Theme 2: Transla-on
4.2 Transla+on: How Proteins Are Synthesized
• It is a protein's primary structure that governs how the molecule folds
into a stable 3-D configura>on
4.2.3 Transla+on consists of ini+a+on, elonga+on, and
termina+on.
◦ Ini>a>on: The ini>ator AUG codon is recognized and Met is
established as the first amino acid in the new polypep>de chain
• Requires ini+a+on factors that bind to the mRNA
▪ In eukaryotes, one group of ini>a>on factors binds to
the 5' cap that is added to the mRNA during
processing
• These recruit a small subunit of the ribosome
• Other ini>ator factors bring up a transfer RNA charged with
Met
• Imita>on complex then moves along the mRNA un>l it
encounters the first AUG triplet
▪ Posi>on of AUG established reading frame
• When first AUG codon is encountered, large ribosome
subunit joins the complex using energy from GTP hydrolysis,
the ini>a>on factors are released and then, the next tRNA is
ready to join the ribosome
▪ tRNAMet binds with the P (pep>dyl) site in the
ribosome and that the next tRNA in line comes in at
the A (aminoacyl) site
• Once new tRNA is in place, the bond connec>ng
the Met to its tRNA is transferred to the amino
group of the next amino acid in line as the first
pep>de bond is formed
• New pep>de is aUached to the tRNA in
the A site
, • Forma>on of the pep>de bond requires mul>ple proteins in
the large subunit, but an RNA in the large subunit is the
actual catalyst
• Ribosome shiWs one codon to the right- moving the
uncharged tRNAMet to the E site and the pep>de-bearing
tRNA to the P site, freeing the A site for the next charged
tRNA in line to come in
• Note: For prokaryotes, the transla>on ini>a>on complex will
assemble at 1/+ ribosome binding sites called Shine-
Dalgarno sequences which tend to be located a few bases
upstream of the AUG codon
▪ Ability for transla>on to occur along many regions of
polycistronic mRNA sequence, allows for specific open
reading frames for more than one protein on a single
mRNA strand
◦ Elonga>on: Successive amino acids are added one by one to the
growing chain
• Immediately aWer the forma>on of a new pep>de bond that
aUaches the growing polypep>de chain to the amino acid
linked to the tRNA in the A site, the small ribosomal subunit
moves one codon farther along the mRNA, the large subunit
shiWs to align with the small subunit, and the tRNA in the E
(exit) site ejects
• While there are three tRNA binding sites, only two are
occupied simultaneously
• Ribosome movement along the mRNA and forma>on of the
pep>de bonds require energy, which is obtained by breaking
the high-energy bonds of the molecule GTP bound with
proteins called elonga>on factors (Note: Each incoming
charged tRNA is delivered in associa>on with a GTP-bound
elonga>on factor)
▪ Specifically, when correct codon-an>codon pairing has
been made, GTP is hydrolyzed and aminoacyl end of
tRNA is released from the elonga>on factor)
4.2 Transla+on: How Proteins Are Synthesized
• It is a protein's primary structure that governs how the molecule folds
into a stable 3-D configura>on
4.2.3 Transla+on consists of ini+a+on, elonga+on, and
termina+on.
◦ Ini>a>on: The ini>ator AUG codon is recognized and Met is
established as the first amino acid in the new polypep>de chain
• Requires ini+a+on factors that bind to the mRNA
▪ In eukaryotes, one group of ini>a>on factors binds to
the 5' cap that is added to the mRNA during
processing
• These recruit a small subunit of the ribosome
• Other ini>ator factors bring up a transfer RNA charged with
Met
• Imita>on complex then moves along the mRNA un>l it
encounters the first AUG triplet
▪ Posi>on of AUG established reading frame
• When first AUG codon is encountered, large ribosome
subunit joins the complex using energy from GTP hydrolysis,
the ini>a>on factors are released and then, the next tRNA is
ready to join the ribosome
▪ tRNAMet binds with the P (pep>dyl) site in the
ribosome and that the next tRNA in line comes in at
the A (aminoacyl) site
• Once new tRNA is in place, the bond connec>ng
the Met to its tRNA is transferred to the amino
group of the next amino acid in line as the first
pep>de bond is formed
• New pep>de is aUached to the tRNA in
the A site
, • Forma>on of the pep>de bond requires mul>ple proteins in
the large subunit, but an RNA in the large subunit is the
actual catalyst
• Ribosome shiWs one codon to the right- moving the
uncharged tRNAMet to the E site and the pep>de-bearing
tRNA to the P site, freeing the A site for the next charged
tRNA in line to come in
• Note: For prokaryotes, the transla>on ini>a>on complex will
assemble at 1/+ ribosome binding sites called Shine-
Dalgarno sequences which tend to be located a few bases
upstream of the AUG codon
▪ Ability for transla>on to occur along many regions of
polycistronic mRNA sequence, allows for specific open
reading frames for more than one protein on a single
mRNA strand
◦ Elonga>on: Successive amino acids are added one by one to the
growing chain
• Immediately aWer the forma>on of a new pep>de bond that
aUaches the growing polypep>de chain to the amino acid
linked to the tRNA in the A site, the small ribosomal subunit
moves one codon farther along the mRNA, the large subunit
shiWs to align with the small subunit, and the tRNA in the E
(exit) site ejects
• While there are three tRNA binding sites, only two are
occupied simultaneously
• Ribosome movement along the mRNA and forma>on of the
pep>de bonds require energy, which is obtained by breaking
the high-energy bonds of the molecule GTP bound with
proteins called elonga>on factors (Note: Each incoming
charged tRNA is delivered in associa>on with a GTP-bound
elonga>on factor)
▪ Specifically, when correct codon-an>codon pairing has
been made, GTP is hydrolyzed and aminoacyl end of
tRNA is released from the elonga>on factor)
