Protein Synthesis The “Stuff of Life”
Proteins
• Proteins are the “workhorse” molecule
found in organisms.
• The blue print for proteins is coded in the
DNA of the organism.
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Uses of Proteins
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History
• 1909 Archibald Gerrod suggested that
genes determine phenotype through
defective enzymes controlling biochemical
pathways.
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Work of Beadle and Tatum
• George Beadle and Edward Tatum established
the link between genes and enzymes in studying
bread mold, Neurospora crassa.
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Conclusion
• One gene produces
one enzyme.
• Later it was modified
• One gene produces
one protein.
• One gene produces
one polypeptide
chain.
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Overview of Protein Synthesis
• DNA is located in the nucleus
• Proteins are made in the cytoplasm
• RNA is the intermediate between the DNA code
and the actual synthesis of a protein
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Structure of RNA versus DNA
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Three Main Types of RNA
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Differences in Protein Synthesis between
Prokaryotes and Eukaryotes
• Prokaryotes do not have introns like
eukaryotes.
• RNA in prokaryotes does not have to
be processed like eukaryotes.
• Transcription and translation can be
simultaneous in prokaryotes.
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Genetic Code
• Amino acids
are coded for
by a triplet of
DNA
nucleotides
called a codon.
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Genetic Code
• Marshall Nirenberg and
Heinrich Matthaei
determined the first
codon for an amino acid.
It was found that UUU
coded for the amino acid
phenylalanine by
creating mRNA entirely
of uracil. The mRNA
(UUU..UUU….) added it to a test tube with amino acids,
ribosomes, RNA polymerase and other needed materials.
It resulted in a protein made of only phenylalanine. Further
research determined the rest of the code.
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Genetic Code
•
The code has redundancy
(GGU, GGC, GGA, and
GGG); all code for the
amino acid glycine.
•
Each codon only codes
for one amino acid.
•
The code is a universal
code meaning almost all
cells use the same code.
A eukaryotic gene can be
expressed in a
prokaryotic cell.
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Specifying or Coding for a Polypeptide
This gene designates that the following
peptide chain be made with the amino acids
in this particular order.
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Transcription
Overview
Transcription-RNA
synthesis from a
DNA template
• Initiation
• Elongation
• Termination
• RNA processing
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Initiation
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Elongation
• Elongation- RNA polymerase unwinds the
DNA and base pairs RNA nucleotides to
the DNA gene. RNA is made 5′ → 3′ so
the DNA gene is 3′ → 5′.
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Termination
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RNA Processing
Eukaryotic RNA processing
• 5' cap is added.
• At the 3' end 30-200 adenine nucleotides are added
(poly-A-tail).
• Introns are removed
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Removing Introns
•
A spliceosome
removes the introns.
•
Spliceosomes are
composed of smaller
particles called snRNP
(made of proteins and
snRNA).
•
The spliceosome will
splice the intron at a
specific RNA
sequence releasing a
"lariat" RNA.
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RNA Processing
• Different exons are recombined in different ways
for certain mRNAs. This increases the number
of different proteins.
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Exon Shuffling and Different Proteins
•
Proteins often have a
modular architecture
consisting of discrete
regions called domains
•
In many cases, different
exons code for the
different domains in a
protein
•
Exon shuffling may result
in the evolution of new
proteins.
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Ready for Translation
• This mRNA has been processed and is
called mature mRNA. It is ready to go to
the cytoplasm for translation.
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