Download Biol 115 DNA, the Thread of Life

Biol115
The Thread of Life
Lecture 7
Lost in translation:
mRNA translation & protein synthesis
“If the dream is a translation of waking life, waking life is
also a translation of the dream.”
~ Rene Magritte
Principles of Biology
• Chapter ‘Gene Expression’
• Chapter ‘Translation’
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Objectives
• Describe the molecular structures involved in translation.
• Explain the process of translation in detail.
• Explain how newly synthesised proteins are delivered to
different destinations (membranes and extracellular space).
• Key terms: anticodon, elongation factor, initiation factor,
polyribosome, release factor, ribosome, signal peptide, signalrecognition particle (SRP), transfer RNA (tRNA)
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Basic principles of translation
• Translation is the synthesis of a polypeptide, which occurs
under the direction of mRNA
• Ribosomes are the sites of translation
• In a eukaryotic cell, the nuclear envelope separates transcription
from translation
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Translation
Conversion from nucleotides, the “language” of mRNA, to amino acids,
the “language” of proteins.
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Codons: triplets of bases
• There are 20 amino acids, but
there are only four nucleotide
bases in DNA
• The flow of information from gene
to protein is based on a triplet
code: a series of non-overlapping,
three-nucleotide words (codons)
• Therefore, 43 (= 64) codons to
specify 64 amino acids.
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Cracking the code
• All 64 codons were deciphered by the
mid-1960s
• Of the 64 triplets, 61 code for amino
acids; 3 triplets are “stop” signals to
end translation
• One codon, AUG, is the universal
“start” codon.
• The genetic code is redundant but
not ambiguous; no codon specifies
more than one amino acid
The amino acids specified by each mRNA codon. Multiple codons can code for the same amino acid.
The codons are written 5' to 3', as they appear in the mRNA. AUG is an initiation codon; UAA, UAG, and
UGA are termination (stop) codons.
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Evolution of the genetic code
• The genetic code is nearly
universal, shared by the simplest
bacteria to the most complex
animals
• Genes can be transcribed and
translated after being
transplanted from one species to
another
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Molecular components of translation
• mRNA: A cell translates an mRNA message into protein with
the help of transfer RNA (tRNA)
• tRNA: Molecules of tRNA are not identical:
• Each carries a specific amino acid on one end
• Each has an anticodon on the other end; the anticodon base-pairs
with a complementary codon on mRNA
• Ribosomes and ribosomal RNAs
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The structure and function
of transfer RNA
A
C
C
• A tRNA molecule
consists of a single
RNA strand that is only
about 80 nucleotides
long
• Contains an amino
acid attachment site
and an anticodon that
recognizes the
complementary codon
on the mRNA.
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Transfer RNA (tRNA)
Adapter molecules that recognize codons in the mRNA and link them
to specific amino acids.
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Ribosomes
• Ribosomes facilitate
specific coupling of tRNA
anticodons with mRNA
codons in protein
synthesis
• The two ribosomal
subunits (large and small)
are made of proteins and
ribosomal RNA (rRNA)
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The amazing complexity of ribosomes
http://bass.bio.uci.edu/~hudel/bs9
9a/lecture22/ribosome70s_a.gif
http://bass.bio.uci.edu/~hudel/bs99a/lecture22/exit_tunnel.jpg
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Ribosomes
• A ribosome has three binding
sites for tRNA:
• The P site holds the tRNA
that carries the growing
polypeptide chain
• The A site holds the tRNA
that carries the next amino
acid to be added to the chain
• The E site is the exit site,
where discharged tRNAs
leave the ribosome
tRNAs move through these
sites in this order.
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Building a polypeptide
• The three stages of translation:
• Initiation
• Elongation
• Termination
• All three stages require protein “factors” that aid in the translation
process
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A gene is expressed through the
processes of transcription and translation.
During transcription, the enzyme
RNA polymerase (green) uses
DNA as a template to produce a
pre-mRNA transcript (pink). The
pre-mRNA is processed to form a
mature mRNA molecule that can
be translated to build the protein
molecule (polypeptide) encoded
by the original gene.
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A DNA transcription unit
A DNA transcription unit is composed, from its 3' to 5' end, of an RNA-coding region (pink rectangle)
flanked by a promoter region (green rectangle) and a terminator region (black rectangle). 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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Translation Begins After the Assembly
of a Complex Structure
When translation begins, the small subunit of the ribosome and an
initiator tRNA molecule assemble on the mRNA transcript. The small
subunit of the ribosome has three binding sites: an amino acid site
(A), a polypeptide site (P), and an exit site (E). The initiator tRNA
molecule carrying the amino acid methionine binds to the AUG start
codon of the mRNA transcript at the ribosome’s P site where it will
become the first amino acid incorporated into the growing polypeptide
chain. 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. In eukaryotes, the free
initiator tRNA first binds the small ribosomal subunit to form a
complex. The complex then binds the mRNA transcript, so that the
tRNA and the small ribosomal subunit bind the mRNA simultaneously.
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Ribosome association and
initiation of translation
• The initiation stage of translation brings together mRNA, a
tRNA with the first amino acid, and the two ribosomal
subunits
• First, a small ribosomal subunit binds with mRNA and a
special initiator tRNA
• Then the small subunit moves along the mRNA until it
reaches the start codon (AUG)
• Proteins called initiation factors bring in the large subunit that
completes the translation initiation complex
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Insert Fig. 17-17 P342
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Elongation of the polypeptide chain
• During the elongation stage, amino acids are added one by
one to the preceding amino acid
• Each addition involves proteins called elongation factors and
occurs in three steps: codon recognition, peptide bond
formation, and translocation
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Insert Fig. 17-18 P343
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Termination of translation
• Termination occurs when a stop codon in the mRNA reaches the
A site of the ribosome
• The A site accepts a protein called a release factor
• The release factor causes the addition of a water molecule
instead of an amino acid
• This reaction releases the polypeptide, and the translation
assembly then comes apart
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Insert Fig. 17-19 P344
Occurs when a stop codon is encountered. Release factors bind to the stop
codon and break the bond between the tRNA and the polypeptide.
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Polyribosomes
• A number of ribosomes can
translate a single mRNA
simultaneously, forming a
polyribosome (or polysome)
• Polyribosomes enable a cell to
make many copies of a
polypeptide very quickly
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Targeting polypeptides to specific
locations
• Two populations of ribosomes are evident in cells: free ribsomes
(in the cytosol) and bound ribosomes (attached to the ER)
• Free ribosomes mostly synthesise proteins that function in the
cytosol
• Bound ribosomes make proteins of the endomembrane system
and proteins that are secreted from the cell
• Ribosomes are identical and can switch from free to bound
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Targeting polypeptides to specific
locations
• Polypeptide synthesis always begins in the cytosol
• Synthesis finishes in the cytosol unless the polypeptide signals
the ribosome to attach to the ER
• Polypeptides destined for the ER or for secretion are marked by a
signal peptide
• A signal-recognition particle (SRP) binds to the signal peptide
• The SRP brings the signal peptide and its ribosome to the ER
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Insert Fig. 17-21 P345
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You should now be able to:
1. Compare transcription and translation in bacteria and eukaryotes.
2. Explain what it means to say that the genetic code is redundant
and unambiguous.
3. Include the following terms in a description of translation: tRNA,
wobble, ribosomes, initiation, elongation, and termination.
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