Nucleotides & Nucleic Acids (OCR AS Biology)

Exam Questions

3 hours35 questions
1a
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3 marks

Figure 1.1 shows the structure of a nucleotide.ocr-a-2-3e-sq-nucleotide

Figure 1.1

Identify the components of a nucleotide. 

 

Component Name
A  
B  
 
1b
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2 marks

State two differences between DNA nucleotides and RNA nucleotides. 

1c
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4 marks

Complete the sentences using the most appropriate terms. Your terms can be single words or short phrases. 

Nucleotides link together to form a ……………. The bonds between nucleotides are called ……………. bonds and are formed through ……………. reactions. In this reaction, the phosphate group at the 5-carbon of the sugar is covalently bonded to the ……………. on the sugar of a different nucleotide. The resulting molecule has a strong sugar-phosphate backbone.

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2a
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6 marks

The bases of nucleotides link together using complementary base pairing.

Complete the table below for ribonucleic acid (RNA). 

Base Type of base Complementary base Number of hydrogen bonds in base pair
Adenine       
Cytosine       
2b
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2 marks

Describe why complementary base pairing is important for the function of deoxyribose nucleic acid (DNA).

2c
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1 mark

State why the DNA molecule is described as antiparallel.

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3a
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2 marks

Figure 3.1 shows the steps of DNA replication.

ocr-a-2-3e-sq-replication

Figure 3.1

Describe the enzymatic reaction which occurs in step A. 

3b
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2 marks

Describe the enzymatic reaction which occurs in step C. 

3c
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1 mark

State why this process of DNA replication is described as semi-conservative.

3d
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4 marks

Figure 3.1 does not show the differences between continuous and discontinuous replication.

Describe continuous and discontinuous replication. 

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4a
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2 marks

Genes are transcribed into RNA by the process of transcription. RNA is then translated into an amino acid sequence by the process of translation.

Describe how the genetic code controls protein structure. 

4b
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3 marks

Describe the characteristics of the RNA end product of transcription.

4c
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5 marks

Complete the sentences using the most appropriate terms. Your terms can be single words or short phrases. 

During transcription, ……………. separates the two DNA molecules. One DNA strand is used for transcription, called the ……………. strand. The other strand is known as the ……………. strand. Free RNA nucleotides form the ……………. strand and are joined together by the enzyme ……………..

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5a
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3 marks

Table 1 below shows the components of the process of translation.

Table 1

Term Definition
Codon  
Ribosome   
tRNA  

Define the terms in Table 1 above. 

5b
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2 marks

Describe how the process of translation ends. 

5c
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3 marks

Explain why the genetic code is described as degenerate.

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1
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4 marks

Fig. 16.1 shows the structure of ATP.q16a-paper-1-nov-2020-ocr-a-level-biology

Fig. 16.1

(i)
Name the circled component in Fig. 16.1.
 [1]
(ii)
Name the type of reaction that occurs when ATP is converted to ADP.

 [1]

(iii)
A teacher told his students that the human body makes the equivalent of its own mass in ATP every day.

Explain why, at the end of the day, only a small proportion of the students’ mass was ATP.
[2]

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2
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5 marks

DNA can be obtained from a variety of plant and animal cells.

A group of students tried to purify some DNA from leek cells using the following method. They decided that exact volumes were not necessary.

1. Grind a leek leaf to a fine pulp using a pestle and mortar.

2. Add salt and cold water and mix again for at least 10 s.

3. Add protease enzyme and mix again for at least 10 s.

4. Filter the liquid into a test tube and stand for at least 10 min.

5. Tilt the test tube and gently pour in ice-cold ethanol.

6. A white layer of DNA forms between the sample and the ethanol.

7. Extract the white layer carefully using a glass rod.

(i)

State the purpose of step 1.

[1]

(ii)

Suggest why a protease enzyme added in step 3 is needed to purify DNA.

[1]

(iii)

The students considered using pineapple juice as a source of protease enzyme.

Suggest why this would not be an appropriate source of protease when attempting to produce a pure sample of leek DNA.

[1]

(iv)

State one important step that the students had left out of their method.

[1]

(v)

Name the process described in step 6.

[1]

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3a
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3 marks

Fig.1 shows the structure of a small section of DNA in a cell during transcription.

U29iN0fg_6-1-fig-1-1Fig. 1

Identify the bases Q, R and S as indicated in Fig.1. Explain how you reached your answer.

3b
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4 marks
(i)
State the importance of hydrogen bonding in the structure of a DNA molecule.
[2]
(ii)
A section of a DNA molecule contains 124 nucleotides.
  
Calculate the maximum number of hydrogen bonds that could be present in this section of DNA. Show your working.

[2]

3c
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3 marks

ATP is the energy-carrying molecule which fuels chemical reactions within the cell.

Compare the structure of ATP with the structure of a DNA nucleotide.

3d
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2 marks

Scientists analysed the nucleotide base sequence of a DNA molecule extracted from a human cheek cell. It was found that 29% of the nucleotides in this DNA molecule contained the nitrogenous base adenine.

Calculate the percentage of nucleotides that would contain the nitrogenous base cytosine.

Show your working.

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4a
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5 marks

The process of transcription is illustrated in Fig. 1.

6-2-fig-2-1

Fig. 1

(i)

Identify molecule X in Fig. 1 and describe its role during transcription.

[3]

(ii)

Identify strand Y in Fig. 1 and state the significance of strand Y during transcription.

[2]

4b
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3 marks

The table below contains a variety of statements that apply to different biological molecules.

Identify which of the statements apply to DNA, messenger RNA and proteins by placing a tick () in the box.

Statement DNA messenger RNA protein
Hydrogen bonds stabilise the molecule      
Subunits are joined by peptide bonds      
The molecule is double stranded      
Contains phosphodiester bonds      
Uracil is present      

4c2 marks

Describe the role of covalent bonds in linking nucleotides together to form a polynucleotide, such as DNA.

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5a
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2 marks

DNA replication is described as being 'semi-conservative'.

Explain what this means.

5b
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3 marks

Explain how the structure of DNA facilitates semi-conservative replication of the molecule.

5c
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2 marks

The new DNA molecules that form during semi-conservative replication are identical to each other.

Discuss the importance of this statement.

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6a
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4 marks

Describe the role of the ribosome in translation.

6b
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3 marks

Fig. 1 shows the base sequence of a gene that codes for a short polypeptide, the RNA codons and the primary structure of this polypeptide.

6-2-fig-4-1Fig. 1

(i)
Complete Fig. 1 to show the missing DNA triplet and the RNA codons.
[2]
(ii)
State the full name of the type of RNA shown in Fig. 1.
[1]
6c
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2 marks

The genetic code is universal across most forms of life.

Explain the importance of this statement.

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1a
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2 marks

Automated gene sequencing is an important technique for investigating base sequences.

Figure 1.1 shows part of the graph produced by an automated gene sequencing machine. The coding strand of DNA was analysed. Figure 1.1 shows the bases from base position 109 to 121 (left to right). Each base is labelled with a different coloured fluorescent dye. Some of the bases are identified and labelled above the corresponding peak.

ocr-a-2-3h-sq-gene-sequencing

Figure 1.1

Identify the sequence of bases from base position 109 to 114, using Figure 1.1.

 

Position 109 110 111 112 113 114
Base C         T

1b
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2 marks

Calculate the percentage of bases on both strands from base position 109 to 121 which are adenine. Give your answer to 1 decimal place. 

1c
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4 marks

 Describe the two structural forms of DNA nitrogenous bases.

1d
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2 marks

Interpret the sequence of amino acids from base position 109 to 114, using the key in Figure 1.2 below.

ocr-a-2-3h-sq-codon-keyocr-a-2-3h-sq-codon-key

Figure 1.2

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2a
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2 marks

Meselson and Stahl were two scientists who investigated DNA replication during the 1950s. 

They grew E. coli bacteria in dishes containing culture media. 

Some bacteria were grown in a culture containing an isotope of nitrogen with a higher molecular weight (15N) than normal nitrogen (14N). 

The bacteria started incorporating the nitrogen isotopes from the culture media into their DNA. As the bacterial colonies grew, the DNA of bacteria grown in a 15N culture became denser than bacteria grown in a normal (14N) culture.

Some bacteria were first grown in the 15N culture and then transferred to the normal (14N) culture.

After letting the bacteria grow for several generations, the scientists extracted and centrifuged the DNA. During centrifugation, the heavier, denser molecules settled towards the bottom of the tube. Figure 2.1 shows some of their results.

Key:

A Bacteria grown in a 15N culture 

B Bacteria first grown in a 15N culture and then transferred to the normal (14N) culture

C Bacteria grown in a 14N cultureocr-a-2-3h-sq-meselson-and-stahl-tubes

Figure 2.1

Some bacteria were first grown in the 15N culture and then transferred to the normal (14N) culture (tube B). They were left there with enough time to replicate once before the DNA was extracted. 

Suggest where a band would appear on this tube (B) in comparison to tubes A and C. 

2b
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2 marks

Justify your answer to part (a). 

2c
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5 marks

Explain how Meselson and Stahl’s experiment demonstrated the semi-conservative replication of DNA, concerning the results shown in Figure 2.1.

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3a
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2 marks

Figure 3.1 shows the hydrolysis reaction of ATP to release energy.

ocr-a-2-3h-sq-atp-reaction

Figure 3.1

The amount of energy released from the hydrolysis of 1 mole of ATP in a living cell is 57 kJ.

Explain why ATP is described as the “universal energy currency”. 

3b
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2 marks

Calculate the amount of free energy in kJ/mol released from the hydrolysis of 0.7 moles of ATP in a living cell. Give your answer to 1 decimal place. 

3c
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4 marks

Compare and contrast the structures and functions of ATP and ADP. 

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4a
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2 marks

A DNA sequence coding for a specific protein was analysed. The base composition of the template strand is given in the table below.

  Number of bases
  Adenine Cytosine Guanine Thymine Uracil
DNA 25 19 46 29 0
mRNA          


Deduce the number of bases in the complementary mRNA strand. 

4b
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2 marks

Calculate the percentage base composition of thymine in the DNA. Give your answer to 1 decimal place. 

4c
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5 marks

mRNA provides a template for protein synthesis. Protein synthesis requires tRNA molecules.

Explain how tRNA is used in the translation stage of protein synthesis. 

4d
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1 mark

State the first tRNA molecule that is used in protein synthesis.

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5
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6 marks

Transcription is the first step of protein synthesis in cells.

Discuss the similarities and differences of the processes of transcription and DNA replication. 

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