Question

Imagine you want to study human crystallins, proteins present in the lens of the eye. To obtain a sufficient amount of the protein, you decide to clone the crystallin gene. Would you construct a genomic library or a cDNA library? What material would you use as a source of DNA or RNA?

Answer 1

Answer: Construct a cDNA library, using mRNA from human lens cells, to have many copies of crystallin mRNAs

Explanation:

Proteins are molecules produced by a process called translation, which uses a RNA molecule. RNA is a Ribonucleic acid synthesized from DNA (Deoxyribonucleic acid), which contains the genetic instructions used in the development and functioning of all living organism, and it is also responsible for hereditary transmission. Then, in the lens of the eye, the necessary DNA gene is transcribed to form messenger RNA, that will give rise to the crystallin protein.  So, the "central dogma of biology" establishes that genetic information flows in the following direction: DNA → RNA→ proteins.

To clone such a gene, it is necessary to use a cDNA library. Complementary DNA (cDNA) is a double-stranded DNA molecule produced by an enzyme called reverse transcriptase, in which one of its strands constitutes a sequence that is fully complementary to the messenger RNA from which it has been synthesized. It is often used for the cloning of eukaryotic cell-specific genes because, given the nature of their synthesis, it lacks introns.

Therefore, as a source of this cDNA for the library, scientists use the transcribed mRNA that already went through the splicing process in which those introns that do not encode protein are removed. And thus the cDNA synthesized from this mRNA only possesses the gene of interest, without useless sequences.

In addition, RNA is chemically unstable, so that in an aqueous solution it is easily hydrolyzed. On the other hand, this does not happen with DNA, which is a stable molecule and that is why cDNA and not mRNA is used to build the library.