Learning Objective: To understand the molecular process of transcription, including DNA template selection, promoter and terminator recognition, RNA polymerase function, and the directionality and regulation of RNA synthesis.

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Introduction to Transcription

Transcription is the first major step in gene expression, in which the information encoded in the DNA sequence is transcribed into a complementary RNA sequence. Only one of the two DNA strands is used as a template for transcription—this is known as the template (antisense) strand. The opposite strand is the coding (sense) strand, which has the same sequence as the RNA transcript except that RNA contains uracil (U) instead of thymine (T).

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Directionality of Transcription

• RNA polymerase synthesizes RNA in the 5′ → 3′ direction.
• This occurs because the enzyme moves along the DNA template strand in the 3′ → 5′ direction.
• The synthesized RNA is therefore antiparallel and complementary to the DNA template.

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Key Elements Required for Transcription

1. Template Strand

Only one DNA strand is transcribed per gene.

• This strand provides the nucleotide sequence that RNA polymerase follows.
• The complementary strand mirrors the RNA sequence.

2. Promoters (Start Signals)

Promoters are specific nucleotide sequences upstream of each gene where transcription begins.
Promoters perform two roles:

• Determine where transcription starts
• Determine which DNA strand acts as the template

3. Terminators (Stop Signals)

These sequences signal RNA polymerase to stop transcription and release the RNA transcript.

4. Transcription Units on Chromosomes

Genes are arranged along chromosomes with:

• Gene → Spacer DNA → Gene → Spacer DNA, etc.
• Each gene has its own promoter, template region, and terminator.

The orientation of each gene may differ, meaning each gene can be transcribed in either direction, depending on promoter orientation.

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Overview of the Transcription Process

Step 1: Promoter Recognition

• RNA polymerase binds to promoter sequences on DNA.
• This establishes the transcription start site.

Step 2: DNA Strand Separation

• RNA polymerase locally unwinds a small section of DNA, forming a transcription bubble.

Step 3: RNA Chain Initiation

• RNA synthesis begins at the +1 nucleotide.
• No primer is required (unlike DNA replication).

Step 4: RNA Chain Elongation

• RNA polymerase moves along the template strand (3′ → 5′).
• Adds ribonucleotides to the growing RNA chain (5′ → 3′ direction).

Step 5: Termination

• When RNA polymerase reaches a terminator sequence, it releases:
  • The newly made RNA transcript
  • The DNA template

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Clinical Relevance (High-Yield Points)

• Mutations in promoter regions may reduce transcription and cause disease.
• Some antibiotics (e.g., rifampin) selectively inhibit prokaryotic RNA polymerase.
• Transcriptional dysregulation contributes to many cancers and genetic disorders.

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