Gene Expression: From Gene to Protein

Flow of Genetic Information

  • Genetic information flows from DNA to RNA to Protein.
  • Gene expression is how DNA directs protein synthesis, linking genotype and phenotype.
  • Transcription uses DNA to make mRNA; translation uses mRNA to make protein.
  • In bacteria, transcription and translation occur simultaneously due to the lack of a nucleus.
  • In eukaryotes, the nuclear envelope separates transcription and translation, allowing for RNA processing.

Transcription

  • Transcription uses DNA as a template to synthesize mRNA, facilitated by RNA polymerase.
  • RNA polymerase builds mRNA in the 5’ to 3’ direction without needing a primer.
  • RNA differs from DNA by being single-stranded, containing ribose instead of deoxyribose, and using uracil instead of thymine.

Transcription Initiation in Bacteria

  • RNA polymerase binds to the promoter, the start of the gene
  • Transcription bubble opens
  • RNA synthesis begins.

Transcription Elongation in Bacteria

  • RNA polymerase adds nucleotides to the 3’ end of mRNA.
  • The template DNA strand guides mRNA synthesis.
  • The coding DNA strand is nearly identical to the mRNA, except with thymine instead of uracil.

Transcription Termination in Bacteria

  • RNA polymerase transcribes a terminator sequence, leading to its release and the mRNA's release.

Transcription Initiation in Eukaryotes

  • Transcription factors (TFs) bind to the TATA box in the promoter.
  • TFs recruit RNA polymerase and help open the transcription bubble.

mRNA Processing in Eukaryotes

  • mRNA undergoes modifications: capping, tailing, and splicing.
  • A modified guanine cap is added to the 5’ end.
  • A polyA tail (50-250 adenine nucleotides) is added to the 3’ end.
  • The 5’ cap and polyA tail protect mRNA, facilitate export from the nucleus, and aid ribosome attachment.
  • Introns are removed, and exons remain.

Translation

  • Translation uses mRNA to produce a polypeptide, relying on the genetic code.
  • Amino acids are encoded by three-nucleotide codons in the mRNA.
  • mRNA, tRNA, and ribosomes are essential for translation.
  • Each mRNA codon contains three nucleotides.
  • AUG is the start codon; there are three stop codons.
  • The genetic code is redundant but not ambiguous.

tRNA

  • Transfer RNA (tRNA) carries amino acids to the ribosome.
  • The anticodon binds to a codon on mRNA.
  • The amino acid attaches to the amino acid attachment site.

Ribosome

  • The ribosome, found in the cytoplasm or on the rough ER, has large and small subunits made of ribosomal proteins and rRNAs.
  • The ribosome has three tRNA binding sites: A, P, and E.

Translation - 3 stages

  • Initiation
  • Elongation
  • Termination

Translation Initiation

  1. The small ribosomal subunit binds to the mRNA at the start codon, aligning it with the P site
  2. The initiator tRNA, carrying methionine, binds to the start codon
  3. The large ribosomal subunit binds to the complex, using GTP for energy.

Translation Elongation

  • Codon recognition – a tRNA enters the A site and the anticodon bonds to the codon
  • Peptide bond formation – The amino acid on that tRNA is covalently attached to the growing polypeptide chain
  • Translocation – The ribosome moves down one codon, allowing the empty tRNA to exit through the E site

Translation Termination

  • Elongation continues until a stop codon in the mRNA reaches the A site.
  • A release factor binds to the stop codon.

Targeting Polypeptides to Specific Locations

  • A signal peptide sequence notifies the ribosome to attach to the ER.
  • The signal recognition particle (SRP) escorts the translation complex to the rough ER.

Making Multiple Polypeptides

  • Multiple ribosomes can translate the same mRNA simultaneously, increasing efficiency.
  • In prokaryotes, transcription and translation can occur simultaneously.

Mutations

  • Mutations in nucleotide sequences can alter polypeptide amino acid sequences.

Nucleotide-Pair Substitutions

  • Silent mutations have no effect.
  • Missense mutations change one amino acid to another.
  • Nonsense mutations change an amino acid to a stop codon.

Frameshift Mutations

  • Insertions or deletions cause frameshift mutations, offsetting the codon reading frame.