PCR has 3 key steps: Denaturation, annealing, and amplification. In denaturation, DNA is heated at 75 degrees, followed by addition of RNA primers which bind or anneal to 5' end and Taq polymerase is added that amplifies strand resulting to amplification of DNA strand.

C. In vitro replication of DNA segment

Alternative selectable markers have been developed which differentiate recombinants from non-recombinants on basis of ir ability to produce colour in presence of a chromogenic substrate. In this, a recombinant DNA is inserted within coding sequence of an enzyme, ( -galactosidase. This results into inactivation of enzyme, which is referred i to as insertional inactivation. The presence of a chromogenic substrate gives blue coloured colonies if plasmid in bacteria does not have an insert. Presence of insert results into insertional inactivation of -galactosidase and colonies do not produce any colour, se are identified as recombinant colonies.

Interferons are  glycoproteins that are secreted by a host cell to serve as signalling molecules. They block multiplication of viruses in cells by inducing  immune response of neighbouring cell. Engineered interferons are used in medicines to treat cancer and or diseases. Charles Weissmann clone human interferon $?$ genes and produce interferon in bacteria. 

• In DNA and RNA, phosphodiester bond connects a 3'-carbon to a 5'-carbon which is hydrolyzed by endonuclease restriction enzyme. 
• Endonuclease makes cuts within DNA strands while exonuclease removes nucleotides from end part of DNA. 
• DNA polymerase enzymes synsize DNA molecules from deoxyribonucleotides.

• Sticky ends are formed when restriction enzymes cut DNA molecule at such a position that it leaves an overhanging stretch of nitrogenous base which can pair with complementary base pairs. 
• EcoRI cut through DNA strands at nucleotides that overhangs at end.
• This overhanging nucleotide strand is called a sticky end because it can easily bond with complementary DNA fragments. So, correct answer is option B.