Transcription

Transcription is the first stage of gene expression, in which the DNA sequence of a gene is copied into a complementary messenger RNA (mRNA) molecule. In eukaryotes, transcription occurs in the nucleus and is followed by mRNA processing before the mature mRNA is exported to the cytoplasm for translation. This lesson covers the mechanism of transcription, the roles of key enzymes, and the post-transcriptional modifications that produce functional mRNA.

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Overview of Transcription

Transcription produces a single-stranded mRNA copy of a gene. The key features are:

• Only one strand of the DNA double helix is used as a template — the template strand (also called the antisense strand).
• The other strand is called the coding strand (or sense strand) because its sequence matches the mRNA (except with thymine instead of uracil).
• The enzyme RNA polymerase catalyses the synthesis of mRNA.
• Free RNA nucleotides (containing uracil instead of thymine) are used as substrates.
• mRNA is synthesised in the 5' to 3' direction, while RNA polymerase reads the template strand in the 3' to 5' direction.

Key Definition: Transcription is the process by which the base sequence of a gene on one strand of DNA is used as a template to synthesise a complementary strand of messenger RNA (mRNA).

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The Mechanism of Transcription

Initiation

1. Transcription factors bind to the promoter region — a specific DNA sequence upstream (before the 5' end) of the gene.
2. Transcription factors recruit RNA polymerase to the promoter. RNA polymerase cannot bind to the promoter on its own in eukaryotes; it requires the transcription factor complex.
3. RNA polymerase binds to the promoter and causes the DNA double helix to unwind and separate (the hydrogen bonds between the two strands break), creating a transcription bubble.

Elongation

4. RNA polymerase moves along the template strand in the 3' to 5' direction.
5. Free RNA nucleotides are aligned with their complementary bases on the template strand:
   • Template A → mRNA U (uracil)
   • Template T → mRNA A
   • Template C → mRNA G
   • Template G → mRNA C
6. RNA polymerase catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides, extending the mRNA chain in the 5' to 3' direction.
7. As RNA polymerase moves forward, the DNA double helix re-forms behind it.

Termination

8. RNA polymerase reaches a terminator sequence on the DNA.
9. RNA polymerase detaches from the DNA template.
10. The newly synthesised mRNA (called pre-mRNA in eukaryotes) is released.

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Differences Between Transcription in Prokaryotes and Eukaryotes

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm (no nucleus)	Nucleus
RNA polymerase	Single type	Three types (RNA Pol I, II, III); Pol II transcribes mRNA
Transcription factors	Not required (RNA polymerase binds directly)	Required for RNA polymerase to bind to promoter
mRNA processing	No processing; translation begins immediately	Extensive processing (5' cap, poly-A tail, splicing)
Coupling with translation	Yes — translation begins while transcription is still occurring	No — mRNA must be processed and exported to cytoplasm first
Introns	Rare	Common — must be spliced out

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Post-Transcriptional Modifications (mRNA Processing)

In eukaryotes, the primary transcript (pre-mRNA) undergoes three key modifications before it becomes mature mRNA:

1. Addition of a 5' Cap

• A modified guanine nucleotide (7-methylguanosine, m⁷G) is added to the 5' end of the pre-mRNA.
• Functions:
  • Protects the mRNA from degradation by exonucleases.
  • Helps the ribosome recognise and bind to the mRNA during translation initiation.
  • Assists in the export of mRNA from the nucleus through nuclear pores.

2. Addition of a Poly-A Tail