AQA A Level Biology

Topic Questions

4.2 DNA & Protein Synthesis

1a1 mark

Determine the mRNA sequence that is coded by the following strand of DNA.

Sense strand:           A T G G G T T C A

Antisense strand:     T A C C C A A G T 

1b4 marks

Replication and transcription both involve DNA. 

Complete Table 1 by using a tick (✓) to indicate which features apply to each of the processes. Use a cross (X) for features that do not apply. The first row has been completed for you. 

Table 1

Feature

Replication

Transcription

 a double-stranded molecule is produced

X

 hydrogen bonds are broken

   

 Only one strand of DNA acts as a template

   

 phosphodiester bonds are formed

   

 RNA polymerase is used

   

1c2 marks

Figure 1 shows the exposed bases (anticodons) of two tRNA molecules involved in the
synthesis of a protein.

Figure 

UGC

 

AAC

     
Complete the boxes to show the codon sequence of bases found along the corresponding section of the mRNA strand.
1d3 marks

Name the structures labelled X, Y and Z in Figure 2

Figure 2

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2a6 marks

Figure 1 below represents a model for the production of a polypeptide. 

Figure 1

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Name and outline what occurs in:

i) Process 1

ii) Process 2

2b1 mark

Figure 2 below shows a section of a pre-mRNA molecule.

Figure 2

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Name the sugar that makes up part of X.

2c1 mark

Using Figure 2 give the sequence of bases on the DNA strand from which this pre-mRNA has been transcribed.

2d2 marks

The structure of an mRNA molecule differs from the structure of a tRNA molecule. Give two differences between the two molecules. 

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3a1 mark

State the role of RNA polymerase in transcription.

3b3 marks

Figure 1 below illustrates the protein synthesis process in eukaryotic cells. The diagrams are not in the correct order. 

Figure 1

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List the letters in order to show the steps of protein synthesis, starting with the earliest.

________  ________  ________  ________ 

3c2 marks

The diagram in Figure 2 below shows some of the molecules involved in protein synthesis.

Figure 2

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Names the processes involved in protein synthesis that are occurring at X and Y.

3d2 marks

Alternative splicing occurs during protein synthesis. Describe what happens during this process.

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4a2 marks

Complete Table 1 below to show the differences between DNA, mRNA and tRNA.

Table 1 

Type of nucleic acid

The nitrogenous base uracil present (✓) or not present (?)

Hydrogen bonds present (✓) or not present (?)

DNA

   

mRNA

   

tRNA

   

4b1 mark

Give one difference between transcription in prokaryotic and eukaryotic cells.

4c2 marks

The diagram in Figure 1 below shows the base sequence on part of a template DNA
strand. 

Figure 1

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Give the sequence of bases on the pre-mRNA transcribed from this strand.

4d2 marks

Use the diagram in Figure 1 from part (c) to answer the following questions:

i) Define exon.

ii) Give the sequence of bases on the mRNA produced by splicing this piece of pre-mRNA.

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5a1 mark

Name the organelle involved in translation.

5b3 marks

The diagram in Figure 1 below represents one process that occurs during protein
synthesis.

 Figure 1

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i) Identify the region X and the molecule labelled Y.

ii) Name the process shown.

5c2 marks

In Figure 1, the first codon is CAU. 

Give the base sequence of:

i) The complementary DNA base sequence of the template strand 

ii) The missing region X  

5d2 marks

During transcription of a gene in eukaryotic cells, a pre-mRNA molecule is produced.

Name the process that removes base sequences from pre-mRNA to form mRNA and state whether it occurs inside or outside the nucleus.

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1a2 marks

In the early 1990s a company in the United States attempted to market a genetically engineered tomato (Solanum lycopersicum) that had an extended shelf life. The gene they inserted that interfered in the production of the enzyme Beta polygalacturonase was just one of the 31,760 genes that a tomato plant has. 

Explain what the 31,760 genes represent in a tomato. 

1b4 marks

Describe the structure of transfer RNA (tRNA) which is involved in the process of
translation.

1c3 marks

The Flavr Savr tomato plant was genetically engineered to ripen and soften more slowly. The inserted gene prevented the enzyme Beta polygalacturonase from breaking down pectin which softened the tomatoes. 

The matching parts of the base sequences for the mRNA produced from the transcription of the softening gene (in the normal tomato plant) and the mRNA base sequence of the inserted gene are shown in the diagram below.

Softening gene mRNA                 …AAUCGGAAU…

Inserted gene mRNA                   …UUAGCCUUA…

Suggest how the inserted gene reduces the production of the softening enzyme.

1d1 mark

Figure 1 shows the sequence of DNA bases coding for seven amino acids in the enzyme papain.

Figure 1

5’ C  A  A  T  T  T  C  A  A  A  G  T  T  G  C  T  T  T  T  T  G 3’  

Figure 2 shows the genetic code (mRNA codons).

Figure 2

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Use Figure 2 to determine the sequence of amino acids in this part of the enzyme. Write your answer in the boxes below.
             

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2a4 marks

Use information in the passage below and your own knowledge to answer the questions below.

In 2018 the US Food and Drug Administration and the European Medicines Agency approved two drugs that targeted mRNA molecules. These drugs were designed to treat a rare genetic disease that causes a blood protein produced by the liver to form clumps (amyloid deposits) in the cardiovascular system, digestive system and around nerve fibres. This disease called hereditary transthyretin (hATTR) amyloidosis affects about 50,000 people in the world. It is an autosomal dominant inherited condition that causes the mutation of the blood protein called transthyretin. The two drugs were designed to bind with the mRNA molecules.

Suggest why the two drugs could be used as a treatment for hereditary transthyretin (hATTR) amyloidosis.

2b2 marks

Distinguish between the genome and proteome of a cell.

2c3 marks

Transcription occurs differently between eukaryotic and prokaryotic cells. 

Explain why there is a difference.

2d2 marks

The KRT35 gene codes for an acidic protein which is a type I hair keratin. Along with type II keratin proteins, it forms the hair and nails on our body. This gene is found on chromosome 17.

Explain how a gene like KRT35 would code for a protein.

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3a3 marks

Ricin is a protein produced by castor beans. In animal cells, ricin acts as an enzyme. This enzyme removes the adenine molecule from one of the nucleotides in the RNA of ribosomes. As a result, the ribosome changes shape.

Ricin causes the death of cells and is very poisonous to many animals. Suggest how the action of ricin on ribosomes could cause the death of cells.   

3b4 marks

Draw labelled diagrams comparing an mRNA and a tRNA molecule.

3c2 marks

Table 1 below shows some mRNA codons and the amino acids for which they code.

Table 1

Codon

Amino Acid

ACG

Threonine

UUA

Leucine

CCA

Proline

GUA

Valine

GCU

Alanine

AAU

Asparagine

i) Give the DNA sense strand sequence for leucine.

ii) Name the amino acid carried by the tRNA with the anticodon CAU.

3d2 marks

Cycloheximide binds to specific sites on the eukaryotic ribosomes. This is shown in Figure 1 below.

Figure 1

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Explain how cycloheximide stops protein synthesis.

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4a2 marks

Scientists studied two Diptera animals Anopheles gambiae (mosquito) and Drosophila melanogaster (fruit fly) who diverged 250 million years ago. Their study was to determine how similar their genes and proteins were. It was known that D. melanogaster has 17,864 genes which code for 30,717 proteins whereas A. gambiae has 13,247 genes coding for 14,102 proteins. The study showed that almost half of their genes were similar and that they had diverged more rapidly than vertebrates.

Using examples from the passage to explain the difference between genome and proteome.

4b3 marks

The skin is the organ that separates our body from the environment and therefore from foreign antigens, thus the skin not only acts as a physical barrier but it also acts as an immunological barrier. Keratinocytes are the primary cells found in the skin epidermis. These cells contain genes that code for cytokines and when the cells are triggered the genes become activated and mRNA transcripts are produced. 

Explain why the mRNA transcripts leaving the nucleus have fewer nucleotides than the RNA initially produced by DNA transcription.

4c1 mark

Explain the role ATP has in translation.

4d3 marks

Mitochondrial diseases (MD) are a group of genetic disorders where the body’s cells cannot aerobically respire properly. MD causes a wide range of symptoms especially in the organs that need the most energy (e.g. brain, heart and muscles) making them hard to diagnose. Multi-organ failure is a common sign that the person has a mitochondrial disorder. 

Mitochondrial DNA (mtDNA) is inherited from our mothers, although recent scientific studies suggest that people may inherit paternal mitochondria. Most mitochondrial diseases occur due to mutations of nuclear genes that are involved in the functioning of mitochondria, although some result from mutations of mitochondrial genes inside the mitochondria. One example of an MD is caused by the mutation of a mitochondrial gene that codes for a tRNA. The mutation changes the anticodon on the tRNA, the guanine base is substituted with adenine. This results in the formation of a non-functional protein in the mitochondrion.

Suggest how the change in the anticodon of a tRNA leads to an MD. 

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5a5 marks

Lactate dehydrogenase (LDH) has a key role in ensuring glycolysis continues. The enzyme catalyses the reversible conversion of pyruvate into lactate, oxidising the cofactor NAD+. LDN is a tetrapeptide containing two types of polypeptides: polypeptide M (muscle), which is coded for by the LDH-A gene and polypeptide H (heart), which is coded for by the LDH-B gene. These two types of polypeptides in various combinations make up five isoenzymes. Isoenzymes are enzymes that differ in amino acid sequence but catalyse the same chemical reaction. 

The base sequences of the LDH-A and LDH-B genes and the sequences of the amino acids encoded by these genes were determined. Figure 1 shows the first ten amino acids of polypeptides M and H and the corresponding base sequences of one of the DNA strands of each gene.

Figure 1 

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Table 1 shows the genetic code (mRNA codons). 

Table 1

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Compare and contrast both sequences shown in Figure 1, using the information given.

5b3 marks

Read the following passage.

Herpes simplex virus (HSV) infects nerve cells in the face. HSV can remain inactive inside the body for years, but when it becomes active, it causes cold sores around the mouth. 

Human cells infected with a virus may undergo programmed cell death. While HSV is inactive inside the body, only one of its genes is transcribed. This gene is the latency-associated transcript (LAT) gene that prevents programmed cell death of an infected nerve cell. Scientists have found that transcription of the LAT gene produces a microRNA. This microRNA binds to some of the nerve cell’s own mRNA molecules. These mRNA molecules are involved in the programme cell death of nerve cells. 

Use information from the passage and your own knowledge to answer the following question.

Scientists have concluded that the production of microRNA allows HSV to remain in the body for years. Suggest how the presence of the microRNA allows HSV to remain inactive for years.

5c2 marks

Scientists have genetically engineered maize plants to resist attack by insects known as stem borers. The scientists inserted a gene from Bacillus thuringiensis (Bt), a soil bacterium, that produces proteins that are highly toxic to the stem borers. These proteins bind to an embedded enzyme aminopeptidase (APN 1) on cell-surface membranes causing the membranes to become more permeable affecting the cells ability to maintain an optimal osmotic potential. 

In Kenya, a study was carried out to see which types of Bt genes and their protein products would be most efficient against three species of stem borer. The stem borers were allowed to feed on nine types of maize (A–I), modified with Bt genes. Figure 2 below shows the leaf areas damaged by the stem borers after feeding on maize leaves for five days.

Figure 2

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Calculate the percentage difference in leaf area damaged by Busseola fusca between the control and maize type H. Show your working.

5d3 marks

Recently, scientists have seen insects like Busseola fusca (maize stalk borer) developing resistance to genetically engineered maize plants. Cornell researchers have discovered that the insects with resistance have a different type of aminopeptidase (APN 6) which allows insects to digest their food and Bt without harm. 

Explain how two different aminopeptidases could evolve resulting in some insects developing resistance to the Bt toxin.

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1a1 mark

State one difference between the genome and proteome of a cell.

1b4 marks

Describe how the process of translation leads to the production of a polypeptide.

1c4 marks

Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how
mRNA is produced in the nucleus of a cell. 

1d2 marks

Give two differences between the structure of mRNA and the structure of tRNA.

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2a4 marks

Describe the function of ribosomes in protein synthesis.

2b2 marks

Explain the role of transfer RNA (tRNA) in the process of translation.

2c2 marks

The amino acid sequence of the enzyme lactase has been determined. The first five amino acids of this sequence are shown in Table 1.

Table 1

 Amino acid sequence 

Met

Glu

Leu

Ser

Tyr

 Student’s nucleotide sequence

AUG

GAG

CCU

AGC

UAA

 Correct =

 Incorrect = X

         

Table 2 shows the genetic code (mRNA codons).

A student was asked to use Table 2 to determine an mRNA nucleotide sequence that would correspond to the first five amino acids of lactase, their results are shown in Table 1

Table 2

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The student has not completed Table 1 correctly. Determine which amino acids have been incorrectly labelled by the student and complete the table using a or a X to indicate this.

2d2 marks

Figure 1 below shows the exposed bases (anticodons) of two tRNA molecules involved in the synthesis of a protein.

Figure 1

UAU

 

GAC

     

Complete the boxes to show the sequence of bases found along the corresponding section of the coding DNA strand.

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3a2 marks

Describe how a gene codes for a polypeptide.

3b3 marks

Figure 1 is the template strand of a DNA molecule that codes for six amino acids.

Figure 1

TAC GCG TCA ATG ATA CTG 

Identify the sequence of anticodons on the transfer RNAs that would synthesise the string of six amino acids?

3c2 marks

Complete Table 1 to show the differences between DNA, mRNA and tRNA.

Table 1

Type of nucleic acid

Number of polynucleotide strands in the molecule

The nitrogenous base uracil present (✓) or not present (X)

DNA

   

mRNA

   

tRNA

   

3d2 marks

Suggest which group of molecules in Figure 2 (A or B) best represents the proteome of a cell. Give a reason for your answer.

Figure 2

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4a2 marks

Contrast the chemical structure of DNA and RNA nucleotides.

4b1 mark

Tobacco plants have been genetically modified to produce human haemoglobin. The first three triplets of the antisense strand of the human haemoglobin gene are:

ATG GTG CAT

Identify the anticodons of the tRNA that would be part of the second step in producing the haemoglobin protein.

4c4 marks

Table 1 shows some of the events which take place in protein synthesis.

Table 1

A

mRNA nucleotides join with exposed DNA bases and form a molecule of mRNA

B

Peptide bonds form between the amino acids

C

tRNA molecules bring specific amino acids to the mRNA molecule

D

The introns are spliced from the pre-mRNA to produce mRNA

E

A ribosome attaches to the mRNA molecule

F

The two strands of a DNA molecule separate

G

The mRNA molecule leaves the nucleus

List the letters in the correct order to show the sequence of events during protein
synthesis, starting with the earliest.

________  ________  ________  ________  ________  ________  ________


 

4d3 marks

Identify the steps in Table 1 that are involved in transcription.

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5a3 marks

Compare the structure of DNA with RNA.

5b1 mark
Figure 1 shows the sequence of bases in a section of the mRNA strand used
to synthesise papain.

Figure 1

G   U   U   A   A   A   G   U   U   U   C   A   A   C   G   A   A   A   A   A   C

Give the DNA sequence which would be complementary to the first six bases in this section of mRNA.
5c1 mark

Using Figure 1, deduce how many different types of tRNA molecules would attach to the section of mRNA shown in the diagram?

5d3 marks

A piece of mRNA is 972 nucleotides long but the DNA coding strand from which it was transcribed is 1215 nucleotides long.

i) Explain why there is a difference in the number of nucleotides.

ii) State the maximum number of amino acids in the polypeptide translated from this piece of mRNA and explain your answer.

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