Learning Objective: By the end of this topic, the learner will be able to explain the process of transcription, differentiate prokaryotic and eukaryotic RNA polymerases, and apply key transcription concepts such as template strand, coding strand, promoter, and termination signals.

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Transcription is the first step in gene expression. It involves the synthesis of an RNA molecule from a DNA template. Only one strand of DNA is used as a template at a time.

• Template strand → copied by RNA polymerase
• RNA synthesized → 5′ → 3′ direction
• RNA polymerase moves → 3′ → 5′ along the template

RNA produced is complementary and antiparallel to the DNA template. The coding (non-template) strand has the same sequence as RNA, except that T is replaced by U.

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Gene organization & directionality

Promoter

A specific DNA sequence where RNA polymerase binds and transcription begins. Determines:

• Start point (+1)
• Direction of transcription
• Which strand is the template

Numbering of DNA bases

• +1 → first transcribed base
• Upstream (5′) → −1, −2, −3
• Downstream (3′) → +2, +3, +4

Termination

Transcription stops when the polymerase reaches a termination sequence.

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Activity

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Types of RNA

rRNA – Ribosomal RNA

• Most abundant RNA
• Combines with proteins to form ribosomes
• Site of protein synthesis

tRNA – Transfer RNA

• Carries amino acids to ribosomes
• Matches anticodon to mRNA codons

mRNA – Messenger RNA

• Encodes protein sequence
• Only RNA that is translated

hnRNA (pre-mRNA)

• Found only in eukaryotic nuclei
• Precursor to mature mRNA

snRNA

• Used for splicing (removal of introns)

Ribozymes

• RNA with enzymatic activity
• Found in both prokaryotes and eukaryotes

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RNA Polymerases

Prokaryotic RNA Polymerase

• Single polymerase: α₂ββ′ + σ factor
• Sigma (σ): required to initiate transcription at the promoter
• Rho (ρ): sometimes required for termination
• Inhibitors:
  • Rifampin → inhibits RNA polymerase
  • Actinomycin D → binds DNA, blocks transcription

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Eukaryotic RNA Polymerases

Key points

• Require transcription factors (e.g., TFIID for RNAP II)
• No sigma or rho factors
• Inhibitors:
  • α-amanitin → inhibits RNA Pol II
  • Actinomycin D → inhibits all transcription

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Transcription: Key terminology

Template Strand

• Used by RNA polymerase
• Read 3′ → 5′

Coding Strand

• Not used
• Sequence matches RNA (T → U)

Direction of synthesis

• RNA synthesized 5′ → 3′ only
• RNA polymerase does not proofread

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Flow of genetic information

DNA (template) → Transcription → mRNA → Translation → Protein

• mRNA read 5′ → 3′ during translation
• Protein made N-terminus → C-terminus

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Activity