Gene Expression

Central Dogma Information flows from: DNA  RNA  Protein Exception: reverse transcriptase (retroviruses) RNA  DNA  RNA  Protein

3 kinds of RNA Messenger RNA (mRNA) Carries info for making protein Transfer RNA (tRNA) Bonds to 1 kind of amino acid and carries it to ribosome Ribosomal RNA (rRNA) Catalytic; protein synthesis

RNA Synthesized from DNA template - transcription Nucleotides differ from DNA nucleotides in that: Ribose instead of deoxyribose Uracil instead of thymine Single stranded

“The Code”: Important terminology Codon – sequence of 3 consecutive mRNA bases – specifies 1 amino acid Anticodon – 3 consecutive bases on tRNA which are complementary to codon on mRNA

Transcription: Copying DNA’s code to RNA RNA synthesis catalyzed by RNA polymerases DNA serves as a template – only 1 side of DNA strand 5’  3’ direction; no primer needed

Initiation: RNA polymerase binds to promoter (not transcribed) – upstream from gene Unwinds DNA, begins transcription Elongation: Additional RNA nucleotides are added to 3’ end of growing RNA molecule Double helix rewinds Termination: RNA polymerase recognizes termination sequence RNA polymerase separates from DNA template and RNA transcript

Post-transcription modification Eukaryotic cells produce pre-mRNA from transcription This will be modified before leaving the nucleus smenu_s.swf smenu_s.swf

mRNA made by transcription… Non-coding leader sequence at 5’ end Recognition sites to bind to ribosome Start codon – AUG Coding sequence Stop codon – UAA, UGA, UAG Non-coding trailing sequence at 3’ end

Translation Overview Conversion of triplet code into polypeptide Takes place at ribosome in cytoplasm Involves all 3 types of RNA

Ribosome Made of 2 subunits – large and small 4 binding sites mRNA binds to small subunit Large subunit has 3 sites: A site – tRNA delivering next amino acid P site – hold growing polypeptide E site - exit

tRNA Binds to specific amino acid at 3’ end Anticodon – complementary to codon on mRNA Structure held by H- bonding

Translation: Initiation: Initiation factors (proteins) attach to small subunit mRNA binds to small subunit around AUG tRNA with UAC anticodon binds to start codon Large subunit binds and initiation factors released (initiator tRNA at P site)

Elongation: Next tRNA binds to codon at A site Peptide bond forms between 2 amino acids (released from P site tRNA and attaches to A site tRNA) Translocation – ribosome moves down 1 codon

Termination: stop codon in A site – no complementary tRNA Release factor binds to A site – freeing polypeptide smenu_s.swf smenu_s.swf