BIL 255 STUDYS CORRECT EXAMS QUESTIONS
AND ANSWERS SURE A+
✔✔Aminoacyl-tRNA - ✔✔•A tRNA with an amino acid covalently bonded to it's 3' end
•This is made by an animoacyl-tRNA synthetase specific to the amino acid being
attached
•Has an anticodon loop, that is complementary to the mRNA that corresponds to the
mRNA
✔✔Aminoacyl-tRNA synthetase - ✔✔•Enzymes couple tRNA to correct amino acid
•One for each amino acid (*highly specificity*)
•Enzyme binds very tightly to the amino acid and the tRNA, which reduces mistakes
(*proof-reading*)
•Can only add a smaller, similar amino acid mistakingly (ex. valine in place of
isoleucine)
-There is an editing site where the smaller amino acid will fit into and this will indicate to
the tRNA that the incorrect amino acid was binded to, so it needs to be removed
•Error rate of 1 in 10,000
•There are 20 different tRNA synthetase, one for each amino acid
✔✔A researcher chemically converts tRNACys to tRNAAla and adds the product to a
cell-free translation system. Consequently, every cysteine in the protein was converted
to Alanine. This shows: - ✔✔b) "Proofreading" doesn't occur in the ribosome
✔✔Cell-free translation system - ✔✔•In a test tube, you can add ribosomes and the
other necessary components, and proteins will be made
✔✔Ribosome - ✔✔•Cellular machines that attach themselves to mRNA and then
capture the correct tRNA, then position them and form the covalent bods with the amino
acids and make the peptide bonds
•Ribosome decodes mRNA message
•Consist of *Ribosomal RNA* (needed for function) and Ribosomal Proteins (used for
structure)
,•Two subunits:
-Prok (70S): 50S + 30S
- Euk (80S): 60S + 40S
-S is the sedimentation rates and correlates with the shape and mass
•Eukaryote ribosomes are super heavy, the large subunit has 2,800,000 daltons (~40
proteins + 3 RNA molecules) and the small subunit has 1,400,000 daltons (~33 proteins
+ 1 RNA molecules) and are 4,200,000 daltons combined
✔✔Why is ribosome considered a ribozyme? - ✔✔• rRNAs fold into highly compact
structure
• Ribosomal proteins (*mostly for structure*) bind the outside of rRNAs
• *Catalytic site in 23S rRNA* (part of large subunit of ribosome); where the peptide
bonds are formed for the nascent proteins
✔✔How does a ribosome work? - ✔✔•Ribosome has one mRNA binding site and three
tRNA binding sites
• mRNA binding site on small subunit
• tRNA binding sites and peptide canal on large subunit
• tRNA sites:
- A-site: aminoacyl-tRNA
- P-site: peptidyl-tRNA
- E-site: exit
•First the large subunit comes in
✔✔Translation steps - ✔✔1. *Initiation*
•The *small ribosomal subunit* binds with *mRNA* and a special initiator RNA
•The initiator tRNA carries the amino acid *methionine*
•Then the small subunit moves along the mRNA until it reaches the start codon (AUG);
it *starts at the P-site*
•Proteins called *initiation factors* bring in the large subunit that completes the
*translation initiation complex*
2. Elongation
•During elongation, amino acids are added one by one to the *C-terminus* of the
growing chain
•Each addition involves proteins called elongation factors
•Elongation occurs in three steps: codon recognition, peptide bond formation, and
translocation
•Energy expenditure occurs in the first and third steps
3. Termination
•Elongation continues until a *stop codon* in the mRNA reaches the A site
•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,
releasing the polypeptide and the translation assembly
, ✔✔Initiation of Translation - ✔✔• The mRNA is scanned for the first AUG (methionine)
by:
- Small ribosomal subunit
- tRNAMet
- Initiation factors
(which assemble together first)
• The complex then binds to the AUG and slides towards the 3' end until it reaches the
AUG of the mRNA and the methionine from the tRNAMet starts off the polypeptide and
initial factors dissociate
• *Prokaryotes*: Specific sequence prior to AUG aids in finding translation initiation site
since there's multiple proteins in one mRNA (polycistronic mRNAs). So the *ribosomes
will bind directly to a ribosome-binding site* preceding the initiation codon
✔✔Step 1 of Translation - ✔✔I. Spent tRNA in E-site released and aminoacyltRNAEF-
Tu bound to GTP enters A-site
II. If tRNA anticodon matches mRNA codon, then *EF-Tu* hydrolyses GTP into GDP
and Pi , aminoacyl-tRNA remains bound to codon in the A-site
✔✔Step 2 of Translation - ✔✔I. Carboxyl group is uncoupled from tRNA in P-site and
transferred to amino group of aminoacyltRNA in A-site
✔✔Step 3 of Translation - ✔✔I. Ribosome is reset by the translocation of Large subunit
followed by the translocation (shifts to the 3' end) of Small subunit;
*Mediated by hydrolysis of GTP by EF-G*
✔✔Termination of Translation - ✔✔• *Release factor* recognizes stop codon (not tRNA)
• *Ribosome transfers last peptidyl bond to water*
• Protein is no longer tethered to tRNA and is released
• Ribosome disassembles
✔✔Does it require energy for the peptide bonds for amino acids to connect to each
other? - ✔✔False, they are spontaneous and happen automatically
✔✔Where is energy required during translation? - ✔✔When the aminoacyl-tRNA binds
to the codon after *EF-Tu* hydrolyses GTP into GDP and Pi and leaves, then during
translocation when *EF-G* hydrolyzes GDP
✔✔How do antibiotics work? - ✔✔•Prevent growth by stopping some cellular process:
Cell wall formation
DNA replication
Protein production
etc.
AND ANSWERS SURE A+
✔✔Aminoacyl-tRNA - ✔✔•A tRNA with an amino acid covalently bonded to it's 3' end
•This is made by an animoacyl-tRNA synthetase specific to the amino acid being
attached
•Has an anticodon loop, that is complementary to the mRNA that corresponds to the
mRNA
✔✔Aminoacyl-tRNA synthetase - ✔✔•Enzymes couple tRNA to correct amino acid
•One for each amino acid (*highly specificity*)
•Enzyme binds very tightly to the amino acid and the tRNA, which reduces mistakes
(*proof-reading*)
•Can only add a smaller, similar amino acid mistakingly (ex. valine in place of
isoleucine)
-There is an editing site where the smaller amino acid will fit into and this will indicate to
the tRNA that the incorrect amino acid was binded to, so it needs to be removed
•Error rate of 1 in 10,000
•There are 20 different tRNA synthetase, one for each amino acid
✔✔A researcher chemically converts tRNACys to tRNAAla and adds the product to a
cell-free translation system. Consequently, every cysteine in the protein was converted
to Alanine. This shows: - ✔✔b) "Proofreading" doesn't occur in the ribosome
✔✔Cell-free translation system - ✔✔•In a test tube, you can add ribosomes and the
other necessary components, and proteins will be made
✔✔Ribosome - ✔✔•Cellular machines that attach themselves to mRNA and then
capture the correct tRNA, then position them and form the covalent bods with the amino
acids and make the peptide bonds
•Ribosome decodes mRNA message
•Consist of *Ribosomal RNA* (needed for function) and Ribosomal Proteins (used for
structure)
,•Two subunits:
-Prok (70S): 50S + 30S
- Euk (80S): 60S + 40S
-S is the sedimentation rates and correlates with the shape and mass
•Eukaryote ribosomes are super heavy, the large subunit has 2,800,000 daltons (~40
proteins + 3 RNA molecules) and the small subunit has 1,400,000 daltons (~33 proteins
+ 1 RNA molecules) and are 4,200,000 daltons combined
✔✔Why is ribosome considered a ribozyme? - ✔✔• rRNAs fold into highly compact
structure
• Ribosomal proteins (*mostly for structure*) bind the outside of rRNAs
• *Catalytic site in 23S rRNA* (part of large subunit of ribosome); where the peptide
bonds are formed for the nascent proteins
✔✔How does a ribosome work? - ✔✔•Ribosome has one mRNA binding site and three
tRNA binding sites
• mRNA binding site on small subunit
• tRNA binding sites and peptide canal on large subunit
• tRNA sites:
- A-site: aminoacyl-tRNA
- P-site: peptidyl-tRNA
- E-site: exit
•First the large subunit comes in
✔✔Translation steps - ✔✔1. *Initiation*
•The *small ribosomal subunit* binds with *mRNA* and a special initiator RNA
•The initiator tRNA carries the amino acid *methionine*
•Then the small subunit moves along the mRNA until it reaches the start codon (AUG);
it *starts at the P-site*
•Proteins called *initiation factors* bring in the large subunit that completes the
*translation initiation complex*
2. Elongation
•During elongation, amino acids are added one by one to the *C-terminus* of the
growing chain
•Each addition involves proteins called elongation factors
•Elongation occurs in three steps: codon recognition, peptide bond formation, and
translocation
•Energy expenditure occurs in the first and third steps
3. Termination
•Elongation continues until a *stop codon* in the mRNA reaches the A site
•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,
releasing the polypeptide and the translation assembly
, ✔✔Initiation of Translation - ✔✔• The mRNA is scanned for the first AUG (methionine)
by:
- Small ribosomal subunit
- tRNAMet
- Initiation factors
(which assemble together first)
• The complex then binds to the AUG and slides towards the 3' end until it reaches the
AUG of the mRNA and the methionine from the tRNAMet starts off the polypeptide and
initial factors dissociate
• *Prokaryotes*: Specific sequence prior to AUG aids in finding translation initiation site
since there's multiple proteins in one mRNA (polycistronic mRNAs). So the *ribosomes
will bind directly to a ribosome-binding site* preceding the initiation codon
✔✔Step 1 of Translation - ✔✔I. Spent tRNA in E-site released and aminoacyltRNAEF-
Tu bound to GTP enters A-site
II. If tRNA anticodon matches mRNA codon, then *EF-Tu* hydrolyses GTP into GDP
and Pi , aminoacyl-tRNA remains bound to codon in the A-site
✔✔Step 2 of Translation - ✔✔I. Carboxyl group is uncoupled from tRNA in P-site and
transferred to amino group of aminoacyltRNA in A-site
✔✔Step 3 of Translation - ✔✔I. Ribosome is reset by the translocation of Large subunit
followed by the translocation (shifts to the 3' end) of Small subunit;
*Mediated by hydrolysis of GTP by EF-G*
✔✔Termination of Translation - ✔✔• *Release factor* recognizes stop codon (not tRNA)
• *Ribosome transfers last peptidyl bond to water*
• Protein is no longer tethered to tRNA and is released
• Ribosome disassembles
✔✔Does it require energy for the peptide bonds for amino acids to connect to each
other? - ✔✔False, they are spontaneous and happen automatically
✔✔Where is energy required during translation? - ✔✔When the aminoacyl-tRNA binds
to the codon after *EF-Tu* hydrolyses GTP into GDP and Pi and leaves, then during
translocation when *EF-G* hydrolyzes GDP
✔✔How do antibiotics work? - ✔✔•Prevent growth by stopping some cellular process:
Cell wall formation
DNA replication
Protein production
etc.
