Question

What is complementary DNA (cDNA)?

Answer 1

This strand of DNA is synthesised from the mRNA by the method of Reverse transcription in the laboratory. cDNA is different from genomic DNA because cDNA is produced after the splicing of intron and as a result, a mature mRNA. cDNA only contains some genes which only codes for proteins.

Complete step by step answer: The central dogma, proposed by Francis Crick stated that the gene information flows from DNA to RNA to protein. But in some virus-like retroviruses the gene information flows from RNA to DNA. The process by which RNA is converted into DNA is called Reverse Transcription.
By using this method scientists created a DNA strand from mRNA strands known as complementary DNA or cDNA. The cDNA is synthesized from mRNA by using the enzyme reverse transcriptase.
Let us understand briefly how cDNA is synthesis:
At the $3'$ end of the mRNA, the poly-adenylated tail is attached. A Complement to the poly-adenylate tail the primer is added along with the reverse transcriptase enzyme. This starts forming the cDNA strand. The resulting strands are mRNA-DNA hybrid strands. After that, the mRNA is degraded by some alkaline solution. The free $3'$end of the cDNA forms a hairpin loop like structure; by using primer and DNA polymerase III the second strand is formed. Finally, the $S1$ nuclease enzyme cleaves the hairpin loop. Finally, the double-stranded cDNA is formed from the original copy of mRNA.

Note: There is much application of cDNA because the cDNA does not contain the intron. The cDNA is used as a gene probe, gene cloning and also to create the cDNA library. It is used to clone the eukaryotic gene in prokaryotes.