The Central Dogma: DNA to RNA to Protein

The Central Dogma

• DNA is used to make RNA, which is used to make Protein

Gene Structure

• A gene has a transcription start site (+1) at the 5' end of the RNA product
• A transcribed region is located downstream of the transcription start site (positive base positions)
• A regulatory domain called a promoter is located upstream of the transcription start site (negative base positions)

Regulation of Gene Expression

Prokaryotes

• Rely largely on negative regulation using repressor proteins

Eukaryotes

• Rely on positive regulation of gene expression
• Use transcription factors as activators or repressors of RNA polymerase activity
• Basal transcription factors bind to the promoter to recruit RNA polymerase, acting as an on/off switch for transcription
• Regulatory transcription factors (activators and repressors) bind to proximal control elements, modifying transcription
• Enhancers greatly increase transcription when bound by transcriptional activators

Transcription

• One strand of DNA acts as a template for RNA production by RNA polymerase
• RNA polymerase:
  • Unwinds DNA locally
  • Assembles complementary RNA nucleotides along the DNA template strand
  • Links nucleotides with phosphodiester bonds into a polynucleotide
  • Can only add to the 3'-end of the growing polymer
  • Doesn't require a primer

Stages of Transcription

• Initiation: RNA polymerase interacts with the promoter at the TATA box to establish a transcription complex
• Elongation: RNA polymerase extends the new RNA by adding to the 3' end
• Termination: A sequence in the DNA (terminator) halts transcription

Eukaryotic Transcription

• Produces pre-mRNA that must be processed to mature mRNA for translation
• Processing involves:
  • Adding a 5' cap
  • Adding a 3' polyA tail
  • Splicing to remove introns and ligate exons
• Mature mRNA is exported to the cytoplasm for translation

Translation

• mRNA has a protein coding sequence using codons
• Codons are translated using tRNA and enzymes for each of the 20 amino acids
• tRNA hydrogen bonds with the proper codon on mRNA through its anti-codon
• Codons represent a degenerate code, with more than one codon representing the same amino acid
• Reading codons in order determines the polypeptide sequence

Ribosome and Translation

• Translation occurs in the ribosome, a large multi-protein complex
• Initiation: the small ribosomal subunit binds to mRNA near the 5'-end
• Elongation:
  • A charged tRNA enters the A-site
  • Ribosomal enzymes cut the growing polypeptide free from the P-site tRNA
  • Attach the polypeptide to the new amino acid on the tRNA in the A-site with a peptide bond
  • The ribosome slides down the mRNA one codon, shifting the tRNAs over (P to E, and A to P)
  • The process repeats until a stop codon lines up with the A-site and binds to a release factor, halting translation