Transcript SURVEY OF BIOCHEMISTRY

SURVEY OF BIOCHEMISTRY
Transcription and Translation
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PRS
Which of the following is not an
example of a DNA-binding motif in
proteins?
1.Beta Sheet
2.Helix-turn-Helix
3.Zinc Finger
4.Leu Zipper
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PRS
• Which amino acid contains a
hydroxyl in its side chain?
1. Threonine
2. Methionine
3. Aspartic Acid
4. Phenylalanine
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Transcription
Transcription is the process by which RNA is produced
from a DNA template.
Steps involved in
transcription
 Initiation
 Elongation
 Termination
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Transcription
DNA has to separate at the position where
transcription occurs.
Fig. 26-3
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Transcription
Sense
Strand
Newly
synthesized
RNA has the
same general
sequence as the
sense strand;
U in RNA,
T in DNA
5’-…GATCCGTAGGTC…-3’
3’-…CTAGGCATCCAG…-5’
Antisense
Strand
(template)
5’-…GATCCGTAGGTC…-3’
5’----GAUCCGUAGGUC…3’
3’-…CTAGGCATCCAG…-5’
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Transcription Initiates at
Promoter Sequences
What is a gene?
Example of E.coli Lactose Operon Organization
Fig. 26-4
P: Promoter
O: Operator
I: Inhibitor
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Structural genes follow promoter
sequences
In E. coli, the lactose operon consists of 3 structural
genes:
 Beta-Galactosidase (Z)
 Galactoside Permease (Y)
 Thiogalactoside Transacetylase (A)
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E. coli Promoter Sequences
Notice how sequence is numbered!
Usually
single
purine
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Prokaryotic RNA Polymerases
 2  '
Core enzyme
The sigma factor binds
specifically to the promoter.
Core enzyme affinity to
DNA ~5x10-12 M.
Holoenzyme

= sigma factor
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Elongation: RNA Chain Growth
3’-OH group acts as a nucleophile,
attacking the alpha phosphate group,
with subsequent release of inorganic pyrophosphate.
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DNA Supercoiling
Negative
supercoiling
Positive
supercoiling
1. RNA polymerase is processive
2. Once an RNAP has moved from the promoter, another
can bind
3. Transcription is fast:
constitutive enzymes vs. inducible enzymes
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Termination of Transcription
Because the newly synthesized RNA is single-stranded,
hairpin structures can easily form from the
termination sequence.
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Hairpin Structure in RNA
G-C rich
bp region
What happens when a
known terminator
sequence is missing?
Poly-U tail
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Termination: Rho Factor
Helicase: RNA-RNA and RNA-DNA complex unwinding.
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Transcription in Eukaryotes
• RNA Polymerase I
– Precursors to rRNA
• RNA Polymerase II
– Precursors to mRNA
• RNA Polymerase III
– Precursors to 5S rRNA, tRNA
– Precursors to other small RNA’s
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Eukaryotic Transcription Factors
Transcription factors are req’d to initiate transcription,
and facilitate elongation in eukaryotes.
Eukaryotic Termination: Not well understood
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Portion of the Genetic Code
Wobble
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Reading Frames
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Translation: Protein Synthesis
• Messenger RNA
– Template
• Transfer RNA
– Carries amino acids
• Ribosomal RNA
– Aids protein synthesis
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tRNA Structures
2D
3D
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tRNA’s have modified bases
Be able to name, draw, recognize structures from
Figure 27-4.
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Aminoacyl tRNA Synthetases
Aminoacyl tRNA synthetases add aa’s to tRNA.
Step 1.
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Aminoacyl tRNA Synthetases
Step 2.
+
tRNA
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Ribosome
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Sites in the Ribosome
A
Aminoacyl
E
P
A
P
Peptidyl
E
Exit
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Chain Initiation in Translation
Start Codon: AUG
Methionine
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Shine-Dalgarno Sequence
upstream
downstream
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Initiation
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Translation: Polypeptide Synthesis
Peptidyl
Transferase
Activity
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Elongation
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Administrative Updates
• Exam 1 on Thursday, 4-5 pm
– 20 questions
– Short answers and Multiple Choice
– Closed-book
– Calculators allowed
– No cell phones, blackberries, etc.
• Office Hours: M, T, R 2-4 pm
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