Which enzyme is responsible for the synthesis of small $RNA$ fragments during $DNA$ replication?

During $DNA$ replication,one strand is formed continuously and the second strand is formed discontinuously because:

Which of the following statements is incorrect $:-$

Replication was allowed to take place in the presence of radioactive deoxynucleotide precursors in $E. coli$ that was a mutant for $DNA$ ligase. Newly synthesised radioactive $DNA$ was purified and strands were separated by denaturation. These were centrifuged using density gradient centrifugation. Which of the following would be a correct result?

Synthesis of leading and lagging strand require

$A$ $DNA$ molecule containing radioactive thymidine is allowed to replicate in a non-radioactive environment. What will be the exact number of $DNA$ molecules containing radioactive thymidine after three replication cycles?