Transcript Molecular Genetics

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