct the synthesis of complementary RNA. The genetic code is read again on the principle of complementarity by tRNA, which acts as an adapter molecule. Protein synthesis is done by ribosomes. Both transcription and translation are energetically very expensive processes. Regulation of gene expression refers to a very broad term that may occur at various levels. The genes in a cell are expressed to perform a particular function or a set of functions. Human Genome Project was a mega project that aimed to sequence every base in human genome. Many new areas and avenues have opened up as a consequence of the project.
In Hershey-Chase experiment, bacteria was infected with viruses that had its DNA and protein coated with ....... and ........ respectively.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Radioactive phosphorous and radioactive sulphur
Hershey and Chase used bacteriophages to find out whether DNA or protein enters the bacterium and carries the genetic information. They had to label DNA and protein differently. DNA contains phosphorus but no sulphur. Protein contains sulphur but no phosphorus. So radioactive phosphorus marks the DNA and radioactive sulphur marks the protein coat.
(1) Radioactive lead and radioactive phosphorous — Incorrect. Lead is not used. Phosphorus marks the DNA, not the protein.
(2) Radioactive phosphorous and radioactive sulphur — Correct. DNA is labelled with phosphorus and protein with sulphur, in that order.
(3) Heavy isotope of nitrogen and carbon — Incorrect. Heavy isotopes belong to the Meselson and Stahl experiment, which used 15N. Hershey and Chase used radioactive isotopes.
(4) Radioactive thymine and Radioactive sulphur — Incorrect. The second label is right, but the DNA was labelled with phosphorus, not thymine.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
The Genetic Material Is DNA – …
