Molecular Biology: Central Dogma and Transcription

Central Dogma of Molecular Biology

• States genetic information flows from DNA to RNA to protein (or RNA directly to protein)

RNA Structure and Stability

• RNA is a single-stranded nucleic acid, unlike the double-stranded DNA.
• RNA backbone has alternating phosphate groups and ribose sugars (in contrast to deoxyribose in DNA)
• Single-stranded nature contributes to RNA's lower stability compared to DNA.

Transcription Overview

• Transcription copies a DNA strand into mRNA.
• DNA acts as a template/reference for genetic material.
• RNA synthesis always occurs in the 5' → 3' direction.
• Template/non-template strands are relative to the gene's direction.
• Transcription can occur bidirectionally.

Gene Structure and Transcription

• Promoter: Region where RNA polymerase binds; not transcribed, determines transcription rate (strong vs. weak)
• Often contains a "TATA" box (Pribnow box in prokaryotes)
• Transcription: A three-step process: initiation, elongation, and termination
• Initiation: involves binding of RNA polymerase and general transcription factors to the promoter.
• Elongation: RNA polymerase catalyzes sequential addition of ribonucleotides. Transcription bubble holds the DNA template strands apart, typically 17 bp long.
• Termination: Different Mechanisms
  • Rho-independent termination: A GC-rich stem-loop followed by a run of U's in mRNA destabilizes RNA, causing the RNA polymerase to dissociate.
  • Rho-dependent termination: Pause points in DNA become termination points.
• Eukaryotic Polymerases: Different enzymes (RNA polymerase I, II, III) handle transcription of different RNA types.
  • Pol I transcribes ribosomal RNA
  • Pol II transcribes protein-coding genes.
  • Pol III transcribes tRNA and other small RNAs

Transcription Initiation (Eukaryotes)

• General transcription factors and RNA polymerase II bind to the promoter.
• DNA unwinding occurs.
• RNA polymerase II's carboxyl terminal domain is phosphorylated.
• First 5-10 phosphodiester bonds are synthesized.
• RNA polymerase II and general transcription factors are released.

Eukaryotic Promoter Elements

• TATA box: A conserved DNA sequence similar to the prokaryotic Pribnow box, a binding site for RNA polymerase II after several steps.
• Initiator Site: Binding site for TATA-binding protein (TBP), assisted by TAFs (TBP-associated factors).

RNA Processing (Eukaryotes)

• Eukaryotic RNA is initially transcribed as a longer precursor.
• Pre-mRNA must be processed to remove introns and add additional features to facilitate export
  • Introns are removed
  • Exons are the coding regions that remain and are translated into proteins.
  • 5' Cap: a modified guanine nucleotide added to the 5' end – crucial for stabilizing RNA and initiating translation
  • Poly A Tail: A sequence of adenine nucleotides added to the 3' end to help with stability and export

Splicing

• Introns are removed, and exons are joined
• Catalyzed by a complex called the spliceosome (composed of multiple RNA-protein subunits (U1, U2, U4/U6).
• Exons: Expressed regions of the gene.
• Introns: Intervening sequences within the gene.

Alternative Splicing

• A single gene can produce multiple protein isoforms with different functions due to different splicing patterns.

Explore the essential concepts of the Central Dogma of Molecular Biology, focusing on DNA, RNA, and protein synthesis. This quiz covers RNA structure, transcription processes, and gene structure, providing an overview of how genetic information flows and is expressed in living organisms.