Gene Expression

Flow of genetic information Crick: 1 gene = 1 protein, 1 gene = 1 enzyme Transcription: info. In DNA used to synthesize mRNA Translation: ribosome uses mRNA to make polypeptide (in ER and folded) RNA composition/types uracil(U): pyrimidine (pairs with adenine) mRNA: messenger, encodes AA seq. rRNA: ribosomal, part of ribosome tRNA: transfer, carries AA to ribosome, reads 3 nucleotides and has cognate Genetic code Codon: 3 nucleotides mRNA, each triplet = 1 AA, UUU = phenylalanine Read in sets of 3 Mutation: 1 letter deleted Universal genetic code Unambiguous, degenerate (redundant), start codon = AUG, 3 stop codons (UAG/UGA)--protein Transcription (DNA -> RNA) -> translation (RNA -> protein) Translation Function: tRNA binds to AA and brings to mRNA in ribosome Structure: RNA molecule Anticodon: complementary/antiparallel to codon Enzymes: aminoacyl: tRNA synthetase -> 20 different enzymes Links carboxyl group of AA to 3’ end of tRNA before reaching active site of rRNA Ribosomes: enzyme complex binds to mRNA template/catalyzes peptide bonds Large and small subunit Large has binding site for tRNA interactions with mRNA

Replication Transcription (DNA -> RNA) Translation (RNA -> protein) Location Nucleus Nucleus Cytoplasm Initiation -starts at origin of replication, replication bubble -DNA helicase unzips mol., breaks H-bonds -SSBP keep strands apart -topoisomerase relaxes helix/prevents from breaking -DNA polymerase lays down complementary bases by attaching to primer -RNA primer needed -RNA primase puts down primer on lagging strand which is later removed and replaced with DNA -synthesized in 5’ -> 3’ -DNA is template, 1 strand template and other is not transcribed -RNA polymerase unzips molecule and binds to promoter (specific DNA seq on transcribed strand @ start of gene @ 3’ end) -NO PRIMER NEEDED -begins transcribing mRNA 5’ -> 3’ -rRNA units disassembled -mRNA arrives with loaded tRNA from cytoplasm -tRNA reads start codon AUG and adds anticodon that forms H-bonds -initiation complex formed

Elongation -nucleoside triphosphate added along with phosphodiester bonds (poly), release pyrophosphate -5’ phosphate attached to 3’ OH -DNA poly adds nucleotides to 3’ OH side -leading strand: continuously synthesized towards replication fork -lagging strand: synthesized in short fragments (Okazaki fragments) away from fork -adds complementary RNA bases and nucleoside triphosphates -template read 3’ -> 5’ -A site: where tRNA enters with AA; ribozyme catalyzes peptide bond -P site: holds growing polypeptide -E site: tRNA leaves -each cycle adds 1 AA -translocation: ribosome moves down 1 codon

Termination -RNA removed -strands are glued together with DNA ligase -2 identical daughter molecules of DNA -reads termination sequence and transcript falls off -mRNA processing (Eukaryotes only): -pre-mRNA: not in correct form to give info for protein synthesis, modified -mature mRNA: alternation of pre-mRNA ends + RNA splicing -introns (not translated) spliced out and extrons (expressed, coding regions) glued together -5’ cap (modified guanine with 3 phosphates) and poly A tail added -for export from nucleus, protect from degradation, help ribosome attach -stop codon read by releasing factors and binds to stop codon in A site and protein is released -rRNA parts disassemble -tRNA goes back to be recharged