Q4. List the four criteria a molecule must meet to act as genetic material.
It should replicate itself; be chemically and structurally stable; allow slow changes (mutation) needed for evolution; and express itself as Mendelian characters (NCERT page 87).
Q5. Why do viruses with an RNA genome evolve faster?
NCERT: RNA, being unstable, mutates at a faster rate, so viruses having an RNA genome and a shorter life span mutate and evolve faster (page 87). Beyond the NCERT text: the RNA-copying enzymes of most RNA viruses cannot check and correct their own errors, as DNA polymerases do (Wikipedia, “RNA virus”, facts in our words); rapid replication adds to this, and evolutionary rate is not the same as mutation rate (our reasoning). Give NCERT’s reason in an NCERT-based answer.
Q6. In the Hershey–Chase experiment, what were the blender and the centrifuge used for?
The blender removed the viral coats from the bacteria as the infection proceeded; the centrifuge separated the virus particles from the bacteria (NCERT page 86).
Q7. In the Hershey–Chase experiment, where was the radioactivity found after centrifugation for each batch of phages?
Bacteria infected with the radioactive-DNA (P) phages were radioactive; those infected with the radioactive-protein (S) phages were not, because the proteins did not enter the bacteria (NCERT page 86); their label stayed with the viral coats that the blender removed and the centrifuge separated from the bacteria (reasoned from NCERT page 86). Beyond the NCERT text: the separation was incomplete; about 20 per cent of the P stayed outside the cells (Wikipedia, “Hershey–Chase experiment”, facts in our words), and a 2016 history judges the experiment too contaminated to have shown that DNA is the genetic material of viruses (Cobb, PLOS Biology, 2016, CC BY).
Q8. Why is DNA, not RNA, preferred for storing genetic information, and RNA for transmitting it?
DNA is chemically less reactive and structurally more stable (no reactive 2′–OH, thymine in place of uracil, complementary strands), so it is better for storage; RNA can directly code for the synthesis of proteins, so it can easily express the characters and is better for transmission (NCERT pages 87–88).