Question

Given below are two statements :
one is labelled as
Assertion (A) and the other is labelled as
Reason (R).
Assertion (A) :
A cDNA library contains only expressed genes.
Reason (R) :
A cDNA library is made from the mRNA molecules of a cell population.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Remember:
• Genomic library \(\rightarrow\) Contains entire genome
• cDNA library \(\rightarrow\) Contains only expressed genes \[ \text{mRNA} \xrightarrow{\text{Reverse Transcriptase}} \text{cDNA} \] cDNA libraries are tissue-specific because different tissues express different genes.

Answer 1

The Correct Option is A

Concept: A cDNA (complementary DNA) library is a collection of DNA copies synthesized from messenger RNA (mRNA) molecules present in a cell or tissue. Since mRNA is produced only from genes that are actively expressed: \[ \text{cDNA library contains only expressed genes} \] Unlike genomic libraries, cDNA libraries do not contain:
• Introns
• Promoters
• Intergenic regions
• Non-expressed genes

Step 1: Evaluating Assertion (A). The assertion states: \[ \text{A cDNA library contains only expressed genes} \] This is correct because cDNA is synthesized from mature mRNA transcripts produced by active genes. Therefore: \[ \boxed{(A) \text{ is correct}} \]

Step 2: Evaluating Reason (R). The reason states: \[ \text{A cDNA library is made from mRNA molecules} \] This is also correct. The process involves:
• Isolation of mRNA
• Synthesis of complementary DNA using reverse transcriptase Thus: \[ \boxed{(R) \text{ is correct}} \]

Step 3: Determining whether (R) explains (A). Because: \[ \text{Only expressed genes produce mRNA} \] and cDNA libraries are prepared from mRNA: \[ \text{cDNA libraries contain only expressed genes} \] Hence: \[ \boxed{(R) \text{ correctly explains } (A)} \] Therefore, the correct answer is: \[ \boxed{(A)\ \text{Both (A) and (R) are correct and (R) is the correct explanation of (A)}} \]