Question

Explain the process of transcription in prokaryotes. How is it different from transcription in eukaryotes?

Answer 1

Transcription is the process of copying DNA messages into mRNA strands. Transcription involves three steps:

1. Initiation: The enzyme RNA polymerase initiates transcription by binding to the DNA promoter region. This results in the local unwinding of the DNA double helix. The RNA polymerase's initiation factor (σ) initiates RNA synthesis.
2. Elongation: The RNA chain is synthesised in the 5'-3' direction.
   • RNA polymerase unzips the DNA double helix and forms an open loop.
   • One of the strands, called the sense strand, acts as a template for mRNA synthesis.
   • RNA polymerase converts ribonucleoside triphosphates (ATP, GTP, UTP, and CTP) into mRNA using the complementary rule.
   • This polynucleotide chain elongation and helix opening process continues until the enzyme reaches the terminator gene.
3. Termination:
   • RNA polymerase identifies the terminator gene through a termination factor known as the rho (σ) factor.
   • The freshly synthesised mRNA transcript and enzyme are released once RNA polymerase reaches the termination region.
   • After leaving the nucleus, the mRNA then moves into the cytoplasm of the cell instead.

In eukaryotes, there are two additional complexities:

1. There are three RNA polymerases: The first RNA polymerase, which is responsible for the transcription of rRNAs (285, 185, and 5.85), the second RNA polymerase, which is responsible for the transcription of the heterogeneous nuclear RNA (hnRNA), and the third RNA polymerase, which is responsible for the transcription of tRNA, 55 rRNA, and snRNAs (small nuclear RNAs).
2. Post-transcriptional processing (occurs inside the nucleus): Before being transported out of the nucleus, eukaryotic pre-mRNA is subjected to post-transcriptional processing, which includes splicing, polyadenylation, and capping, among other processes. The mRNA molecule is stabilised as a result of these alterations, which also promote its export to the cytoplasm and regulate gene expression.