Molecular Biology and Genomics — MCQs

Question 1: Transcription is the synthesis of:

• A. A single-stranded complementary copy of DNA (Correct Answer)
• B. A double-stranded complementary copy of DNA
• C. A complementary copy of RNA
• D. A complementary copy of rRNA

Explanation: **Explanation:** Transcription is the fundamental process of gene expression where the genetic information stored in **DNA** is copied into **RNA**. **Why Option A is Correct:** During transcription, the enzyme **RNA polymerase** reads the template strand of DNA (3' to 5') and synthesizes a **single-stranded** RNA molecule in the 5' to 3' direction. This RNA transcript is **complementary** to the DNA template strand (with Uracil replacing Thymine) and identical in sequence to the coding (non-template) strand. **Why Other Options are Incorrect:** * **Option B:** DNA is double-stranded, but the resulting RNA transcript is always single-stranded. Double-stranded DNA synthesis occurs during **Replication**, not transcription. * **Option C:** Synthesis of a complementary copy of RNA is characteristic of **RNA-dependent RNA replication** (seen in certain RNA viruses) or **Reverse Transcription** (where DNA is made from RNA). * **Option D:** While rRNA is indeed produced via transcription, it is only one specific type of RNA. Transcription encompasses the synthesis of **all** RNA types, including mRNA and tRNA; therefore, "a complementary copy of DNA" is the more accurate general definition. **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** Transcription always proceeds in the **5' → 3' direction**. * **Enzymes (Eukaryotes):** * **RNA Pol I:** Nucleolus; synthesizes 28S, 18S, and 5.8S **rRNA**. * **RNA Pol II:** Nucleoplasm; synthesizes **mRNA** and snRNA (Inhibited by **α-amanitin** from *Amanita phalloides* mushrooms). * **RNA Pol III:** Nucleoplasm; synthesizes **tRNA** and 5S rRNA. * **Prokaryotes:** A single RNA polymerase (multimeric enzyme) synthesizes all types of RNA. **Rifampicin** inhibits the β-subunit of this bacterial RNA polymerase.