EK Bio1 Ch 2 (cont..)
 Regulation of transcription o In prokaryotes
 mRNA typically includes several genes in a single transcript
 Jacob-Monod model
 Operon = operator + promoter + genes contributing to single prokaryotic mRNA o Genes outside of operon may code for activators/repressors
 Lac operon in e. coli o Codes for enzymes that allow e. coli to import & metabolize lactose when glucose is nor present in sufficient quantities o Activated when glucose is scarce and lactose is present o Mechanism: (38-39)
 Low glucose levels -> high cAMP levels
 cAMP activates CAP
 CAP binds to CAP site adjacent to promoter on lac operon
 Positive control: CAP activates promotor
 Gene repression: when lactose not present, lack repressor protein binds to operator, prevents expression of lac genes o In eukaryotes
 mRNA includes only 1 gene per transcript
 modification of RNA o post-transcriptional modifications offur in both eukaryotic and prokaryotic cells
 eukaryotes: only in nucleus
 bacterial genome does not contain introns o purpose of modifications:
 help molecules that initiate translation to recognize the mRNA
 protect mRNA from degradation
 eliminate extraneous sequences of nucleotides from the transcript before translation
 provide mechanism for variability in protein products produced by a single transcript o primary transcript = first nucleotide sequence arrived at through transcription
 much longer than mRNA that will be translated into protein
 portions of transcript (introns) are spliced out before exiting nucleus
 exons are joined end-to-end

 **introns stay IN nucleus, exons Exit nucleus to be translated
 snRNPs = small nuclear ribonucleoproeins, act as ribozymes o spliceosome = complex of snRNP + associate proteins o 5’ end of eukaryotic transcription is capped via GTP before transcription is finished
 5’ cap serves as attachment site in protein synthesis & as protection against enzyme degradation (via exonucleases)
 3’ end has long series of adenine nucleotides (poly A trail) o alternative splicing in eukaryotic cells
 allows cell to incorporate different coding sequences into mature mRNA
 can create variety of mRNA molecules for translation from single DNA coding seq
 human genome is 20,000 – 25,000 coding regions, but codes for >100,000 protein products
 introns help determine possible splicing patterns & promote protein production
 introns tend to be highly conserved between species
 translation: nucleotide sequence of mRNA to amino acid seq of corresponding protein o triplet code: 3 nucleotides (codon) to code for 1 amino acid
 degenerative… >1 series of 3 nucleotides can code for same AA
 **Stop codons: UAA, UGA, UAG
 **Start codon: AUG
 Number of possible combinations of any three nucleotides: 4 3
= 64 o Types of RNA
 mRNA = template that carries genetic code from nucleus to cytosol in form of codons
 tRNA
 two ends: o one contains anticodon, series of 3 nucleotides that binds to complementary codon sequence on
RNA o other carries AA that corresponds to codon, to be added to growing polypeptide chain
 some flexibility in bonding at the third-bp position
 wobble pairing o all translation uses ribosomes
 small subunit + large subunit
 prokaryotic: 30s + 50s = 70s
 eukaryotic: 40s + 60s = 80s

 the nucleoleus manufactures ribosomes (euk only) o mechanism: (p 43)
 initiation
 after post-transcriptional processing (euk), mRNA leaves nucleus though nuclear pores, enters cytosol
 initiation factors help attach 5’ end to small ribosomal subunit
 tRNA containing 5’-CAU-3’ gathers methionine and settles into p-site
 this signals large subunit to join & form initiation complex o elongation
 ribsosome slides down mRNA one strand at a time 5’->3’ while matching each codon to complimentary tRNA anticodon
 corresponging AAs attached to tRNA are bound together into growing polypeptide
 elongation requires energy
 when tRNA attaches to P-site, new tRNA attaches to neighboring A site, peptide bond forms between the two via peptidyl transferase activity
 tRNA with methionine moves to E-site, and then exits ribosome, tRNA carrying new dipeptide moces to p side, a site is open for next until stop codon reaches p o termination
 when stop codon reaches A site, release factor proteins add to end of polypeptide chain, which frees it from tRNA and ribosome
 ribosome breaks into subunits to be used later o polypeptide begins folding during translation, assisted by chaperons
 post-translational modification o regulate gene expression by affecting which translational products become proteins o may add sugars, lipids, or phosphate groups, or cleave the polypeptide o final destination = related to location of translation o 20 AA seq called signal peptide near front of polypeptide is recognized by protein called RNA signal-recognition particle (SRP) that carries entire ribosome complex to receptor protein on ER
 protein grows across membrane, then either released into lumen or remains partially attached to ER
 signal peptide removed by enzyme
 signal peptides may also be attached to polypeptides to target them to mitochondria, nucleus, or other organelles
 DNA Replication: Mitosis o Semiconservative: each DNA copy contains one strand of original DNA o Mechanism governed by group of proteins called replisome

o Replication begins in middle of chromosome, at the origin of replication
 Single origin on prok, multiple on euk.
 Bidirectional process, two replisomes proceed in opposite directions
