Which bonds does helicase break in $DNA$?

$DNA$ polymerase enzyme is responsible for the synthesis of

What does the experiment of Matthew Meselson and Franklin Stahl suggest?

Give scientific reasons: $DNA$ replication is a semi-conservative process.

Given below are two statements,one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ $\text{DNA}$ replication occurs within a small opening of the $\text{DNA}$ helix,referred to as the replication fork.
Reason $(R):$ During transcription,the discontinuously synthesized fragments of $\text{DNA}$ are joined by the enzyme $\text{DNA}$ ligase.
In the light of the above statements,choose the most appropriate answer from the options given below:

Mutagens that are effective in inducing spontaneous $DNA$ mutations are .....