Transcription Overview in Biology

Transcription Overview

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• Textbooks are essential for complete understanding and exam preparation
• Transcription is the synthesis of mRNA from a DNA gene
• The enzyme responsible for this process is RNA polymerase
• mRNA is a messenger RNA, carrying the genetic code from DNA to the ribosomes for protein synthesis.

Learning Objectives

• Define translation
• Describe transcription initiation, elongation, and termination
• Compare and contrast transcription in eukaryotes and prokaryotes
• List transcription inhibitors
• Describe post-transcriptional modification process

Transcription

• RNA polymerase binds to the promoter region of a gene, a specific sequence on the DNA template that signals the start of transcription
• Only one DNA strand is transcribed (template strand)
• The transcribed strand creates a complementary strand of mRNA
• RNA polymerase synthesizes RNA by attaching nucleotides in the 5' to 3' direction.

Transcription Steps (Prokaryotes)

• Chain initiation
• Chain elongation
• Chain termination

Chain Initiation

• RNA synthesis is initiated at specific sites on the DNA called promoter regions (often containing Pribnow box and -35 sequence)
• The DNA strand that gives rise to mRNA is the template/anti-sense/non-coding strand
• The other (non-template) strand has a sequence identical to the mRNA (coding strand).

Chain Elongation

• DNA unwinds at the origin, creating a transcription bubble
• RNA polymerase synthesizes RNA within the bubble
• The RNA-DNA hybrid is formed during elongation.

Chain Termination

• Occurs at a termination site
• Two types of termination:
  • Rho dependent (requires rho protein)
  • Rho independent (intrinsic)
• Rho independent terminators have a sequence of 4-10 AT base pairs after a GC rich region

Transcription in Eukaryotes

• Eukaryotes have multiple RNA polymerases (I, II, III) each responsible for different types of RNA synthesis:
  • RNA polymerase I: produces rRNA
  • RNA polymerase II: produces mRNA
  • RNA polymerase III: produces tRNA and some rRNA
• Eukaryotic promoters contain TATA box and CCAAT box, and enhancer sequences.
• Transcription factors (proteins) are required to recognize and bind to the promoter sequences
• Eukaryotic genes lack specific termination sequences

Inhibitors of Transcription

• Rifampicin is an antibiotic used against tuberculosis that binds to prokaryotic RNA polymerase and inhibits chain elongation.
• α-amanitin is a potent toxin from mushrooms that binds tightly to eukaryotic RNA polymerase II, inhibiting its function

Post-Transcriptional Modifications (RNA Processing)

• In prokaryotes, mRNA is directly ready to synthesize protein
• In eukaryotes, nascent mRNA is processed before becoming mature mRNA. This includes:
  • mRNA Capping: adding a methylated guanine nucleotide to the 5' end (protection from degradation)
  • Polyadenylation: adding a poly-A tail to the 3' end (protection from degradation, ribosomal RNA recognition)
  • Splicing: removal of introns (non-coding regions) and rejoining exons (coding regions)

Eukaryotes vs. Prokaryotes (Transcription Differences)

• Overall the process is similar, but differences include:
  • Prokaryotic mRNA is often polycistronic (multiple genes on one mRNA) and translated immediately, while eukaryotic mRNA is monocistronic, processed, and translated in separate compartments
  • Eukaryotic mRNA has introns that are removed post-transcription, unlike prokaryotic mRNA, which does not
  • Eukaryotic transcription occurs in the nucleus, while prokaryotic transcription occurs in the cytoplasm:
  • Eukaryotes have multiple RNA polymerases each for different transcripts, while prokaryotes have only one