Nucleic Acids: Structure & DNA Replication (AQA A Level Biology)

Draw and label a single RNA nucleotide.

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.

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.

Figure 1 shows DNA replication. 

Figure 1

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

Compare the structures and functions of DNA and RNA.

i)  How are the two strands of the DNA molecule held together? 

ii) Which part of a DNA molecule contains nitrogen?

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

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.

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

Figure 1

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

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

Table 1

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

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. Using data from Table 1 and your knowledge of DNA suggest what this difference is. Explain your answer.

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

Figure 1

Name part P of each nucleotide.

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?

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

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. 

Describe the structure 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. Figure 1 shows Meselson and Stahl’s results.

Figure 1

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

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

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

Explain why DNA replicates semi-conservatively.

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

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

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.   

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. 

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

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 .

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.  

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.  

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. 

Compare and contrast the structures of DNA and RNA. 

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. 

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. 

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.  

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

Figure 1

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

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

Describe and explain the structure and function of a ribosome.

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

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.

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

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.