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

u_s.swf 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 u_s.swf 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

Summary class.unl.edu/biochem/gp2/m_biology/animat ion/gene/gene_a2.html class.unl.edu/biochem/gp2/m_biology/animat ion/gene/gene_a2.html