AQA A Level Chemistry

Topic Questions

7.7 Amino acids, Proteins & DNA (A Level only)

1a3 marks

State the IUPAC name of the following amino acids shown in Figure 1.

Figure 1

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i)
Alanine

ii)
Glycine

iii)
Lysine
1b1 mark

Amino acids are solid at room temperature and exist as the following structure shown in Figure 1.

Figure 1

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What is the name given to this type of compound?

1c1 mark

Aspartic acid and valine can form a dipeptide as shown in Figure 2.

Name this type of bond and label it on Figure 2.

Figure 2

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1d1 mark

State the type of reaction occurs when two amino acids bond together.

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2a1 mark

Amino acids can form dipeptides, draw the dipeptide formed from two molecules of alanine. 

2b2 marks

Alanine exists as a zwitterion at room temperature, draw the structure of the zwitterion formed by alanine.

2c2 marks

When amino acids are placed in different pH’s, their structure changes.

i)
Draw the structure of alanine when it is reacted with excess sodium hydroxide, NaOH.

ii)
Draw the structure of alanine when it is reacted with excess hydrochloric acid, HCl.
2d3 marks

State the conditions required for hydrolysis of a peptide.

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3a2 marks

Proteins have complex shapes and are held in place by bonds. Name the two types of bond formed to maintain the shape of the protein.

3b2 marks

Draw two of the four amino acids that would be formed by the hydrolysis of the polypeptide shown in Figure 1.

Figure 1
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3c2 marks

After hydrolysis amino acids can be separated and identified by a technique called thin-layer chromatography (TLC). From the TLC plate, the Rf values of the amino acids can be identified. 

State the meaning of Rf value and show how it can be calculated.

3d2 marks

Explain why some amino acids have different Rf values.

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4a3 marks

Figure 1 shows the composition of a nucleotide. Label each part of the nucleotide. 

Figure 1

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4b1 mark

A strand of DNA has the sequence CCAGTT. 

State the complementary strand sequence.

4c2 marks

One part of the nucleotide contains phosphorus and this will act as an acid. Explain how this group can act as an acid.

Referring to Figure 1 in part (a) may assist you in your answer.

4d1 mark

State the type of polymerisation that occurs when two nucleotides of DNA bond together.

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5a1 mark

State the name of the anti-cancer drug shown in Figure 1.

 Figure 1

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5b1 mark

Draw an isomer of the molecule in Figure 1 which is ineffective in treating cancer.

5c1 mark

The use of this anti-cancer drug has side effects for the patient. 

Suggest one way in which these side effects can be reduced when administering the drug. 

5d3 marks

Explain how the anticancer drug shown in Figure 1 in part (a) successfully treats cancer.

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1a1 mark

Use your data sheet to answer the following questions

The amino acid alanine will react with excess chloromethane, draw the structure of the product formed in this reaction.

1b2 marks

Alanine can also react with serine to produce two different dipeptides, draw both dipeptides formed in this reaction.

1c3 marks

Explain the conditions in which alanine exists as a solid, draw the structure of the amino acid in these conditions.

1d3 marks

Serine is a water soluble amino acid, whereas some amino acids are insoluble in water. Explain which part of the structure of an amino acid influences how soluble the molecule is in water. 

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2a1 mark

The tripeptide shown in Figure 1 is made of three different amino acid molecules. Draw the zwitterion of the central amino acid in the tripeptide shown.

Figure 1

1-16

2b1 mark

Give the IUPAC name of the central amino acid in the tripeptide in part (a). 

2c1 mark

Cysteine is included in the tripeptide shown in Figure 1 in part (a). Draw cysteine at a high pH.

2d3 marks

Polypeptide chains are held together by hydrogen bonds. Draw a diagram to show how cysteine can form a different type of bond between polypeptide chains.

2e4 marks

Explain how ‘R’ groups influence the structure of a tertiary protein.

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3a3 marks

A single strand of DNA is a polymer made up from four different monomers. DNA exists as two strands of nucleotides in the form of a double helix with hydrogen bonding between the strands.

Guanine and uracil are shown in Figure 1. Use the data sheet to draw a diagram to show how a molecule of guanine bonds with another molecule on the other strand of DNA.

Figure 1
2-15

 

3b1 mark

Adenine forms a base pairing in RNA, explain how this pairing differs from your answer to part (a).

3c2 marks

Explain why the bonding between adenine and it’s compatible base in RNA will bond in the same way as adenine’s compatible base in DNA.

3d4 marks

The formula for urea is NH2CONH2 and when heated it can be used in alkaline conditions to separate the double helix structure of DNA.

Draw the displayed formula of urea and explain why the molecule can separate the DNA double helix. 

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4a4 marks

Isoleucine is shown in Figure 1 and has four stereoisomers. Draw each of the four stereoisomers in 3D representation.

Figure 1

5-13

4b1 mark

Isoleucine can form a tripeptide with alanine and phenylalanine. State the maximum number of different tripeptide molecules can be formed from these three amino acids. Each tripeptide must have three different amino acids in.

4c2 marks

The structure of the anticancer drug cisplatin is shown in Figure 2. Cisplatin was discovered in 1965 and is one of the most successful cancer treatments.

 Figure 2
6-11

Write an equation to show the formation of the complex produced when cisplatin enters the body and then reacts with water.

4d3 marks

Draw a simplified diagram to show how cisplatin bonds with the DNA double helix.

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5a5 marks

In all living organisms a large variety of proteins are formed naturally from amino acids.

i)
State the general formula of an amino acid

ii)
Describe how amino acids can be combined to give a variety of polypeptides and proteins. 
5b2 marks

Explain why DNA can behave as an acid. 

5c2 marks

The amino acid proline is shown in Figure 1. Draw both dipeptides formed when proline reacts with phenylalanine.

Figure 1

7-7

5d1 mark

Draw the product of the reaction between proline and ethanol in the presence of concentrated sulfuric acid.

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1a2 marks

Two amino acids, alanine and valine are drawn in Figure 1. 

Figure 1
1-15

  

i)
Give the IUPAC name for valine.

ii)
Draw the species formed when alanine is in a solution of high pH.
1b2 marks
i)
When alanine and valine, drawn in Figure 1, react together a dipeptide is formed. Give the structured formula of two dipeptides which are formed when these two molecules react together.

 

ii)
Give the formula of the molecule that is eliminated when these two amino acids
1c2 marks

Name the type of reaction and the structural formula of alanine when it reacts with propanol, CH3CH2CH2OH in the presence of concentrated sulphuric acid, H2SO4.

1d4 marks

Valine exists as a pair of stereoisomers. 

Explain the meaning of the term stereoisomers and explain how the stereoisomers of valine can be distinguished from one another.

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2a7 marks

A student hydrolyses a dipeptide to form a solution containing a mixture of amino acids. This mixture is then analysed by silica gel thin-layer chromatography (TLC) using a toxic solvent. The individual amino acids are then identified from their Rf values.

Outline the main steps in producing the chromatogram that will enable the student to calculate the Rf values. Include any safety precautions that the student should take.

2b3 marks

Thin Layer Chromatography is a commonly used analytical technique. Once a chromatogram has been produced, often an Rf value will be calculated.

i)
Give the equation to which can be used to calculate Rf values.

ii)
A chromatogram is shown in Figure 1. Calculate the Rf values for both compounds shown in the diagram. Give your answer to two decimal places.

 Figure 1

2-14

2c2 marks

Explain why the amino acids in the solution have different Rf values.          

2d3 marks

Give three advantages of Thin Layer Chromatography (TLC) as an analytical technique.

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3a4 marks

A section of a beta pleated sheet is shown in Figure 1. Explain how the interactions of the structure are formed.

Figure 1

3-15

3b6 marks

A single strand of DNA is made from many nucleotides linked together.
Draw the structure of the nucleotide that contains cytosine and name the three components. You will need to clearly show the bonding between the three different components. 

3c2 marks

Complete Table 1 by placing a Y in the box that indicates which compound is involved in the process of making a protein or a strand of DNA. 

Table 1

Compound

Protein Synthesis

DNA Synthesis

Adenine

 

 

Alanine

 

 

Aspartic acid

 

 

Phosphate

 

 

3d4 marks

Explain how the double helix structure of DNA is held together. In your answer you should make reference to the type of bonding and intermolecular forces that are present.

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4a3 marks

Use your data sheet to help you answer the following questions about amino acids,

i)
Draw the zwitterion ion of lysine

ii)
Give the IUPAC name of serine

iii)
Draw the structural formula of the species formed by alanine at pH 1.
4b7 marks

Cis-platin can act as an anti cancer drug. It works by attaching itself to DNA and not allowing it to function properly. Cis-platin and an isomer, trans-platin, are drawn in Figure 1 below. 

Figure 1 

4-15

 

i)
Explain how cis platin is able to stop the growth of cancer cells.

ii)
Suggest why trans-platin would not be able to do the same.

iii)
Give one risk associated with taking cis-platin as an anti cancer drug
4c1 mark

Naproxen is a drug used to treat pain from arthritis and is drawn in Figure 2. Drugs treat medical conditions that work in many different ways. One way is for the drug to bind to the active site of an enzyme. State the name given to the type of molecule that binds to the active site of an enzyme.

Figure 2

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4d4 marks

Naproxen has a chiral carbon, meaning that it exists as enantiomers.

i)
Explain why only the correct enantiomer of Naproxen will be catalysed by a particular enzyme.

ii)
Suggest why it is important for the patient taking the drug to the correct enantiomer.  

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5a2 marks

When the two amino acids, phenylalanine and serine, react together a dipeptide is formed.

Give the structural formula of the dipeptide formed when phenylalanine and serine react together.

5b2 marks

When serine is at room temperature it exists as a solid. Name the type of compound that is formed and draw the displayed formula of the compound. 

5c4 marks

Explain why amino acids such as serine are soluble in water and insoluble in organic solvents such as hydrocarbons.

5d6 marks

The dipeptide formed from serine and phenylalanine is heated with concentration hydrochloric acid and a mixture of amino acids was formed. Thin Layer Chromatography (TLC) was used to separate the mixture.

i)
Explain why a mixture of amino acids was formed.

ii)
Outline briefly why TLC is able to separate these amino acids in this mixture.

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