From mRNA to DNA—Identifying the Product of Reverse Transcription Which type of DNA is synthesized when processed messenger RNA (mRNA) molecules are used as templates with reverse transcriptase?
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AcDNA
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BrDNA (ribosomal DNA sequences from the genome)
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CmDNA (a non-standard term)
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DtDNA (transfer DNA; non-standard usage here)
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EGenomic DNA containing introns and promoters
Answer
Correct Answer: cDNA
Explanation
Introduction / Context:Converting mRNA into complementary DNA enables cloning and expression of protein-coding sequences without introns. This is essential for expressing eukaryotic genes in prokaryotic hosts and for transcriptome analyses.
Given Data / Assumptions:
- Template is processed mRNA (polyadenylated, introns removed).
- Enzyme is reverse transcriptase.
- Priming can use oligo-dT, random primers, or gene-specific primers.
Concept / Approach:Reverse transcriptase copies the mRNA sequence into single-stranded DNA, which is then converted to double-stranded complementary DNA (cDNA). Because the template is processed mRNA, cDNA lacks introns and often reflects only the transcribed, spliced exons and untranslated regions.
Step-by-Step Solution:
Bind primer (commonly oligo-dT to the poly(A) tail).Synthesize first-strand DNA using reverse transcriptase → mRNA–DNA hybrid.Remove RNA and synthesize second-strand DNA with DNA polymerase → cDNA.Verification / Alternative check:cDNA libraries are screened to study gene expression; expression of eukaryotic ORFs in bacteria typically uses cDNA to avoid introns that bacteria cannot splice.
Why Other Options Are Wrong:
- rDNA refers to genomic rRNA gene repeats, not reverse-transcribed products.
- mDNA/tDNA as listed are non-standard or misleading in this context.
- Genomic DNA contains introns; it is not synthesized from mRNA templates.
Common Pitfalls:Assuming cDNA equals genomic DNA; cDNA represents expressed sequences and lacks introns, making it ideal for coding-region studies.
Final Answer:cDNA