General Biology/Genetics/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 “Central Dogma” 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 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 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 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 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 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 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 Ribosome structure Large ribosome subunit Translation Initiation complex Small ribosomal subunit mRNA fMet-tRNA (prokaryotes only; met-tRNA in eukaryotes) Initiation factors Elongation Ribosome mRNA tRNAs Elongation factors Initiation complex 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 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 References This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.