General Biology/Genetics/Gene Expression

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General Biology | Getting Started | Cells | Genetics | Classification | Evolution | Tissues & Systems | Additional Material


Gene expression[edit]

Flow of genetic information

  • DNA → mRNA → polypeptide
  • Transcription: DNA → mRNA
    • RNA polymerase
    • Nucleus in eukaryotes
    • Transcription also makes rRNA and tRNA
  • Translation: mRNA → polypeptide
    • Ribosomes: protein and rRNA
    • Genetic code and tRNA

“Central Dogma”[edit]

The Genetic Code[edit]

  • Triplet codon
    • 64 triplet codons (43)
    • Experimentally deciphered in 1961
  • Nearly universal
    • Implies common ancestor to all living things
    • Minor exceptions: certain ciliates, mitochondria, chloroplasts
  • Still evolving

Transcription[edit]

  • RNA polymerase
    • NTP substrates
    • Synthesizes single stranded RNA complementary to template strand of DNA
    • New nucleotides to 3’ end
  • Begins at promoter site
    • no primer necessary
  • Ends at terminator site
  • Much posttranscriptional modification in eukaryotes

Transcription bubble[edit]

Promoter site

  • Prokaryotes
    • -10 nt, TATA box
    • -35 nt, additional signal
  • Eukayotes
    • -25, TATAAA box
    • Additional signals upstream
  • Promoters may be strong or weak
  • In eukaryotes, access to promoter depends upon state of chromatin coiling

Eukaryote mRNA[edit]

  • Synthesized as pre-mRNA, processed in nucleus
  • 5’ end: GTP cap placed in inverted position
    • Essential for ribosome recognition
  • 3’ end: poly-A tail; non-templated addition of ~50-250 A nucleotides; stability
  • Introns: intervening sequences removed

Translation[edit]

  • Requires:
    • mRNA
    • tRNA
    • ribosomes
    • translation factors (various proteins)
  • In prokaryotes, takes place on growing mRNA
  • In eukaryotes, in cytoplasm on free ribosomes and RER
  • AUG start codon to stop codon

Translation in bacteria[edit]

tRNA

  • Transfer RNA
  • Two important parts
    • Anticodon
  • Hydrogen bonds with mRNA codon
    • 3’ end
  • Accepts amino acid (using energy of ATP)
  • Aminoacyl-tRNA synthetase

Aminoacyl tRNA synthase[edit]

  • Enzyme used to bind amino acid from the cytoplasm to tRNA, which then transfers that amino acid to the ribosome for polypeptide formation

Ribosome structure[edit]

Large ribosome subunit[edit]

Translation[edit]

  • Initiation complex
    • Small ribosomal subunit
    • mRNA
    • fMet-tRNA (prokaryotes only; met-tRNA in eukaryotes)
    • Initiation factors
  • Elongation
    • Ribosome
    • mRNA
    • tRNAs
    • Elongation factors

Initiation complex[edit]

Elongation, translocation[edit]

  • incoming tRNA enters the A site
  • rRNA catalyzes peptide bond formation. Note that growing peptide is attached to what was incoming tRNA at P site after translocation.
  • empty tRNA leaves via E site; recycled
  • A site ready for next charged tRNA


Introns/exons[edit]

  • In eukaryotes, coding regions of gene may be interrupted by introns, noncoding regions of DNA (RNA)
  • Introns
    • 22- >10,000 nt in length
    • 5’ GU … 3’ AG removal sequence
    • Not essential to genes
    • May constitute >90% of gene
  • removed from pre-mRNA to form mRNA
  • Exon: often codes for functional domain of protein
    • translatable mRNA

This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.