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DNA TRANSLATION
DNA to mRNA to Protein:
The genes in DNA encode protein molecules, which are the "workhorses" of
the cell, carrying out all the functions necessary for life
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In the simplest sense, expressing a gene means manufacturing its corresponding
protein, and this multilayered process has two major steps
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During transcription, the DNA of a gene
serves as a template for complementary base-pairing, and
an enzyme called RNA polymerase II catalyses the formation of a pre-mRNA
molecule, which is then processed to form mature mRNA (Figure 1)
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During transcription, the enzyme RNA polymerase (green) uses DNA as a
template to produce a pre-mRNA transcript (pink)
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Figure 1: A gene is expressed through the processes of transcription and
translation
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Each group of three bases in
mRNA constitutes a codon, and each codon specifies a particular amino acid
(hence, it is a triplet code)
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Figure 2: The amino acids specified by each mRNA codon
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The codons are written 5' to 3', as they appear in the mRNA
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Figure Detail
But where does translation take place within a cell? What individual substeps
are a part of this process? And does translation differ between prokaryotes and
eukaryotes? The answers to questions such as these reveal a great deal about the
essential similarities between all species
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In eukaryotes, mature mRNA molecules must leave
the nucleus and travel to the cytoplasm, where the ribosomes are located
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In this situation, translation begins at the 5' end of the
mRNA while the 3' end is still attached to DNA
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Each subunit exists separately in
the cytoplasm, but the two join together on the mRNA molecule
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Within the ribosome, the mRNA and aminoacyl-tRNA complexes are held
together closely, which facilitates base-pairing
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The Beginning of mRNA Is Not Translated
Interestingly, not all regions of an mRNA molecule correspond to particular
amino acids
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This portion of
mRNA is located between the first nucleotide that is transcribed and the start
codon (AUG) of the coding region, and it does not affect the sequence of amino
acids in a protein (Figure 3)
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In bacteria, this site is
known as the Shine-Dalgarno box (AGGAGG), after scientists John Shine and
Lynn Dalgarno, who first characterized it
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In
bacterial mRNA, the 5' UTR is normally short; in human mRNA, the median
length of the 5' UTR is about 170 nucleotides
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Figure 3: A DNA transcription unit
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Regions to the left, or moving towards the
3' end, of the transcription start site are considered \"upstream;\" regions to the
right, or moving towards the 5' end, of the transcription start site are considered
\"downstream
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First, three initiation factor proteins (known as IF1, IF2, and IF3)
bind to the small subunit of the ribosome
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Figure 4: The translation initiation complex
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The small subunit of the
ribosome has three binding sites: an amino acid site (A), a polypeptide site (P),
and an exit site (E)
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Here, the initiator tRNA molecule is shown binding
after the small ribosomal subunit has assembled on the mRNA; the order in
which this occurs is unique to prokaryotic cells
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The complex
then binds the mRNA transcript, so that the tRNA and the small ribosomal
subunit bind the mRNA simultaneously
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In fact, if a large number of
proteins are sequenced and compared with their known gene sequences,
methionine (or formylmethionine) occurs at the N-terminus of all of them
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For example, many proteins begin with methionine
followed by alanine
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However, if the second amino acid is lysine, which is also frequently the
case, methionine is not removed (at least in the sample proteins that have been
studied thus far)
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, 1986)
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, 1986)
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Table 1: N-Terminal Sequences of Proteins
NTerminal
Sequence

Percent of
Prokaryotic
Proteins with This
Sequence

Percent of
Eukaryotic
Proteins with This
Sequence

MA*

28
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17%

MK**

10
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50%

MS*

9
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67%

MT*

7
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67%

* Methionine was removed in all of these proteins
** Methionine was not removed from any of these proteins
Once the initiation complex is formed on the mRNA, the large ribosomal
subunit binds to this complex, which causes the release of IFs (initiation
factors)
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The A (amino acid) site is the location at which the
aminoacyl-tRNA anticodon base pairs up with the mRNA codon, ensuring that
correct amino acid is added to the growing polypeptide chain
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Finally, the E (exit) site is the location
at which the "empty" tRNA sits before being released back into the cytoplasm
to bind another amino acid and repeat the process
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The ribosome is thus
ready to bind the second aminoacyl-tRNA at the A site, which will be joined to
the initiator methionine by the first peptide bond (Figure 5)
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The initiator tRNA molecule, carrying the methionine amino acid that will serve
as the first amino acid of the polypeptide chain, is bound to the P site on the

ribosome
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First,
the ribosome moves along the mRNA in the 5'-to-3'direction, which requires the
elongation factor G, in a process called translocation
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coli, these are called EF-Tu and EF-Ts), as well as
guanosine triphosphate (GTP) as an energy source for the process
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Next, peptide bonds between the now-adjacent first and second amino acids are
formed through a peptidyl transferase activity
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After the
peptide bond is formed, the ribosome shifts, or translocates, again, thus causing
the tRNA to occupy the E site
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In addition, the A site is now empty and ready to
receive the tRNA for the next codon
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At this point,
translation must be terminated, and the nascent protein must be released from
the mRNA and ribosome
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No tRNAs recognize these
codons
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Comparing Eukaryotic and Prokaryotic Translation
The translation process is very similar in prokaryotes and eukaryotes
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As previously noted, in bacteria, transcription and
translation take place simultaneously, and mRNAs are relatively short-lived
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