From Gene to Phenotype

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From Gene to Phenotype
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Lecture Outline 11/4/05

• The central dogma
• DNA-gtRNA-gtprotein
• Control of gene expression in prokaryotes vs
  eukaryotes
• Transcription
• Initiation, Elongation,Termination
• mRNA processing
• Introns and exons
• Other types of RNA

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In prokaryotes

• Transcription and translation occur simultaneously

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In Eukaryotes

• RNA transcripts are modified before becoming true
  mRNA
• Transcription and translation occur in separate
  compartments of the cell

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One Gene -gt One Enzyme
Beadle and Tatum studied mutants of the bread
mold Neurospora crassa and showed that each gene
specified a particular enzyme
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One Gene -gt One Enzyme
Which mutants can grow with which
supplement? Mut 1 Mut 2 Mut 3 none 0 0 0 Ornith
ine 1 0 0 Citrulline 1 1 0 Arginine 1 1 1
Normal cells can synthesisze arginine from
precursors in the minimal medium
Arginine
Ornithine
Citrulline
Precursor
Specific enzymes (arrows) catalyze each step
Arginine is an essential amino acid, required for
growth
Each mutant blocked a particular step of the
pathway
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One Gene -gt One Enzyme
Which mutants can grow with which
supplement? Mut 1 Mut 2 Mut 3 none 0 0 0 Ornithin
e 1 0 0 Citrulline 1 1 0 Arginine 1 1 1
Mutant 2 can grow only if supplemented with
citrulline or arginine
Arginine
Ornithine
Citrulline
Precursor
Therefore mutant 2 must not make the enzyme to
convert Ornithine to Citrulline
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One strand of DNA is copied to make messenger RNA
This is the sense or coding strand, because
it reads the same as the mRNA
This is the strand that is actually copied

• 5 . . . ATGAATGTC . . . 3 coding
• 3 . . . TACTTACAG . . . 5 template
• 5 . . . augaauguc-gt . . 3 RNA copy

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Synthesis of an RNA Transcript
RNA polymerase binds to a promoter sequence

• Initiation
• Elongation
• Termination

mRNA copy of gene is synthesized 5 to 3
Termination sequence causes transcription to stop
Figure 17.7
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RNA Polymerase
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RNA synthesis

• RNA synthesis is similar to DNA synthesis except
  
• Does NOT need a primer
• No proofreading
• Why not? mRNA has high turnover. An error in one
  molecule will not be inherited
• Both strands of DNA can serve as the template
• Some genes are on one strand, other genes are on
  the other

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(No Transcript)
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See an animation from www.dnai.org
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Initiation in eukaryotes
TATA box
Several transcription factors must bind to
promoter sequences upstream of the gene
Then RNA polymerase can bind
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RNA processing in eukaryotes
2. Add poly A tail to 3 end
1. Add 5 cap
A modified GTP is added, backwards, on the 5 end
About 200 As added at 3 end
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RNA processing in eukaryotes
3. Splice out introns
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Spliceosomes
Pre- mRNA
Special small nuclear RNA molecules do the
splicing
Mature mRNA
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Most Eukaryotic genes have introns
6 exons 5 introns
Example Red/Green colorblindness
13 kb
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Alternative splicing

• Make different mRNAs (and proteins) from same
  gene by splicing out certain exons

Cell-type specific RNA splicing
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Some have many introns and alternative forms

• Human Dystrophin gene

260 kb intron
2.4 Mb
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Correspondence between exons and protein domains
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Four types of RNA

• mRNA
• Messenger RNA, encodes the amino acid sequence of
  a polypeptide
• rRNA
• Ribosomal RNA, forms complexes with protein
  called ribosomes, which translate mRNA to protein
• tRNA
• Transfer RNA, transports amino acids to ribosomes
  during protein synthesis
• snRNA
• Small nuclear RNA, forms complexes with proteins
  used in eukaryotic RNA processing

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RNA will fold to specific shapes

• Because RNA is single-stranded, parts of the
  molecule can base pair with other parts of the
  same molecule, causing it to fold into defined
  shapes.
• Some RNA molecules can even act as enzymes
  (ribozymes)

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Summary