Biology Transcription Overview

Transcription Overview

• Transcription is the process of creating mRNA from a DNA template.
• DNA acts as a template for transcription.
• RNA polymerase is required.
• Unlike DNA replication, transcription is highly selective and tightly regulated.
• Transcription is a cellular process that synthesizes RNA.
• This includes coding RNA and non-coding RNA.

Transcription Process

• The process occurs in various stages:
  • Initiation: Formation of an initiation complex. This differs between prokaryotes and eukaryotes.
  • Elongation: The RNA polymerase moves along the DNA template, synthesizing RNA in a 5' to 3' direction. The base-pairing rule from DNA to RNA applies here but with uracil (U) replacing thymine (T).
  • Termination: The process of detaching the newly synthesized RNA strand from the DNA template. Different mechanisms of termination exist in prokaryotes and eukaryotes. In Prokaryotes termination can be rho-dependent or rho-independent. In eukaryotes, it less well understood and more complex than in prokaryotes
• In prokaryotes, one RNA polymerase synthesizes all types of RNA
• In eukaryotes, there are three different RNA polymerases, each responsible for different types of RNA synthesis.

Coding vs. Non-coding RNA

• Coding RNA (mRNA): Encodes proteins that act as various components like enzymes and structures in cells. It provides instructions for making proteins.
• Non-coding RNA (e.g., rRNA, tRNA, siRNA, miRNA, snRNA): Regulates cellular activities but doesn't directly code for proteins. It determines when and where genes turn on and off.

Gene Structure

• A gene is defined as the entire DNA sequence needed for synthesizing a functional polypeptide.
• DNA sequences include coding regions (exons) and non-coding regions (introns and regulatory elements).

Regulatory Elements

• Cis-acting elements: DNA sequences that regulate the expression of genes on the same molecule (e.g., core promoter, proximal promoter elements, enhancer, silencer).
• Trans-acting elements: Non-DNA sequences that regulate gene expression by binding to cis-acting DNA sequences (e.g., transcription factors: general and gene-specific). Includes activators and repressors.

RNA Polymerase

• RNA polymerase is an enzyme with multiple subunits, crucial for transcription.
• RNA polymerase recognizes and binds to the promoter region.
• Synthesizes mRNA using a DNA template.
• Prokaryotes and eukaryotes have different RNA polymerases.

Transcription Inhibitors

• Rifamycin: Binds to bacterial RNA polymerase, treating gram-positive and gram-negative bacterial infections. Not effective against eukaryotic RNA polymerases.
• Actinomycin D: Binds to DNA, inhibiting its use as a template for RNA synthesis. This has antibacterial and antitumor activity.
• a-amanitin: Specifically inhibits eukaryotic RNA polymerase II, which is toxic to eukaryotic cells, but not to prokaryotes.

Post-transcriptional Modifications (mRNA)

• Prokaryotic mRNA isn't modified after transcription, while eukaryotic mRNA undergoes processing.
• Three primary processing steps include 5' capping, 3' polyadenylation, and splicing.

Post-transcriptional Modifications (rRNA and tRNA)

• rRNA and tRNA of eukaryotes are modified after transcription, but bacterial rRNA and tRNA are not similarly modified.
• rRNA primary transcript is called preribosomal RNA
• tRNA and rRNA in eukaryotes undergo post-transcriptional modifications.

Transcription Initiation Complex

• Formation of the initiation complex differs between prokaryotes and eukaryotes. Prokaryotic initiation involves specific RNA polymerase binding. Eukaryotic initiation, on the other hand, involves additional transcription factors that help recruit the RNA polymerase.

Transcription Process Overview (Eukaryotes)

• Occurs in the nucleus
• Transcription factors form an initiation complex
• Pre-mRNA undergoes splicing
• Three RNA polymerases (I, II, and III)
• Monocistronic mRNA (one gene codes for one mRNA)