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Transcript of Transcription and Translation. Central Dogma of Molecular Biology Proposed by Crick DNA RNA ...
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