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Translation-1 [v6.0]
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Translation
A) Components
mRNA - conveys the genetic code to site of protein synthesis
ribosomes - organelles where protein synthesis occurs
a. two parts - large and small subunits- that are characterized by "s" values
b. bacterial ribosomes are typically 70s when the two components are combined; the small
subunit is 30 s and the large 50 s when measured apart.
c. ribosomes in eukaryotic cytoplasm are slightly larger; 80s overall with 35s and 60 s
subunits
d. each component has a long piece of RNA (rRNA) that serves as a scaffolding for 20 or
more proteins
bacterial rRNAs are 16s and 23s while those in eukaryotic ribosomes are 18s and
28s in the Large and Small subunits, respecttively
there is also a small 5s rRNA that helps link the subunits (and a 5.8s rRNA in
eukaryotes)
e, rRNA is coded by multiple genes in the genome
the genes are tandomly arrayed, one after the other with a short spacer between
(7 copies in E. coli, many copies in eukaryotes where they form the nucleolus).
each rRNA gene is transcribed into one long pre-rRNA that contains all three
subunits
the large, small, and 5s rRNA fragments are released from the long transcript by
specific enzymes
• tRNAs transfer amino acids to the ribosome
each tRNA carries a specific amino acid
activating enzymes connect the amino acid to the tail of the tRNA with a high energy
bond
each tRNA-amino acid combination requires a different activation enzyme
the energy comes from ATP (the activation uses 2 phosphate high energy
bonds.
a. each tRNA is a single strand about 77 bases in length but flolds back on itself by
hydrogen bonding into a "cloverleaf" structure\
b. the 3 bases at the end of the middle leaf are called "anticodons"; they form base
pairs with the code words (codons) in the mRNA
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c.
(anticodon)
"charged tRNA" carrying histidine
d.) tRNAs are also coded by DNA, with several copies per genome and are also
modified extensively after transcription of the "pre-tRNAs".
1. ) Mechanism (recall that translation takes the information from a polymer of nucleotides
to make a polymer of amino acids)
1. • initiation
1. The 5' leader of the message attaches to the small subunit of a ribosome (in
prokaryotes, base pairing between the 5' mRNA leader and an exposed section of
the 16s rRNA is asssted by IF1).
2. In eukaryotes, one of the initiation factors binds the "CAP" at the beginning of
mRNA to get it located onto ribosomes
3. In the presence of additional initiation factors, the large ribosomal subunit is
added, which aligns the first AUG in the message with a P site of the ribosome
4. A tRNA called f-met-tRNA is postioned on the ribosome so that it's anticodon
pairs with the AUG in the mRNA.
in prokaryotes, the tRNA carries a modified methionine that has a C=O
attached to its amino group (= formyl methionine or f-met)
In eukaryotes, the f-met tRNA is used to start but the met is not formylated.
4. The next tRNA with an anticodon complementary to the next codon in the message is
positioned in the A site. This requires an elongation factor and uses a GT; for animations of
the events in translation see:
http://www.dnalc.org/resources/3d/16-translation-advanced.html
• elongation
1. the high energy bond on the last tRNA used holds the growing chain of
amino acids in the P site and is used to make a new peptide bond by linking
to the amino group of the amino acid on the tRNA in the A site.
2. the now-uncharged tRNA in the P site is released for re-use
3. the peptide bond is formed by an enzyme called peptidul synthase which
includes both protein and RNA from the large subunit, making it a
"ribozyme"
4. the tRNA with the now-longer chain of amino acids moves from the A site
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to the P site with the aid of elongation factor G (EFG)
5. the energy needed to break the H-bonds for moving tRNAs and for inching
the message along the ribosome comes from GTP, not ATP
6. once enough of the message has been read, another ribosome can attach
through the initiation process and start translating the same message
7. A total of 4 high energy phosphates are used for each amino acid added to
the chain
8.
Stop action photo of translation with the 6th codon and cognate tRNA in position for
making a peptide bond
• termination
whenever one of 3 codons, UAG, UUA, UGA enters the A-site of the
ribosome, translation will end
these 3 codons are called stop codons for their effect on translaton, or
nonsense codons since they do not call for amino acids.
there are no tRNAs with corresponding anticodons
a release factor enters the A site and triggers the peptidyl synthase,
releasing the polypeptide chain
In E. coli, UAA and UAG are recognized by release factor 1 (RF1)
while UGA uses RF2
in eukaryotes one RF functions far all 3 stop codons
the ribosome disassembles and can be used over and over
once translation has proceded far enough, another ribosome can
attach at the initiation site.
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