Transcription, RNA Processing, and Translation: Core Concepts and Mechanisms

Transcription: The Synthesis of RNA from DNA

Overview of Transcription

Transcription is the process by which RNA polymerases synthesize an RNA copy of the genetic instructions stored in DNA. This process is fundamental to gene expression and occurs in both prokaryotic and eukaryotic cells.

• RNA polymerases use ribonucleoside triphosphates (NTPs) as substrates to build RNA.

• Only one strand of DNA serves as the template strand; the other is the non-template (coding) strand, which matches the mRNA sequence (except U replaces T).

• Transcription proceeds in the 5' to 3' direction.

Comparison: DNA vs. RNA Polymerases

• Similarities: Both perform template-directed synthesis.

• Differences: RNA polymerases do not require a primer to begin transcription.

• Bacteria: One RNA polymerase.

• Eukaryotes: At least three distinct RNA polymerases (I, II, III).

Initiation of Transcription in Bacteria

Role of Sigma Factor and Holoenzyme Formation

Transcription initiation is the first phase, requiring the assembly of a holoenzyme composed of RNA polymerase and a sigma protein.

• Sigma protein binds to RNA polymerase, forming the holoenzyme.

• Sigma recognizes promoter sequences, determining where transcription begins.

• Promoter orientation determines the template strand and direction of RNA synthesis.

Bacterial Promoters

Bacterial promoters are specific DNA sequences that signal the start of transcription.

• Promoters are typically 40–50 base pairs long.

• -10 box: TATAAT sequence, ~10 bases upstream of the transcription start site.

• -35 box: TTGACA sequence, ~35 bases upstream.

• Downstream: Direction RNA polymerase moves; Upstream: Opposite direction.

Mechanism of Transcription in Bacteria

Events Inside the Holoenzyme

• Transcription begins when sigma binds to the -35 and -10 boxes.

• RNA polymerase opens the DNA double helix, creating a transcription bubble.

• The template strand is threaded through the active site; NTPs diffuse in and pair with complementary DNA bases.

Elongation and Termination

• Elongation: RNA polymerase adds nucleotides to the 3' end of the growing RNA strand.

• Termination: Occurs when a transcription-termination signal is transcribed, causing the RNA to form a hairpin structure that leads to separation of RNA polymerase from the RNA transcript.

Transcription in Eukaryotes

Key Differences from Bacterial Transcription

• Three distinct RNA polymerases (I, II, III).

• Promoters are larger and more diverse, often containing a TATA box.

• General transcription factors (not sigma proteins) recognize promoters.

• Termination involves transcription of a poly(A) signal rather than a hairpin.

• Transcription occurs in the nucleus; translation occurs in the cytoplasm.

RNA Processing in Eukaryotes

Primary Transcript and RNA Processing

In eukaryotes, the initial product of transcription is an immature primary transcript or pre-mRNA, which must undergo processing before translation.

• Primary transcripts contain both coding and noncoding regions.

• RNA processing includes splicing, capping, and addition of a poly(A) tail.

Discovery of Split Genes: Introns and Exons

• Protein-coding genes contain noncoding DNA (introns).

• Introns are removed during RNA processing; exons remain in the mature mRNA.

• Experimental evidence: DNA-RNA hybrids form loops where introns are present in DNA but absent in mRNA.

RNA Splicing

Mechanism and Significance

• Splicing removes introns from primary transcripts.

• Catalyzed by small nuclear ribonucleoproteins (snRNPs), which form a spliceosome.

• Splicing enables production of different mRNAs and proteins from a single gene (alternative splicing).

Steps of RNA Splicing

1. snRNPs bind to exon-intron boundaries and to an A nucleotide near the end of the intron.

2. Other snRNPs join to form the spliceosome.

3. The intron forms a lariat (loop) structure with A as the branch point.

4. The lariat is cut out, exons are linked, and the intron is degraded.

Adding Caps and Tails to Transcripts

5' Cap and 3' Poly(A) Tail

• 5' cap: Modified guanine nucleotide added to the 5' end; aids ribosome binding and protects from degradation.

• Poly(A) tail: 100–250 adenine nucleotides added to the 3' end; required for translation and stability.

• After splicing and addition of cap and tail, the product is a mature mRNA.

• Mature mRNAs contain untranslated regions (UTRs) at both ends.

Summary Table: Key Differences in Transcription

Feature	Bacteria	Eukaryotes
RNA Polymerases	One	Three (I, II, III)
Promoter Elements	-10 and -35 boxes	TATA box, diverse elements
Initiation Factors	Sigma protein	General transcription factors
Termination Signal	Hairpin structure	Poly(A) signal
RNA Processing	None	Splicing, capping, poly(A) tail
Location	Cytoplasm	Nucleus (transcription), cytoplasm (translation)

Key Terms and Definitions

• RNA polymerase: Enzyme that synthesizes RNA from a DNA template.

• Promoter: DNA sequence where transcription begins.

• Sigma factor: Protein required for initiation in bacteria.

• Spliceosome: Complex of snRNPs that catalyzes RNA splicing.

• Exon: Coding region retained in mature mRNA.

• Intron: Noncoding region removed during splicing.

• 5' cap: Modified nucleotide added to the 5' end of eukaryotic mRNA.

• Poly(A) tail: String of adenine nucleotides added to the 3' end of eukaryotic mRNA.

Relevant Equations

• Template-directed synthesis: $\text{(DNA template)} + \text{NTPs} \xrightarrow{\text{RNA polymerase}} \text{RNA}$

• Direction of synthesis: $5' \rightarrow 3'$

Example: Alternative Splicing

Alternative splicing allows a single gene to produce multiple protein variants by including or excluding different exons during RNA processing. This increases protein diversity in eukaryotic organisms.

Additional info: The notes have been expanded with academic context and definitions to ensure completeness and clarity for exam preparation.