Transcript From Gene to Protein

From
Gene
to
Protein
Chapter 15
Gene Info
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Beadle & Tatum – determined one
gene codes for one enzyme
• Not all proteins are enzymes, so
refined to be one-gene-onepolypeptide hypothesis
Crick – Central Dogma of Genetics
• DNA  RNA Protein
•Modified since
Protein Synthesis Basics
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RNA – ribose, phosphate group, A-U
and G-C
•Single stranded
Transcription – synthesis of RNA under
direction of DNA template
•Called mRNA (messenger RNA)
Translation – synthesis of polypeptide
from the mRNA
In prokaryotes, both happen at same
time
Protein Synthesis Basics
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Triplet code on RNA (codon) =
one amino acid
Ex: GGC = glycine
64 codons
Codons read in
5’-3’ direction
of mRNA
Protein Synthesis Basics
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61 of 64 codons for a.a. only
1 also tells ribosome to start (AUG)
3 tell it to stop
Genetic code
is nearly
universal
Overview of
Protein Synthesis
Step One: Transcription


RNA Polymerase
opens DNA strands
& connects RNA
nucleotides together
Three steps –
initiation,
elongation,
termination
Step One: Transcription

DNA sequence where this begins –
promoter
•Also determines which strand is
used
•Collection of proteins bind to
promotor (transcription
factors)
Step One: Transcription
• One strand of DNA has TATA box –
TATA sequence
before promotor
• Where
transcription
factors bind
• The other
strand
becomes the
template
Step One: Transcription

Transcription factors + RNA
polymerase bound to promotor =
transcription initiation complex

End sequence – terminator
Step One: Transcription

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Part transcribed – transcription unit
mRNA considered pre-mRNA
•Needs processing
Step Two: mRNA Processing
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Enzymes modify pre-mRNA in nucleus
5’ end gets 5’-cap
• Modified guanine nucleotide
• Protects m-RNA from hydrolysis
• Shows ribosome where to attach
3’ end gets poly (A) tail
• 50-250 adenine nucleotides
• Same protective function
Step Two: mRNA Processing

RNA splicing
•RNA has sections that code for A.A.
(exons) and non-coding sections
(introns)
•Interspersed
•Introns are cut out – exons spliced
together
Step Two: mRNA Processing
• How?
• Ends of introns
– short nucleotide
sequence
• Recognized by snRNPs
• Small nucleotide ribonucleoproteins
• RNA (snRNA) & proteins
• Join with introns to make spliceosome
• Cuts introns & joins exons
• snRNA acts as ribozyme (RNA that acts
like enzymes)
RNA
Processing
Animation
Step Three: Translation
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In cytoplasm
mRNA read to make
protein
tRNA brings A.A. to
ribosome
tRNA has anti-codon
(complementary
sequence to codon)
Step Three: Translation

Some tRNA have flexible 3rd b.p. rules
•“Wobble”
•U of anti-codon can pair with A or G
as 3rd b.p.
Step Three: Translation

•
Ribosomes
2 subunits (mostly
rRNA)
•
1 binding site for
mRNA
•
3 for tRNA
•
A site (aminoacyl-tRNA site)
•
Holds tRNA with next A.A.
to add
Step Three: Translation
•P site (peptidyl-tRNA site)
–Holds tRNA carrying
polypeptide
•E site (exit site)
•tRNA leaves ribosome
3 Stages of Translation

1) Initiation – uses GTP
ATP
GTP
3 Stages of Translation

2) Elongation
• A.A. added
• 3 steps:
• Codon
recognition
• Peptide bond
formation
• Translocation
(ribosome
moves tRNA
from A to P site)
3 Stages of Translation

3) Termination
•Stop codon
•Release factors needed
•Bind to stop codon at A site
•Adds H2O to chain – hydrolyzes
polypeptide from tRNA
Protein Synthesis Animation
Mutations

Point mutations – change in one base
pair of a gene
• 1) Base-pair substitution
• Replacement of one nucleotide &
complement with another on DNA
• Missense mutation
•Altered codon codes for a.a., but
not right one
•If turns to stop codon – nonsense
mutation – nonfunctional protein
Mutations
• 2) Insertions & Deletions
• Often detrimental
• Leads to frameshift

mutations
Mutagens can cause
DNA mutations