2.5 Nucleic Acids: Structure & DNA Replication

Explain why only bases that are complementary to the bases on the template strand can be added to the new DNA strand during DNA replication. 

Ultraviolet exposure can cause guanine to be oxidised to 8-oxyguanine, which is no longer complementary to cytosine. Instead, during replication, 8-oxyguanine can form bonds with adenine, resulting in a base pair.

Outline the possible consequences of this change.  

In the absence of mutagens, the rate of mutations during DNA replication 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. 

Even the smallest DNA molecules are very long.

• A kilobase (Kb) is a unit equivalent to 1000 base pairs of a DNA molecule.
• One Kb of double stranded DNA has a length of 0.34 μm.

The DNA in the nucleus of a cell from a fruit fly (Drosophila) is 5.6 cm long.

Calculate the number of Kb in the DNA of the fruit fly. Give your answer to the nearest whole number.

The amount of DNA found in the nucleus of cells can vary amongst people, with each human chromosome containing between 5 x 10⁴ and 26 x 10⁴ Kb of DNA.

Suggest one reason why people might have different quantities of DNA to each other.

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

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. 

The image below shows a section of the skeletal formula of a DNA molecule.

Number the carbon atoms of all the pentose sugars shown in the image using the standard numbering format.

The DNA nucleotides are covalently bonded together in the sugar-phosphate backbone between the pentose sugar and the phosphate group, however, they are hydrogen bonded together between the bases. 

Explain why both types of bonds are important for the functioning of DNA. 

During DNA replication both DNA strands act as a template, whereas in transcription only one strand acts as a template. 

Outline what is meant by the word 'template' in this context. 

One mark is available for clarity of communication throughout this question. 

ATP is a source of energy used in cells and is produced from processes such as respiration. 

The structure of ATP is shown in the diagram below.

Use the information in the diagram, as well as your own knowledge, to compare and contrast the structure of ATP with an adenine DNA nucleotide.

Explain how the structure of DNA allows replication.

Outline the steps in the experiment that Meselson and Stahl carried out to determine the semi-conservative nature of DNA replication. 

Using appropriate shapes to represent chemical structures, draw and label a single RNA nucleotide.

A section of DNA was found to contain the following percentages of bases, as shown in the table below.

Use your knowledge of DNA structure to complete the table by filling in the missing boxes.

The diagram below shows DNA replication.

Identify the enzyme shown in the diagram and describe its function.

Indicate with a tick or ticks (✓) in the table below the chemical group(s) that appear(s) at the two ends of a single strand of DNA.
 

The diagram below shows a base pair within a molecule of DNA.

Identify part P of this section of DNA

Scientists sequenced the gene for a hormone, in order to understand more about why some individuals stop producing this hormone. The scientists determined that the gene consisted of 1 500 base pairs; 30% of the total bases were cytosine.

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

Describe the conventional numbering system for carbon atoms in a pentose sugar such as the ribose or deoxyribose sugars found in RNA and DNA. You may sketch a diagram to illustrate your answer.

The diagram below shows the process of DNA replication. The horizontal lines represent the positions of bases.

Identify the parts of the DNA molecule represented by the labels W, X, Y and Z.

The table shows the percentage of different bases in the DNA of some organisms.

Calculate the missing figures for mouse DNA and complete the table.

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.

The DNA of the virus is different from that of the human, caterpillar and mouse. Some viruses contain single-stranded DNA that is not base-paired to a complementary strand. Use data from the table in question 4b) to show evidence for this difference.

One mark is available for clarity of communication throughout this question. 

Describe the structural features of a DNA molecule.

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. The diagram below shows Meselson and Stahl’s results.

The table below shows the types of DNA molecule that could be present in samples 1 to 3. Complete the table with a tick (✓) if the DNA molecule is present in the sample.

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