Author: Philip Colls
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Protein synthesis booklet Revision questions   NAME:
Q1. (a)     Draw the general structure of an amino acid.
 
(1)
Table 1 shows mRNA codons and the amino acids coded for by each codon. It also shows some properties of the R group of each amino acid.
Table 1

Key to the properties of the R group of each amino acid
 
| No overall change | | Positively charged | | Negatively charged

(b)     The genetic code is described as degenerate.
What is meant by this? Use an example from Table 1 to illustrate your answer.
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(2)
A scientist investigated changes in the amino acid sequence of a human enzyme resulting from mutations. All these amino acid changes result from single base substitution mutations.
This enzyme is a polypeptide 465 amino acids long.
Table 2 shows the result of three of the base substitutions.
 
Table 2
Amino acid number | Correct amino acid | Amino acid inserted as a result of mutation
203 | Val | Ala
279 | Glu | Lys
300 | Glu | Lys
(c)     What is the minimum number of bases in the gene coding for this polypeptide?
Answer = ____________________(1)
(d)     Use information from Table 1 to tick (✔) one box that shows a single base substitution mutation in DNA that would result in a change from Val to Ala at amino acid number 203
 
CAA → CGA |
GUU → GCA |
GUU → GUC |
CAC → CGG |
(1)
(e)     A change from Glu to Lys at amino acid 300 had no effect on the rate of reaction catalysed by the enzyme. The same change at amino acid 279 significantly reduced the rate of reaction catalysed by the enzyme.
Use all the information and your knowledge of protein structure to suggest reasons for the differences between the effects of these two changes.
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____________________________________________________(3) (Total 8 marks)
Q2. (a)     Complete the table to show the differences between DNA, mRNA and tRNA.
 
Type of nucleic acid | Hydrogen bonds present () or not present () | Number of polynucleotide strands in molecule
DNA | |
mRNA | |
tRNA | |
(2)
(b)     The diagram shows the bases on one strand of a piece of DNA.
 
(i)      In the space below, give the sequence of bases on the pre-mRNA transcribed from this strand.

 
 
(2)
(ii)     In the space below, give the sequence of bases on the mRNA produced by splicing this piece of pre-mRNA.

 
(1)
(Total 5 marks)
Q3. The diagram shows part of a pre-mRNA molecule.
 
(a)     (i)      Name the two substances that make up part X.
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(ii)     Give the sequence of bases on the DNA strand from which this pre-mRNA has been transcribed.
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(b)     (i)      Give one way in which the structure of an mRNA molecule is different from the structure of a tRNA molecule.
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(ii)     Explain the difference between pre-mRNA and mRNA.
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(1)
(c)     The table shows the percentage of different bases in two pre-mRNA molecules.
The molecules were transcribed from the DNA in different parts of a chromosome.
 
Part of chromosome | Percentage of base
| A | G | C | U
Middle | 38 | 20 | 24 |
End | 31 | 22 | 26 |
(i)      Complete the table by writing the percentage of uracil (U) in the appropriate boxes.
(1)
(ii)     Explain why the percentages of bases from the middle part of the chromosome and the end part are different.
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(2)
(Total 7 marks)
Q4. The diagram below represents one process that occurs during protein synthesis.
 
(a)     Name the process shown.
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(1)
(b)     Identify the molecule labelled Q.
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(1)
(c)     In the diagram above, the first codon is AUG. Give the base sequence of:
the complementary DNA base sequence __________________________________
the missing anticodon _________________________________________________
(2)
The table below shows the base triplets that code for two amino acids.
 
Amino acid | Encoding base triplet
Aspartic acid | GAC, GAU
Proline | CCA, CCG, CCC, CCU
(d)     Aspartic acid and proline are both amino acids. Describe how two amino acids differ from one another. You may use a diagram to help your description.

 
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(1)
(e)     Deletion of the sixth base (G) in the sequence shown in the diagram above would change the nature of the protein produced but substitution of the same base would not. Use the information in the table and your own knowledge to explain why.
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__________________________________________________ (3)  (Total 8 marks)
Q5.
(a)     DNA is a polymer of nucleotides. Each nucleotide contains an organic base.
Explain how the organic bases help to stabilise the structure of DNA.
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(2)
(b)     Triplets of bases in a DNA molecule code for the sequence of amino acids in a polypeptide. The genetic code is frequently written as the three bases on mRNA that are complementary to a triplet on DNA. Table 1 shows what different combinations of bases on mRNA code for. The names of amino acids are abbreviated. For example, ‘Ala’ stands for alanine.
 
Table 1
First base | Second base | Third base
| Guanine (G) | Adenine (A) | Cytosine (C) | Uracil (U) |

G

GGG    Ala
 GGA    Gly
 GGC    Gly
 GGU    Gly

GAG    Glu
 GAA    Glu
 GAC    Asp
 GAU    Asp

GCG    Ala
 GCA    Ala
 GCC    Ala
 GCU    Ala

GUG    Val
 GUA    Val
 GUC    Val
 GUU    Val

G
A
C
U

A

AGG    Arg
 AGA    Arg
 AGC    Ser
 AGU    Ser

AAG    Lys
 AAA    Lys
 AAC    Asn
 AAU    Asn

ACG    Thr
 ACA    Thr
 ACC    Thr
 ACU    Thr

AUG    Met
 AUA    Iso
 AUC    Iso
 AUU    Iso

G
A
C
U

C

CGG    Arg
 CGA    Arg
 CGC    Arg
 CGU    Arg

CAG    Gln
 CAA    Gln
 CAC    Hist
 CAU    Hist

CCG    Pro
 CCA    Pro
 CCC    Pro
 CCU    Pro

CUG    Leu
 CUA    Leu
 CUC    Leu
 CUU    Leu

G
A
C
U

U

UGG    Trp
 UGA    stop
 UGC    Cyst
 UGU    Cyst

UAG    stop
 UAA    stop
 UAC    Tyr
 UAU    Tyr

UCG    Ser
 UCA    Ser
 UCC    Ser
 UCU    Ser

UUG    Leu
 UUA    Leu
 UUC    Phe
 UUU    Phe

G
A
C
U

Suggest one advantage of showing the genetic code as base sequences on mRNA, rather than triplets on DNA.
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(c)     What name is given to a group of three bases on mRNA that codes for an amino acid?
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(d)     Use information from Table 1 to explain why the genetic code is described as degenerate.
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(e)     Suggest the role of the mRNA base triplets UGA, UAG and UAA.
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(f)      Table 2 shows the sequence of mRNA bases forming part of a single gene.
 
Table 2
Base on DNA template | | | | | | | | |
Base on mRNA | G | U | G | U | A | C | U | G | G
Encoded amino acid | | |
Complete Table 2 to show the base sequence of the DNA template from which this mRNA was transcribed and the encoded amino acid sequence.
(2)
(Total 10 marks)
Q6.
(a)     What is the proteome of a cell?
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(1)
(b)     Give two structural differences between a molecule of messenger RNA (mRNA) and a molecule of transfer RNA (tRNA).
1. _________________________________________________________________
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2. _________________________________________________________________
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(c)     Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide.
Do not include descriptions of transcription and splicing in your answer.

(5)
(Total 8 marks)
Q7.
This question should be answered in continuous prose.
Quality of Written Communication will be assessed in the answer.
(i)      Starting with mRNA, describe how the process of translation leads to the production of a polypeptide.
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(4)
(ii)      Normal tomato plants have an enzyme that softens tomatoes as they ripen. Genetically engineered tomatoes ripen and soften more slowly. A gene was inserted which reduces the amount of softening enzyme produced.
The diagram shows matching parts of the base sequences for the mRNA produced by the gene for the softening enzyme and that produced by the inserted gene.
Softening gene mRNA                 …AAUCGGAAU…
Inserted gene mRNA                   …UUAGCCUUA…
Suggest how the inserted gene reduces the production of the softening enzyme.
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(Total 6 marks)
Q8. (a)     (i)      What is the role of RNA polymerase in transcription?
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(ii)     Name the organelle involved in translation.
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(1)
(b)     Figure 1 shows some molecules involved in protein synthesis.
Figure 1
 
Complete Figure 1 to show
(i)      the bases on the DNA strand from which the mRNA was transcribed;
(ii)     the bases forming the anticodons of the tRNA molecules.
(2)
Figure 2 shows the effects of two different mutations of the DNA on the base sequence of the mRNA. The table shows the mRNA codons for three amino acids.
Figure 2
 
(c)     Name the type of mutation represented by mutation 1.
________________________________________________________________(1)
(d)     Use the information in the table to
(i)      identify amino acid X in Figure 1;
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(ii)     explain how each mutation may affect the polypeptide for which this section of DNA is part of the code.
Mutation 1 _____________________________________________________
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(2)
Mutation 2 _____________________________________________________
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(2)
(Total 10 marks)
Q9.
In a eukaryotic cell, transcription results in a molecule of pre-mRNA that is modified to produce mRNA. In a prokaryotic cell transcription produces mRNA directly.
(a)     Explain this difference.
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(2)
(b)     Give two differences between the structure of mRNA and the structure of tRNA.
1. _________________________________________________________________
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2. _________________________________________________________________
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(2)
(Total 4 marks)

Mark schemes
Q1.
(a)
 
Accept other correct representations.
1
(b)     1.      More than one codon codes for a single amino acid;
Accept ‘triplet’ or ‘sequence of 3 bases/nucleotides’ for ‘codon’.
Reject ‘production/produces’ for ‘codes for’.
Do not infer mp1 from mp2.
2.      Suitable example selected from Table 1;
2
(c)     1395;
Accept 1398 and 1401 (for those that include start and/or stop codons)
Allow 2796 or 2802 or 2790
Ignore ‘bases/base pairs/bp/bps’ written after the numerical answer.
1
(d)     ✔CAA → CGA
1
(e)     1.      (Both) negatively charged to positively charged change in amino acid;
2.      Change at amino acid 300 does not change the shape of the active site
OR
Change at amino acid 300 does not change the tertiary structure OR Change at amino acid 300 results in a similar tertiary structure;
Reference to ‘shape’ of active site only needed once.
3.      Amino acid 279 may have been involved in a (ionic, disulfide or hydrogen) bond and so the shape of the active site changes
OR
Amino acid 279 may have been involved in a (ionic, disulfide or hydrogen) bond and so the tertiary structure changed;
OR
Amino acid 279 may be in the active site and be required for binding the substrate;
Reference to ‘shape’ of active site only needed once.
Both parts are required for each mark option.
For ‘a bond’ reject peptide bond.
3
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Q2.
(a)     
DNA | | 2
mRNA | | 1
tRNA | | 1
One mark for each correct column
Regard blank as incorrect in the context of this question
Accept numbers written out: two, one, one
2
(b)     (i)      Marking principles
1 mark for complete piece transcribed;
Correct answer
UGU CAU GAA UGC UAG
1 mark for complementary bases from sequence transcribed;
but allow 1 mark for complementary bases from section transcribed, providing all four bases are involved
2
(ii)     Marking principle
1 mark for bases corresponding to exons taken from (b)(i)
Correct answer
UGU UGC UAG
If sequence is incorrect in (b)(i), award mark if section is from exons. Ignore gaps.
1
[5]
Q3.
(a)     (i)      Phosphate and ribose; 
Accept in either order. Both correct for one mark. 
For phosphate accept PO_4  / Pi /  but not P. 
Do not accept phosphorus. 
Ignore references to pentose / sugar. 
1
(ii)     TAGGCA;
1
(b)     (i)      Does not contain hydrogen bonds / base pairs / contains
codons / does not contain anticodon / straight / not folded / no
amino acid binding site / longer;
Assume that “it” refers to mRNA.
Do not accept double stranded. 
1
(ii)     (pre-mRNA) contains introns / mRNA contains only exons;
Assume that “it” refers to pre-mRNA.
Accept non-coding as equivalent to intron. 
1
(c)     (i)      
Part of chromosome | U
Middle | 18
End | 21
One mark for both figures correct 
1
(ii)     1.      Have different (base) sequences / combinations of (bases); 
2.      (Pre-mRNA) transcribed from different DNA / codes for different proteins; 
2
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Q4.
(a)     Translation.
1
(b)     Transfer RNA / tRNA.
1
(c)     TAC;
UAC.
2
(d)     Have different R group.
Accept in diagram
1
(e)     1.      Substitution would result in CCA / CCC / CCU;
2.      (All) code for same amino acid / proline;
3.      Deletion would cause frame shift / change in all following codons / change next codon from UAC to ACC.
3
[8]
Q5.
(a)     1.      Hydrogen bonds between the base pairs holds two strands together
2.      Many hydrogen bonds provides strength
Reject strong hydrogen bonds
2
(b)     (Because) ribosomes assemble polypeptides using mRNA code
OR
DNA has two strands each with a different (complementary) base sequence;
1
(c)     Codon;
1
(d)     1.      (Because) some amino acids have more than one codon / mRNA code;
2.      Correct example from table.
2
(e)     1.      Stop translation;
2.      Result in detachment of polypeptide chain from ribosome.
2
(f)
 
CAC | ATG | ACC
Val | Tyr | Trp
Mark each row
2
[10]
Q6.
(a)     (The proteome is the full) range of / number of different proteins that a cell is able to produce (at a given time);
OR
(The proteome is the full) range of / number of different proteins the genome / DNA is able to code for;
Do not accept number of proteins unqualified
1
(b)     1.      mRNA does not have hydrogen bonds / base pairing, tRNA does;
OR
mRNA is linear / straight chain, tRNA is cloverleaf;
2.      mRNA does not have an amino acid binding site, tRNA does;
Accept mRNA cannot carry an amino acid, tRNA can
3.      mRNA has more nucleotides;
Accept mRNA is longer or converse
4.      (Different) mRNAs have different lengths, all tRNAs are similar / same length;
5.      mRNA has codons, tRNA has an anticodon;
Statements must be comparative
2 max
(c)     1.      mRNA associates with a ribosome / ribosome attaches to mRNA;
Idea of association is required
2.      Ribosome moves to / finds the start codon / AUG;
3.      tRNA brings / carries (appropriate / specific) amino acid;
Must be explicitly stated and not inferred.
4.      Anticodon (on tRNA complementary) to codon (on mRNA);
5.      Ribosome moves along to next codon;
OR
Ribosome ‘fits’ around two codons / can fit two tRNAs;
Must be explicitly stated and not inferred.
6.      (Process repeated and) amino acids join by peptide bonds / condensation reaction (to form polypeptide);
OR
(Process repeated and) amino acids joined using (energy from) ATP (to form polypeptide);
5
[8]
Q7.
(i)      mRNA attaches to ribosome;
codon on mRNA;
binds to an anti-codon on tRNA;
each tRNA brings a specific amino acid;
sequence of codons / bases on mRNA determines order of amino acids;
formation of peptide bonds / amino acids joined by condensation 
reactions;
4 max
(iii)     inserted gene / mRNA complementary to normal gene / mRNA;
binds to it to prevent protein synthesis / form double strand / prevents 
mRNA binding to ribosomes;
will not stop all translation, some mRNA reaches ribosomes / 
because not all mRNA is bound by inserted gene mRNA;
2 max
[6]
 
Q8.
(a)     (i)      join / attach nucleotides, to form a strand / along backbone / phosphodiester bonds;
(reject reference to H bonds, complementary base pairing)
1
(ii)     ribosome / RER;
1
(b)     (i)      CGTTACCAA;
1
(ii)     CGU UAC CAA;
1
(c)     substitution;
1
(d)     (i)      alanine;
1
(ii)     (mutation 1)
no change(to sequence of amino acids);
codon for alanine / degenerate codon / same amino acid coded for;
2
(mutation 2)
(change in sequence) valine replaced by alanine / codon for alanine;
folding / shape / tertiary structure / position of bonds may change;
(reject peptide bonds)
2
[10]
Q9.
(a)     1.      DNA of eukaryotic cell has non-coding regions / introns within gene
Allow converse: (But) a prokaryotic cell does not have non-coding regions / introns in DNA;
OR
pre-mRNA contains non-coding regions / introns;
2.      (After transcription / during modification) these regions are removed from (pre-)mRNA;
Ignore references to 'cells need / bacteria do not need’
2
(b)     1.      mRNA longer
OR
Has more nucleotides than tRNA;
2.      mRNA is a straight molecule but tRNA is a folded molecule / clover-leaf shaped molecule;
3.      mRNA contains no paired bases / hydrogen bonds but tRNA has some paired bases / hydrogen bonds.
2 max
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