Read the given passage and answer the following question.
The process of copying genetic information from one strand of the DNA into RNA is termed as transcription. Here also, the principle of complementarity governs the process of transcription, except the adenosine complements now forms base pair with uracil instead of thymine. However, unlike in the process of replication, which once set in, the total DNA of an organism gets duplicated, in transcription only a segment of DNA and only one of the strands is copied into RNA. The promoter and terminator flank the structural gene in a transcription unit. The promoter is said to be located towards 5' -end (upstream) of the structural gene (the reference is made with respect to the polarity of coding strand). It is a DNA sequence that provides binding site for RNA polymerase, and it is the presence of a promoter in a transcription unit that also defines the template and coding strands. By switching its position with terminator, the definition of coding and template strands could be reversed. The terminator is located towards 3' -end (downstream) of the coding strand and it usually defines the end of the process of transcription.
In bacteria, the transcription of all types of RNA is catalysed by:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A single DNA-dependent RNA polymerase
Transcription is done by an enzyme that reads DNA and makes RNA, so it is called a DNA-dependent RNA polymerase. Bacteria make three major types of RNA – mRNA, tRNA and rRNA – but they use only one RNA polymerase for all of them. Eukaryotes are different: their nucleus has at least three RNA polymerases, each making its own types of RNA.
Option (1) Three different RNA polymerases, one for each type of RNA — Incorrect. This is the pattern of eukaryotes, not bacteria. The eukaryotic nucleus has RNA polymerases I, II and III with a division of labour, but bacteria have just one enzyme for all their RNAs.
Option (2) RNA polymerase II alone — Incorrect. RNA polymerase II is a eukaryotic enzyme that makes the precursor of mRNA (hnRNA). Bacteria do not have separate RNA polymerases I, II and III, and RNA polymerase II does not make tRNA or rRNA even in eukaryotes.
Option (3) A single DNA-dependent RNA polymerase — Correct. One DNA-dependent RNA polymerase transcribes mRNA, tRNA and rRNA in bacteria. It binds to the promoter, starts transcription and copies the gene into RNA.
Option (4) A DNA-dependent DNA polymerase — Incorrect. A DNA-dependent DNA polymerase builds new DNA on a DNA template during replication. It cannot make RNA, so it plays no part in transcription.
NCERT Extract:
Types of RNA and the Process of Transcription – “In bacteria, there are three major types of RNAs: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA). All three RNAs are needed to synthesise a protein in a cell. The mRNA provides the template, tRNA brings aminoacids and reads the genetic code, and rRNAs play structural and catalytic role during translation. There is single DNA-dependent RNA polymerase that catalyses transcription of all types of RNA in bacteria. RNA polymerase binds to promoter and initiates transcription (Initiation). … In eukaryotes, there are two additional complexities – (i) There are at least three RNA polymerases in the nucleus (in addition to the RNA polymerase found in the organelles).”