Question

Choose the correct answer from the alternatives given:

The RNA polymerase holoenzyme transcribes

(a) The promoter, structural gene, and the terminator region

(b) The promoter, and the terminator region

(c) The structural gene, and the terminator region

(d) The structural gene only

Answer 1

A Transcription unit that comprises the various nucleotide sequences present in the DNA strand which are helpful in the coding and formation of RNA. The enzyme that assembles DNA from its component is known as the DNA polymerase.

Complete answer:

Transcription is the first step of gene expression in living organisms where the DNA after replication is transcribed into RNA to be further translated into proteins.

- The transcription is initiated when the RNA polymerase binds to the promoter which is the upstream DNA sequence found. They are those sequences that are responsible for the initiation of the process of transcription by sending the signals to the binding site where RNA polymerase attaches. Promoters are present at the initial gene pairs present that are around 100-1000 base pairs long DNA.

- RNA Polymerase produces RNA from DNA (structural genes) that either codes for proteins in the form of messenger RNA or non-coding RNA that function as enzymes or regulate other processes. RNA polymerase holoenzyme helps in the transcription of structural genes only,

So, we can say that according to this statement option ‘D’ is the correct answer.

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The process of transcription occurs in three main steps:

1. Initiation – In this step, the double helix DNA strand is unwounded and becomes single-stranded near the initiation site. The other name of the promoter sequence is the Pribnow box / TATA box where the transcription starts by binding RNA polymerase to it. This can be done with the help of sigma factors.

2. Elongation – After the synthesis of RNA that has more than 10 base pairs the sigma factor stops functioning and detaches itself from the promoter region. The RNA polymerase enzyme while synthesizing RNA will move continuously in the 5’ – 3’ direction.

3. Termination – Termination is the last step of transcription. It has two types of mechanisms-

a) -Rho-independent transcription termination – transcription is terminated due to a specific sequence in terminator DNA. The complementary pairing of DNA in hairpin-like structure containing the invert repeats forms transcript RNA through this terminator DNA resulting in the removal of RNA polymerase from the DNA template.

b) -Rho-dependent transcription termination – here with the help of rho protein the process of transcription is stopped. It then attaches to RNA during transcription. It later detaches itself from RNA polymerase from DNA.

Note: In the case of eukaryotes, the transcription unit is monocistronic while in the case of prokaryotes it is polycistronic. The transcription unit consists of a promoter region, structural gene, and termination region. In eukaryotes, exons (interrupting coding sequences) are found in between the monocistronic structural gene.