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/transmenu_s.swf

Translation

• mRNA Protein (facilitated by tRNA)• Happens in a ribosome, in the cytoplasm• Has three stages:(1)Initiation(2)Elongation(3)Termination

tRNACodon – 3 nt sequence on mRNAAnticodon – 3 nt sequence on tRNA

The Ribosome

• made up of 2 subunits

• has three tRNA sites (A site – acceptor siteP site – peptide siteE 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/transmenu_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://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html