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Transcription and Translation Central Dogma of Molecular Biology • Proposed by Crick • DNA RNA Protein Background on RNA Transcription • DNA mRNA • This happens in the nucleus • Has three stages: (1) Initiation (2) Elongation (3) Termination Transcription • Special sequences upstream of the gene determine gene transcription • Promoter is where the RNA polymerase will bond • Contains TATA box, which is located 25 nucleotides away from the site where transcription is initiated. • Transcription Factors attach first • RNA Polymerase then bonds to complete assembly Transcription (1) Initiation: -RNA Polymerase binds to the DNA promoter -DNA is unwound, and a template is exposed (2) Elongation: -mRNA is synthesized from 5’ 3’ from the template -Therefore, DNA template strand is 3’ 5’ -Note: The template strand is called the non-coding strand. -Note: A U and G C Transcription (2) Elongation: -mRNA synthesis continues. DNA that has already been transcribed recoils into a helix -RNA polymerase terminates at the end of the gene (3) Termination: -mRNA transcription finishes, RNA polymerase is released Transcription vs. Replication • Transcription and Replication are different! • Transcription = creation of mRNA, uses RNA Polymerase • Replication = creation of new DNA strand, uses DNA Polymerase RNA Processing • aka Post-transcriptional Modifications • The primary transcript is called hnRNA – 5’ cap (methylated G) – added and a poly-A tail added to the 3’ end – (Note: cap and tail are protection from degradation and recognition by ribosome) – Spliced out introns (non-coding segments; the coding segments are called exons) RNA Processing RNA Processing • http://207.207.4.198/pub/flash/26/transmen u_s.swf Translation • mRNA Protein (facilitated by tRNA) • Happens in a ribosome, in the cytoplasm • Has three stages: (1)Initiation (2)Elongation (3)Termination tRNA Codon – 3 nt sequence on mRNA Anticodon – 3 nt sequence on tRNA The Ribosome • made up of 2 subunits • has three tRNA sites (A site – acceptor site P site – peptide site E site – exit site) Translation (1) Initiation: • (Note: aminoacyl-tRNA synthetase attaches amino acid is attached tRNA) • Small subunit recognizes and binds to 5’ cap upstream of AUG sequence with aid of initiation factors • Initiator tRNA recognizes start codon AUG • Large subunit attaches to complete translational complex • First tRNA is sitting in the A site Translation (2) Elongation cont’d: • The first codon (AUG) is recognized by an anti-codon (UAC) on tRNA • The tRNA enters the ribosome at the A site, bringing an amino acid along with it (AUG = methionine • The tRNA moves to the P-site, while another tRNA comes into the A-site • Peptide bonds are formed between the junction of the P-site and A-site • tRNAs move to the E-site, where they exit Translation (3) Termination: • When a stop codon is encountered by the ribosome, there is no corresponding tRNA for that stop codon • A release factor dismantles the ribosome/mRNA complex, releasing the polypeptide Translation - Visual Summary of Translation • http://207.207.4.198/pub/flash/26/transmen u_s.swf The Genetic Code DEGENERATE! Wobble Hypothesis!!! – the violation of the usual rules of base pairing at the third nucleotide of a codon Homework • • • • Read/review pages 234 – 253 KNOW the 1 Gene - 1 Polypeptide Hypothesis P. 236 # 2, ***5 P. 241 2 Summary • http://wwwclass.unl.edu/biochem/gp2/m_biology/animat ion/gene/gene_a2.html