1 SURVEY OF BIOCHEMISTRY Transcription and Translation.

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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 Sheet2.Helix-turn-Helix3.Zinc Finger4.Leu Zipper

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PRS

• Which amino acid contains a hydroxyl in its side chain?

1. Threonine2. Methionine3. Aspartic Acid4. Phenylalanine

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Transcription

Transcription is the process by which RNA is producedfrom a DNA template.

Initiation Elongation Termination

Steps involved intranscription

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Transcription

DNA has to separate at the position where transcription occurs.

Fig. 26-3

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Transcription

5’-…GATCCGTAGGTC…-3’3’-…CTAGGCATCCAG…-5’

5’-…GATCCGTAGGTC…-3’

SenseStrand

AntisenseStrand

(template)

5’----GAUCCGUAGGUC…3’3’-…CTAGGCATCCAG…-5’

Newly synthesizedRNA has the

same general sequence as

the sense strand;

U in RNA, T in DNA

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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 promotersequences

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

Usuallysingle purine

Notice how sequence is numbered!

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Prokaryotic RNA Polymerases

€

α 2ββ 'ωσCore enzyme

Holoenzyme

The sigma factor bindsspecifically to the promoter.

Core enzyme affinity to DNA ~5x10-12 M.

€

σ = 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

Positivesupercoiling

Negativesupercoiling

1. RNA polymerase is processive2. Once an RNAP has moved from the promoter, another

can bind3. 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

Poly-U tail

G-C richbp region

What happens when a known terminator

sequence is missing?

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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 fromFigure 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

PPeptidyl

EExit

APE

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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

PeptidylTransferase

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