AQA A Level Biology

Topic Questions

1.5 Nucleic Acids: Structure & DNA Replication

1a2 marks

Describe the main role of the molecule, DNA.

1b1 mark

State one nucleic acid; DNA, or RNA, which never leaves the nucleus in a eukaryotic cell. Ignore the events of the stages of mitosis. 

1c3 marks

The presence of nucleotides has been put forward as a definition of whether an entity constitutes a form of ‘life’. Name the three main components of all nucleotides. 

1d1 mark

Figure 1 shows the way in which nucleotides join together to form the backbone of the DNA double helix.

Figure 2

 oRCeGzqz_screenshot-2023-05-22-105429

Name the bond that joins the nucleotides together to form this backbone.  The bond is labelled as bond X in Figure 1.

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2a2 marks

The base pairing of DNA nucleotides is set out in Table 1. Purine bases always base-pair with a pyrimidine base and vice versa

Table 1

Base

Purine or Pyrimidine

Base-pairs to which base

Adenine

Purine

 

Cytosine

Pyrimidine

 

Guanine

   

Thymine

   

Use your knowledge of DNA structure to complete the missing boxes in Table 1.

2b1 mark

State the name of the base that appears in RNA but not in DNA. Also state the name of the DNA base that it replaces. 

2c1 mark

State the purpose of a phosphodiester bond in DNA and RNA structure.

2d1 mark

DNA and RNA are referred to as polynucleotides. Explain the meaning of the prefix, ‘poly’ in the term ‘polynucleotide’. 

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3a3 marks

Table 1 shows some differences between DNA and RNA. Each row contains the terms Low and High to show whether DNA or RNA is greater or lesser than the other for that characteristic. The first row illustrates this, as DNA exists in longer chains than RNA. 

Table 1

Parameter

RNA

DNA

Length of chain

Low

High

Chemical stability

   

Amount found outside the nucleus

   

Amount of double-stranded in the cell

   


Complete Table 1 using the terms Low and High OR L and H in each box.

3b1 mark

Having the base pairs on the inside of the sugar-phosphate backbone helps to protect the important code-carrying parts of the DNA molecule. Suggest one additional structural feature of DNA that boosts its protection from chemical attack. 

3c2 marks

In a section of DNA, 17% of the nucleotides were found to contain cytosine. Calculate the percentage of thymine in this DNA. 

3d1 mark

Give one reason why the calculation from question 3c) could not be performed for a piece of RNA. 

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4a2 marks

Table 1 shows some statements about ribosomes. Some statements are true, others are false. 

Table 1

The ribosome is...

TRUE or FALSE?

...made of a large and a small subunit

TRUE

...the site of translation

 

...the site of transcription

 

...only visible under an electron microscope

 

...surrounded by a membrane

 

Complete Table 1 by indicating whether each statement is TRUE or FALSE (T/F) in each box.

4b1 mark

State the purpose of replication of DNA. 

4c2 marks

The diagrams in Figure 1 show two models of DNA replication. 

Figure 1

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State which diagram from Figure 1, A or B, is correct. Explain your choice.

4d2 marks

Name: 

  1. The enzyme that unwinds DNA by breaking down its hydrogen bonds. 
  2. The enzyme that builds a new DNA strand by complementary base pairing.

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5a3 marks

Figure 1 below shows the structure of DNA. 

Figure 1

screenshot-2023-05-22-113818

Name the parts of the molecule labelled A-C. The specific name of part B is not required.

5b2 marks

Calculate the fraction of a cell’s original DNA (not copies) that will be present in a great-granddaughter cell (i.e. after 3 full cycles of replication).

5c2 marks

In a length of DNA 1,000 nucleotides long, there are 382 guanine nucleotides in one strand of the DNA. Using this information, explain why it is not possible to calculate the number of guanine nucleotides in the opposite strand.  

5d1 mark

Both DNA and RNA contain sugars in their sugar-phosphate backbones. Deoxyribose and ribose sugars are both classed as pentose sugars. Define the term ‘pentose’ sugar.

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1a6 marks

Describe the term, ‘frame-shift mutation’ and evaluate the likely impact of this kind of mutation on the phenotype of the organism.  

1b3 marks

Suggest why most mutations in eukaryotic cells are regarded as ‘neutral’.

1c3 marks

Certain environmental factors can increase the rate of mutation above the normal, very low rate.  Identify two such factors and describe their interaction with DNA that gives them their mutagenic effect.   

1d1 mark

In the absence of mutagens, the rate of mutations is very low, approximately 160 bases per cell cycle. Given that the human genome contains 3.2 billion base pairs, calculate the percentage copying error rate of each cell cycle. 

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2a2 marks

The two strands of DNA are described as ‘antiparallel’. Explain the meaning of  ‘antiparallel’ in this context. 

2b3 marks

The ends of a DNA strand are referred to as the 3’ (“3-prime”) end and the 5’ (“5-prime”) end. Describe the aspects of DNA structure that give rise to this naming system .

2c4 marks

Adenine / thymine and guanine / cytosine are well known to hydrogen-bond to each other in complementary base-pairs within the DNA double helix. These bases can also form hydrogen bonds to other molecules in order to carry out their function. Suggest how other such molecules can bind to the bases and explain how this adds functionality.  

2d6 marks

Watson and Crick first proposed the double-helical structure of DNA and, in later work, the semi-conservative replication of DNA. Meselson and Stahl’s experiment proved Watson and Crick’s theory of semi-conservative replication. Define semi-conservative replication and outline the steps in Meselson and Stahl’s experiment that provided this proof.  

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3a4 marks

A section of DNA contains 1,200 base pairs.

  • The number of guanine molecules on strand one was counted as 156.
  • The number of cytosine molecules on strand one was counted as 209.
  • The number of adenine molecules on strand two was counted as 264.

Complete the table below to include the total number of each base present in the section, and the % composition of each base. 

  Number of molecules present % composition
Adenine    
Cytosine    
Guanine    
Thymine    

3b6 marks

Compare and contrast the structures of DNA and RNA. 

3c2 marks

Identify and explain the structural difference between a single strand of DNA and an RNA strand that makes DNA a better storage molecule than RNA. 

3d3 marks

The technique of DNA sequencing is used to determine the exact nucleotide sequence of an individual gene, chromosome or even an entire organism's genome. One of the applications of this technique is in forensic science, to examine samples of DNA left at crime scenes.  DNA sequencing is preceded by a technique called Polymerase Chain Reaction (PCR) which employs DNA polymerase.

Suggest and explain two advantages of using PCR to produce samples for forensic analysis. 

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4a3 marks

Ultraviolet exposure can cause guanine to be oxidised to 8-oxyguanine, which is unable to hydrogen bond to cytosine. Instead, during replication, 8-oxyguanine can form hydrogen bonds with adenine, resulting in a base pair. Outline the possible consequences of this change.  

4b2 marks

The structures of the four bases of DNA are shown in Figure 1.

Figure 1

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Draw the positions of the hydrogen bonds that form during base-pairing. 

4c2 marks

Explain why it is incorrect to state that RNA is always single-stranded.   

4d4 marks

Describe and explain the structure and function of a ribosome.

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5a6 marks

The structure of DNA has many characteristics that give the molecule excellent functionality.  Describe and explain three such structural features.   

5b2 marks

Account for the long period of time (approximately 80 years) between the identification of a molecule of (what turned out to be) DNA and the proof that DNA carried genetic information.

5c3 marks

Name the pyrimidine and purine bases in DNA and RNA and explain their base-pairing characteristics.  

5d2 marks

During replication of DNA, nucleotides are activated prior to incorporation into a nascent strand of DNA. Describe what is meant by ‘activated’ nucleotides and their role in replication.

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1a2 marks

Draw and label a single RNA nucleotide.

1b2 marks

DNA holds the genetic information of the cell and is kept protected in the nucleus. Within the nucleus, DNA is replicated semi-conservatively in order to produce new cells.

State two features of DNA and explain how these features are important in the process of semi-conservative replication of a cell’s DNA.

1c2 marks

Mitochondrial DNA (mtDNA) replicates differently to nuclear DNA.

Scientists believe replication of the second mtDNA strand begins after two-thirds of the first strand has been replicated. A piece of mtDNA contains 33,140 nucleotides and it takes 0.02 seconds to replicate a nucleotide.

How long it would take to copy this mtDNA to the nearest whole second.

 

1d3 marks

Figure 1 shows DNA replication. 

Figure 1

    screenshot-2023-05-22-201611

Name the enzyme shown in Figure 1 and describe its function.

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2a5 marks

Compare the structures and functions of DNA and RNA.

2b2 marks
i)
 How are the two strands of the DNA molecule held together? 
ii)
 Which part of a DNA molecule contains nitrogen?

2c1 mark

Other than for use in replication, give one advantage of DNA molecules having two strands.

2d1 mark

When scientists initially investigated which biological molecule was responsible for carrying the genetic code it was deemed unlikely that the molecule was DNA. 

Suggest why scientists initially concluded this.

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3a2 marks

Figure 1 shows the process of DNA replication. The horizontal lines represent the positions of bases.

Figure 1

screenshot-2023-05-22-211743

Which parts of the DNA molecule are represented by the labels W, X, Y and Z?

3b2 marks

Table 1 shows the percentage of different bases in the DNA of some organisms.

Table 1

Organism

Percentage of each base

Adenine

Guanine

Cytosine

Thymine

Human

32.8

17.2

17.2

32.8

Caterpillar

33.1

16.9

16.9

33.1

Mouse

 

22.4

 

 

Virus

24.7

24.1

18.5

32.7

Calculate the missing figures for mouse DNA and complete Table 1.
3c3 marks

Humans and caterpillars have very similar percentages of each base in their DNA but are not the same class of organism.

Use your knowledge of DNA structure and function to explain how this is possible.

3d2 marks

The DNA of the virus is different from that of the human, caterpillar and mouse. Using data from Table 1 and your knowledge of DNA suggest what this difference is. Explain your answer.

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4a1 mark

Figure 1 shows a base pair within a molecule of DNA.

Figure 1

screenshot-2023-05-22-222353

Name part P of each nucleotide.

4b2 marks

Scientists sequenced the gene of hormone X in order to understand more about why some individuals stop producing the hormone. The scientists determined that the gene consisted of 1500 base pairs, 30% of which were cytosine.

How many nucleotides of adenine and guanine in this sample of DNA?

4c7 marks

Describe the process of semi-conservative replication of DNA. In your answer give details of any molecules, bonds or enzymes involved.


4d1 mark

Genetic material contains genes that code for proteins, such as enzymes and hormones, that are vital for an organism to function. 

Ribosomes are the site of protein synthesis. State the biological molecules that form a ribosome. 

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5a4 marks

Describe the structure of a DNA molecule.

5b2 marks

Meselson and Stahl grew bacteria for several generations in a solution containing light nitrogen (14N) and obtained DNA from a sample of these bacteria (Sample 1).They then transferred the bacteria to a solution containing heavy nitrogen (15N). This step of the process was timed so that the bacteria were allowed to grow and divide twice. After each division, they obtained DNA from a sample of bacteria (Samples 2 and 3).The DNA from each sample of bacteria was suspended in a solution in separate tubes which were then spun in a centrifuge at the same speed and duration. Figure 1 shows Meselson and Stahl’s results.

Figure 1

1-5-q5b-medium-nucleic-acids

Table 1 shows the types of DNA molecules that could be present in samples 1 to 3. Complete the table with a tick if the DNA molecule is present in the sample.

Table 1

1-5-q5b-medium-nucleic-acids-1

5c2 marks

Hypoxanthine is a drug used to treat certain cancers as it prevents DNA replication and therefore inhibits tumour growth. Figure 2 shows the structures of Hypoxanthine and the DNA base guanine.

Figure 2

screenshot-2023-05-22-225548

Use the information in Figure 2 to suggest how hypoxanthine prevents DNA replication.

5d1 mark

Explain why DNA replicates semi-conservatively.

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