Q.11 Eukaryotic transcription is carried out by
(A) DNA–dependent RNA polymerase
(B) DNA–dependent DNA polymerase
(C) RNA–dependent DNA polymerase
(D) RNA–dependent RNA polymerase
Eukaryotic transcription is carried out by DNA-dependent RNA polymerase. This process synthesizes RNA from a DNA template in eukaryotic cells. The correct answer is option (A).
Option Analysis
DNA-dependent RNA polymerase (A): This enzyme reads a DNA template strand to synthesize complementary RNA during transcription initiation, elongation, and termination in eukaryotes. Three types—RNA Pol I, II, and III—handle rRNA, mRNA, and tRNA/small RNAs, respectively.
DNA-dependent DNA polymerase (B): This replicates DNA by using a DNA template to produce new DNA strands, functioning in DNA synthesis during cell division, not RNA production.
RNA-dependent DNA polymerase (C): Known as reverse transcriptase, this synthesizes DNA from an RNA template, as seen in retroviruses like HIV, opposite to standard transcription direction.
RNA-dependent RNA polymerase (D): This synthesizes RNA from an RNA template, common in RNA viruses for replication, but absent in eukaryotic cellular transcription.
Eukaryotic transcription DNA-dependent RNA polymerase drives the synthesis of RNA from DNA templates in nucleus. This fundamental process, essential for gene expression, uses specialized enzymes to produce mRNA, rRNA, and tRNA.
Transcription Mechanism
RNA polymerase II, the primary enzyme for mRNA, assembles with transcription factors at promoters to unwind DNA and elongate RNA chains in a 5′ to 3′ direction. Unlike prokaryotes, eukaryotes require ATP hydrolysis for promoter melting due to chromatin barriers. Initiation involves abortive cycles until stable transcripts form beyond 10 nucleotides.
Enzyme Specialization
Eukaryotes employ three nuclear DNA-dependent RNA polymerases: Pol I for rRNA, Pol II for protein-coding mRNA and snRNAs, and Pol III for tRNA and 5S rRNA. This division ensures precise regulation of diverse RNA types.
Exam Relevance
For competitive exams like IIT JAM Biotechnology, recognizing DNA-dependent RNA polymerase as the key player distinguishes it from viral enzymes like reverse transcriptase or RdRp. Practice such MCQs strengthens molecular biology concepts.
