From Wikibooks, the open-content textbooks collection

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

---

Contents 1 Gene expression 1.1 “Central Dogma” 1.2 The Genetic Code 1.3 Transcription 1.4 Transcription bubble 1.5 Eukaryote mRNA 1.6 Translation 1.7 Translation in bacteria 1.8 Aminoacyl tRNA synthase 1.9 Ribosome structure 1.10 Large ribosome subunit 1.11 Translation 1.12 Initiation complex 1.13 Elongation, translocation 1.14 Introns/exons

[edit] Gene expression

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

[edit] “Central Dogma”

[edit] The Genetic Code

• 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

[edit] Transcription

• 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

[edit] Transcription bubble

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

[edit] Eukaryote mRNA

• 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

[edit] Translation

• 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

[edit] Translation in bacteria

tRNA

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

[edit] Aminoacyl tRNA synthase

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

[edit] Ribosome structure

[edit] Large ribosome subunit

[edit] Translation

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

[edit] Initiation complex

[edit] Elongation, translocation

• 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

[edit] Introns/exons

• 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.