Chapter 17 From Gene to Protein (Protein Synthesis) From Gene to Protein (Protein Synthesis)
Protein Synthesis Overview
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Sec 5.1 / 5.2
One Gene – One PolypeptideHypothesisearly 20th century – Archibald Garrod
physician that noticed that some metabolic errors were found in numerous members in a family
alkaptonuria – metabolic disorder where tyrosine is not properly broken down
hypothesized that these people had a defective enzyme that usually breaks down tyrosine. He also hypothesized that this enzyme was under the control of a single gene.
1941 – Beadle & Tatum
first hypothesized that genes and enzymes are somehow related
Beadle & Tatum ExptUsed X-rays to cause bread mold (N. Crassa) to
mutate
Wild type (Normal): can survive on minimal medium (agar, inorganic salts, glucose and biotin)
can synthesize all other molecules they need from these simple molecules (e.g. amino acids and nutrients)
Mutant: could not survive on minimal medium could not synthesize amino acids and nutrients they need because
of deficit in specific enzymes
examining arginine synthesis
precursor ornithine citrulline arginineE1 E2 E3
Beadle & Tatum Conclusionsone mutation corresponded to a change in a single
enzyme
As researchers learned more about proteins they made minor revisions to the “one gene-one-enzyme” hypothesis
Not all proteins are enzymes (e.g. keratin – structural protein)
Some proteins are made up of more than one type of polypeptide, each controlled by a different gene (e.g. Hemoglobin – α and β)
More accurately: “one-gene-one-polypeptide”
Central Dogma – the cellular chain of command
Transcription Animation
DNA
mRNA
protein
COMPONENTS LOCATION PROCESS
nucleus transcription
cytoplasm translation
Eukaryotes vs. Prokaryotes
Figure 17.3b
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
mRNA
DNA
Pre-mRNA
Polypeptide
Ribosome
Nuclearenvelope
TRANSLATION
TRANSCRIPTION DNA
mRNA
Ribosome
Polypeptide
Transcription
Figure 17.7
PromoterTranscription unit
RNA polymerase
Start point
53
35
35
53
53
35
53
35
5
5
Rewound
RNA
RNA
transcript
3
3
Completed RNA transcript
Unwound
DNA
RNA
transcript
Template strand of DNA
DNA
1 Initiation. After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand.
2 Elongation. The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5 3 . In the wake of
transcription, the DNA strands re-form a double helix.
3 Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.
Figure 17.4
DNAmolecule
Gene 1
Gene 2
Gene 3
DNA strand(template)
TRANSCRIPTION
mRNA
Protein
TRANSLATION
Amino acid
A C C A A A C C G A G T
U G G U U U G G C U C A
Trp Phe Gly Ser
Codon
3 5
35
A codon in messenger RNA Is either translated into an amino acid or serves as a
translational stop signal
Figure 17.5
Second mRNA baseU C A G
U
C
A
G
UUUUUCUUAUUG
CUUCUCCUACUG
AUUAUCAUAAUG
GUUGUCGUAGUG
Met orstart
Phe
Leu
Leu
lle
Val
UCUUCCUCAUCG
CCUCCCCCACCG
ACUACCACAACG
GCUGCCGCAGCG
Ser
Pro
Thr
Ala
UAUUAC
UGUUGC
Tyr Cys
CAUCACCAACAG
CGUCGCCGACGG
AAUAACAAAAAG
AGUAGCAGAAGG
GAUGACGAAGAG
GGUGGCGGAGGG
UGGUAAUAG Stop
Stop UGA StopTrp
His
Gln
Asn
Lys
Asp
Arg
Ser
Arg
Gly
U
CA
GUCAG
UCAG
UCAG
Fir
st m
RN
A b
ase
(5
end
)
Th
ird
mR
NA
bas
e (3
en
d)
Glu
The following is the sequence of a bases on the template strand of DNA in the transcription unit
3’ – GGATCAGGTCCAGGCAATTTAGCATGCCCC – 5’
a) Transcribe this sequence into mRNA
5’ – CCUAGUCCAGGUCCGUUAAAUCGUACGGGG – 3’
b) List the order of amino acids
Pro - Ser – Pro – Gly – Pro – Leu – Asn – Arg – Thr - Gly
ClassworkSection 5.1 (pg. 236) #1-2,4-6Section 5.2 (pg. 241) #1-3,5-13