Cell and Molecular Biology: Chapter 11 - The Central Dogma

The Central Dogma: Overview

• The central dogma of molecular biology explains the flow of genetic information from DNA to RNA to protein.
  • Three main processes:
  • Transcription: The process of synthesizing RNA from a DNA template.
  • Translation: The synthesis of proteins using the information encoded in mRNA.
  • mRNA: Acts as an intermediary between genes and polypeptides.

Types of RNA

• Messenger RNA (mRNA): Intermediate between DNA and protein synthesis.
• Ribosomal RNA (rRNA): Integral part of ribosome structure.
• Transfer RNA (tRNA): Transfers amino acids to ribosome during protein synthesis.
• Small nuclear RNA (snRNA): Involved in pre-mRNA processing.
• Small nucleolar RNA (snoRNA): Guides chemical modifications of rRNA.
• Small interfering RNA (siRNA): Involves RNA interference mechanisms.
• Piwi-interacting RNA (piRNA): Involved in transposon silencing in germ cells.
• MicroRNA (miRNA): Regulates gene expression.

Transcription Process

RNA Polymerases

• Enzymes: Known as DNA-dependent RNA polymerases.
• Functionality:
  • Synthesizes RNA complementary to the DNA template.
  • Moves in a 3' to 5' direction.
  • Assembles RNA at a rate of 20-50 nucleotides/second.
  • Has proofreading capabilities.

Prokaryotic Transcription

• Bacteria: Contain one RNA polymerase with five subunits.
  • Initiation requires a sigma (σ) factor that binds to promoters.
  • RNA polymerase undergoes conformational changes during transcription.

Eukaryotic Transcription

• Three Types of RNA Polymerases:
  • RNA Polymerase I: Transcribes rRNA.
  • RNA Polymerase II: Transcribes mRNA and most snRNA.
  • RNA Polymerase III: Transcribes tRNA and 5S rRNA.
• RNA Processing: Includes trimming of primary transcripts.

Eukaryotic RNA Synthesis and Processing

• Ribosomal RNA (rRNA):
  • Constitutes over 80% of cellular RNA; synthesized in nucleolus.
  • rRNA genes are clustered in chromosomal regions.
• Transfer RNA (tRNA):
  • Clustered across the genome.
  • Processed from primary transcripts by trimming.
• Messenger RNA (mRNA):
  • Characteristics include continuous coding sequence, 5' cap, and 3' poly(A) tail.
  • Spliced to remove introns.

Split Genes and mRNA Processing

• Exons: Coding sequences that contribute to mature mRNA.
• Introns: Non-coding sequences that must be spliced out.
• Splicing Mechanism: Involves multiple steps; typically favors an ordered removal of introns.

Genetic Code and Codons

• Codon Assignment: Cosmetic analysis of codons derived from artificial mRNAs.
  • The code is universal; first two bases are invariant.
• tRNA Structure:
  • Approximately uniform, with a triplet sequence CCA at the 3' end.
  • Wobble hypothesis explains flexibility in base pairing.

Translation Process

Initiation

• Eukaryotic Initiation Steps:
  • Formation of the preinitiation complex (PIC).
  • Assembly of initiation complex requiring various initiation factors.
  • Interaction between the complex and mRNA.

Role of Ribosome

• Functionality: Ribosome cycles through conformational changes during translation.
• It selects tRNAs based on information in the mRNA for accuracy.

Elongation and Termination

1. Elongation: Sequential addition of amino acids to the growing peptide chain.
2. Termination: Recognizing stop codons and releasing the synthesized polypeptide.

• Nonsense Mutations: Can lead to inherited disorders due to premature stop codons.

Small RNA and RNA Silencing

• RNA Interference (RNAi): Involves small regulatory RNAs that can silence gene expression via mechanisms such as dicer and argonaute complexes.
• Clinical Implications: RNAi can suppress the expression of disease-causing proteins, such as in Huntington's disease.

CRISPR Technology

• CRISPR-Cas9: A bacterial immune strategy utilized for gene editing by creating site-specific breaks in DNA, allowing for targeted modifications.