Protein Synthesis
Protein Synthesis
 Garrod hypothesized that genes specify
proteins by looking at metabolic disorders;
formulated one gene – one enzyme
hypothesis
 Beadle and Tatum lent more support for this
hypothesis with their work on Neurospora
crassa
 Over time this hypothesis has been modified
first to one gene – one protein to now one
gene – one polypeptide
Protein Synthesis
 Transcription and translation are the
processes that link genes to proteins

Transcription is the synthesis of RNA using
DNA as the template
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Messenger RNA (mRNA) is the particular type of
RNA synthesized
Translation is the actual synthesis of proteins
using mRNA
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This takes place at ribosomes
In prokaryotes, transcription and translation are
taking place at the same time
In eukaryotes, transcription and translation are
separated by the nuclear membrane
Protein Synthesis
 In the genetic code, nucleotide triplets specify
amino acids
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The mRNA base triplets (or complementary
DNA base triplets) are called codons
Each codon represents one amino acid
There are 20 amino acids and 64 possible
combinations, so some amino acids are
represented by more than one codon
The codons are read in reading frames of
three bases at a time with no overlap
Transcription
 RNA polymerase unzips the DNA strands
apart and hooks together the DNA template in
the 3’ to 5’ direction
 RNA polymerase attaches to the promoter
region of DNA

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A promoter is a sequence of DNA nucleotides
that initiate transcription
In eukaryotes, proteins called transcription
factors enable RNA polymerase to bind to
DNA and initiate transcription
Transcription
 RNA polymerase moves along the DNA
molecule untwisting the helix and adding
nucleotides to the end of the mRNA molecule;
the DNA molecule reforms as the RNA
molecule “peels” away from its template
 Transcription continues until the RNA
polymerase transcribes a terminator
sequence from the DNA molecule
Transcription
 Eukaryotic cells modify RNA after
transcription
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A 5’cap is added to protect the mRNA
molecule from degradation and it signifies as
an attachment site for the ribosomes (for
translation)
A 3’ poly A tail is added to the also protect the
mRNA molecule and it facilitates the export of
mRNA from the nucleus
Transcription
 RNA splicing also
occurs
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Many segments of
RNA are not
translated; these
intervening
sequences (introns)
are removed from
the sequence
The expressed
sequences (exons)
are spliced together
Translation
 The interpreter for this portion
of protein synthesis is transfer
RNA (tRNA)

Each tRNA molecule has a
different amino acid attached
to it that is specified by the
anticodon on the other end

Anticodons are nucleotide
triplets that complement
codons on mRNA
Translation
 Translation takes place at the ribosomes
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Ribosomes are constructed of proteins and
ribosomal RNA (rRNA)
Consist of two subunits with a binding site for
mRNA and three binding sites for tRNA
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P site- holds growing polypeptide
A site- holds amino acid (tRNA molecule)
E site- exit site
Translation (Ribosome Binding Sites)
Translation
 Initiation of translation

mRNA (with the 5’ cap and AUG downstream)
and tRNA (with methionine) bind to the
ribosome
Translation
 Elongation
 Amino acids are added one by one to the
preceding amino acid in three steps
1.
2.
3.
Codon recogition – mRNA codon in the A site
forms hydrogen bonds with the anticodon tRNA
molecule
Peptide bond formation – rRNA molecule
catalyzes the formation of peptide bond
between amino acid in P site with amino acid in
A site
Translocation – tRNA in A site with attached
polypeptide chain is moved to P site; mRNA and
tRNA remain hydrogen bonded so mRNA
molecule moves along with tRNA molecule
Translation
 Termination

Elongation continues until the stop codon in
the mRNA molecule reaches the A site of the
ribosome