BIOCHEM MODULE 8 EXAM COMPLETE
QUESTIONS AND EXPERTLY VERIFIED
ANSWERS 2026
⩥ How is the expression of the lac operon inhibited by the lac repressor?
Answer: Expression of the lac operon is inhibited by the lac repressor
because in the absence of lactose or galactose in the media, the lac
operon is turned off. The reason for this is because the lac repressor
binds to the operator which is between the promoter and the beginning
of the gene. Because of this, the RNA polymerase is prevented from
transcribing the Z, Y, and A genes. When there is lactose present in the
media, allolactose (a metabolite of lactose) binds to the repressor protein
and causes the repressor to dissociate from the operator, enabling
transcription again.
⩥ Which two characteristics differentiate promoters from enhancers?
Answer: 1. enhancers can be located in a gene, or they can be located
millions of base pairs away from a gene, whereas promoters have a
relatively fixed position in relation to the start point of the gene
2. Enhancers also undergo something called DNA looping, which
promoters do not
⩥ Name the 3 different eukaryotic RNA polymerases and describe the
specific genes they transcribe. Answer: 1. RNA polymerase I:
synthesizes ribosomal RNAs (specifically 18S, 5.8S and 28s)
,2. RNA polymerase II: synthesizes the mRNA and some small nuclear
RNAs
3. RNA polymerase III: transcribes the tRNA genes
⩥ What role do small nucleolar ribonucleoproteins (snoRNPs) play in
RNA processing? Answer: snoRNPs are involved with recognizing the
particular sequences in the tRNA transcript and making modifications
(they're methylated)
⩥ What is the amino acid error rate in protein synthesis? Is this rate
logical in evolutionary terms of function? Answer: About 1/10,000
events. This error rate is logical because most genes are only about 1-
2,000 nucleotides long so there won't be mistakes incorporated into
complete proteins very often.
⩥ What is the first step in the elongation cycle of protein synthesis?
Answer: The first step in the elongation cycle is that the transpeptidation
moves the end of the nascent polypeptide chain to the A-site. Then
there's a translocase, and EFG (elongation factor G) binds and
hydrolyzes GTP to translocate the mRNA, moving it one codon down.
Now, the tRNA that was in the A site is moved to the P-site, the tRNA
that was in the P-site moves into the E-site (exit site), and then can
dissociate from the ribosome.
⩥ Translation can be considered to have three phases or steps: initiation,
protein synthesis, and termination. Please give a detailed summary of the
factors and molecules involved with each step and, where appropriate,
,compare/contrast the differences between prokaryotic and eukaryotic
translation. Answer: Initiation
--> Prokaryotic: prokaryotic translation initiation starts with the 30S
initiation complex staged at the proper AUG start codon by
complementary binding of 16S rRNA and the Shine-Delgarno sequence
in mRNA. IF2-GTP is hydrolyzed when fMet-tRNA binds to AUG and
then IF1 and IF3 are released and the 50S subunit binds to form a 70s
initiation complex.
--> Eukaryotic: in eukaryotic translation you have 80S ribosomes which
are composed of 60S and 40S subunits. The ribosomal initiation
complex is formed on the mRNA cap and then scans down the mRNA in
search of the first AUG, which is where it will begin translation. There is
a special initiator called tRNAi which the first amino acid, methionine,
is charged onto. The binding of Met-tRNA allows the 60S subunit to
bind, which in turn allows the 80S ribosome complex to form.
The main differences between eukaryotic translation initiation and
bacterial translation initiation include the fact that in eukaryotes, the
ribosomal initiation complex forms on the 5' cap of mRNA, translation
begins at the first AUG in the mRNA, the first amino acid is methionine
instead of n-formylmethionine and there is no equivalent of the Shine-
Delgarno sequence
Protein synthesis
--> Prokaryotic: charged tRNA complexed with EF-TU is delivered to
the ribosome acceptor site. EF-TU hydrolyzes bound GTP and
dissociates if there is a correct tRNA fit. Transpeptidation moves the end
of the nascent peptide chain to the tRNA in the acceptor site. EF-G binds
and together with GTP hydrolysis translocates the mRNA one codon
, moving the peptidyl-tRNA to the P-site, and the discharged tRNA to the
E-site.
--> Eukaryotic: ribosomal initiation complex forms on the mRNA cap
and scans down the RNA for the first AUG. First amino acid is a
methionine charged onto as special initiator called tRNAi. Binding of th
⩥ The DNA strand with the same sequence as the mRNA (except with U
in RNA and T in DNA) is called the:
A) sense strand
B) coding strand
C) antisense strand
D) template strand
E) A and B Answer: E) A and B
⩥ What is the mRNA sequence transcribed from the following DNA
strand? __________________________
5´-GGACTTAATTAACCG-3´
3´-CCTGAATTAATTGGC-5´ Antisense Strand Answer: 5'-
GGACUUAAUUAACCG-3'
⩥ What are the two regions of conserved DNA sequences that are used
to bind RNA polymerase and define promoter strength?
QUESTIONS AND EXPERTLY VERIFIED
ANSWERS 2026
⩥ How is the expression of the lac operon inhibited by the lac repressor?
Answer: Expression of the lac operon is inhibited by the lac repressor
because in the absence of lactose or galactose in the media, the lac
operon is turned off. The reason for this is because the lac repressor
binds to the operator which is between the promoter and the beginning
of the gene. Because of this, the RNA polymerase is prevented from
transcribing the Z, Y, and A genes. When there is lactose present in the
media, allolactose (a metabolite of lactose) binds to the repressor protein
and causes the repressor to dissociate from the operator, enabling
transcription again.
⩥ Which two characteristics differentiate promoters from enhancers?
Answer: 1. enhancers can be located in a gene, or they can be located
millions of base pairs away from a gene, whereas promoters have a
relatively fixed position in relation to the start point of the gene
2. Enhancers also undergo something called DNA looping, which
promoters do not
⩥ Name the 3 different eukaryotic RNA polymerases and describe the
specific genes they transcribe. Answer: 1. RNA polymerase I:
synthesizes ribosomal RNAs (specifically 18S, 5.8S and 28s)
,2. RNA polymerase II: synthesizes the mRNA and some small nuclear
RNAs
3. RNA polymerase III: transcribes the tRNA genes
⩥ What role do small nucleolar ribonucleoproteins (snoRNPs) play in
RNA processing? Answer: snoRNPs are involved with recognizing the
particular sequences in the tRNA transcript and making modifications
(they're methylated)
⩥ What is the amino acid error rate in protein synthesis? Is this rate
logical in evolutionary terms of function? Answer: About 1/10,000
events. This error rate is logical because most genes are only about 1-
2,000 nucleotides long so there won't be mistakes incorporated into
complete proteins very often.
⩥ What is the first step in the elongation cycle of protein synthesis?
Answer: The first step in the elongation cycle is that the transpeptidation
moves the end of the nascent polypeptide chain to the A-site. Then
there's a translocase, and EFG (elongation factor G) binds and
hydrolyzes GTP to translocate the mRNA, moving it one codon down.
Now, the tRNA that was in the A site is moved to the P-site, the tRNA
that was in the P-site moves into the E-site (exit site), and then can
dissociate from the ribosome.
⩥ Translation can be considered to have three phases or steps: initiation,
protein synthesis, and termination. Please give a detailed summary of the
factors and molecules involved with each step and, where appropriate,
,compare/contrast the differences between prokaryotic and eukaryotic
translation. Answer: Initiation
--> Prokaryotic: prokaryotic translation initiation starts with the 30S
initiation complex staged at the proper AUG start codon by
complementary binding of 16S rRNA and the Shine-Delgarno sequence
in mRNA. IF2-GTP is hydrolyzed when fMet-tRNA binds to AUG and
then IF1 and IF3 are released and the 50S subunit binds to form a 70s
initiation complex.
--> Eukaryotic: in eukaryotic translation you have 80S ribosomes which
are composed of 60S and 40S subunits. The ribosomal initiation
complex is formed on the mRNA cap and then scans down the mRNA in
search of the first AUG, which is where it will begin translation. There is
a special initiator called tRNAi which the first amino acid, methionine,
is charged onto. The binding of Met-tRNA allows the 60S subunit to
bind, which in turn allows the 80S ribosome complex to form.
The main differences between eukaryotic translation initiation and
bacterial translation initiation include the fact that in eukaryotes, the
ribosomal initiation complex forms on the 5' cap of mRNA, translation
begins at the first AUG in the mRNA, the first amino acid is methionine
instead of n-formylmethionine and there is no equivalent of the Shine-
Delgarno sequence
Protein synthesis
--> Prokaryotic: charged tRNA complexed with EF-TU is delivered to
the ribosome acceptor site. EF-TU hydrolyzes bound GTP and
dissociates if there is a correct tRNA fit. Transpeptidation moves the end
of the nascent peptide chain to the tRNA in the acceptor site. EF-G binds
and together with GTP hydrolysis translocates the mRNA one codon
, moving the peptidyl-tRNA to the P-site, and the discharged tRNA to the
E-site.
--> Eukaryotic: ribosomal initiation complex forms on the mRNA cap
and scans down the RNA for the first AUG. First amino acid is a
methionine charged onto as special initiator called tRNAi. Binding of th
⩥ The DNA strand with the same sequence as the mRNA (except with U
in RNA and T in DNA) is called the:
A) sense strand
B) coding strand
C) antisense strand
D) template strand
E) A and B Answer: E) A and B
⩥ What is the mRNA sequence transcribed from the following DNA
strand? __________________________
5´-GGACTTAATTAACCG-3´
3´-CCTGAATTAATTGGC-5´ Antisense Strand Answer: 5'-
GGACUUAAUUAACCG-3'
⩥ What are the two regions of conserved DNA sequences that are used
to bind RNA polymerase and define promoter strength?
