Process of Transcription in Eukaryotes

1. Initiation

• RNA polymerase binds to the promoter region of the template strand.​
• The DNA double helix unwinds locally at the transcription start site.​
• In prokaryotes, the sigma (σ) factor assists in initiation; in eukaryotes, transcription factors (TFs) help RNA polymerase recognise and bind the promoter.​
• Synthesis of RNA begins in the 5′ → 3′ direction.​

2. Elongation

• RNA polymerase moves along the template strand in 3′ → 5′ direction.​
• Nucleoside triphosphates (NTPs) are used as substrates; complementary ribonucleotides are added.​
• The RNA chain grows in the 5′ → 3′ direction.​
• Only a short stretch of the RNA remains associated with the polymerase–DNA complex at any time.​

3. Termination

• When RNA polymerase reaches the terminator sequence, the nascent RNA is released.​
• The RNA polymerase dissociates from the DNA template.​
• In prokaryotes, the rho (ρ) factor assists termination; in eukaryotes, specific termination signals exist.​
• The released primary transcript in eukaryotes is hnRNA (not ready for translation yet).​

In eukaryotes, the primary transcript (hnRNA) is non-functional and must be processed before it can serve as mRNA for translation.​

Splicing:

• Introns (non-coding intervening sequences) are removed.​
• Exons (coding sequences) are joined together in a defined order.​
• Performed by a molecular machine called the spliceosome (composed of snRNAs and proteins).
• Result: A continuous coding sequence without interruptions.

5′ Capping (Capping):

An unusual nucleotide - methyl guanosine triphosphate (7-methylguanosine, m⁷G) - is added to the 5′ end of hnRNA.​

Functions of 5′ cap:

• Protects mRNA from degradation by 5′ exonucleases
• Aids in ribosome recognition and attachment during translation initiation
• Facilitates export of mRNA from the nucleus

3′ Poly-A Tailing (Tailing)

• 200–300 adenylate (AMP) residues are added to the 3′ end of hnRNA.​
• This occurs in a template-independent manner (poly-A polymerase enzyme).​

Functions of poly-A tail:

• Protects mRNA from 3′ degradation
• Enhances mRNA stability and half-life
• Assists in nuclear export of mRNA

After all three processing steps, the fully processed hnRNA is now called mature mRNA and is transported out of the nucleus to the cytoplasm for translation.

Transcription and Processing of hnRNA to mRNA in Eukaryotes

• Transcription copies DNA → RNA using RNA polymerase, following complementarity rules (A pairs with U).​
• In eukaryotes, 3 RNA polymerases: Pol I (rRNA), Pol II (hnRNA → mRNA), and Pol III (tRNA, 5S rRNA, snRNA).​
• The template strand has 3′→5′ polarity; RNA is synthesised 5′→3′.​
• Transcription unit = Promoter + Structural Gene + Terminator.​
• In eukaryotes, the primary transcript = hnRNA, which contains both exons and introns.​
• 3 processing steps: Splicing (remove introns) → 5′ Capping (m⁷G) → 3′ Poly-A tailing (200–300 A residues).​
• Mature mRNA is transported to the cytoplasm for translation.​