Genetic fingerprinting

Cystic fibrosis (CF) is a recessive inherited disorder caused by a defective CFTR gene.

Table 6 shows the number of people with CF and the estimated population of the UK in 2020 and 2022.

Calculate how many more people had CF than was expected in 2022.

You can assume the relative frequency of CF in a population remains constant.

Show your working.

Additional number of people

CF can be caused by a three-base deletion mutation in the CFTR gene. This deletion

creates a recessive allele called F508del.

DNA probes and gel electrophoresis were used to screen two parents and their two children for the F508del allele. One of the children was diagnosed with CF.

Figure 5 shows the results.

What is a DNA probe?

The identity of each family member can be determined from the relative position of the bands shown in Figure 5.

Use information in Figure 5 to:

• complete Table 7

• explain how you identified the child with CF.

You can use F to represent the dominant CFTR allele and f to represent the recessive F508del CFTR allele.

Bam H1 and Hin dIII are both restriction endonucleases.

Figure 4 shows the positions where these enzymes cut a linear molecule of DNA.

In two experiments, multiple copies of the DNA molecule shown in Figure 4 were all completely cut into fragments using these restriction enzymes. The DNA fragments produced were then separated by electrophoresis.

Experiment 1 – DNA cut into fragments using Bam H1.

Experiment 2 – DNA cut into fragments using Bam H1 and Hin dIII.

Complete Figure 5 to show the relative positions of the bands following

electrophoresis in experiments 1 and 2.

Suggest how you could determine the size of the different DNA fragments produced in these experiments.

How many DNA fragments would be produced in experiment 2 if the original DNA molecule was a plasmid?

Describe how restriction endonuclease and DNA ligase are used to insert a gene into a plasmid.

Short tandem repeats (STRs) are short sequences of DNA, usually 2 to 7 base pairs. STRs are repeated a number of times, one after another. For example, the STR D5S818 is made up of AGAT repeated 7 to 16 times.

STRs are found throughout the whole genome.

The repeated sequences in STRs are common to all humans. Due to variation in the number of repeats, STRs can be used in genetic fingerprinting.

Describe how STRs could be removed from a sample of DNA.

Genetic fingerprinting using STRs requires amplification of the STRs using the polymerase chain reaction (PCR). The short base sequences either side of a specific STR are known.

Explain the importance of knowing these base sequences in PCR.

A single STR molecule consisting of a 12 base pair sequence of AGAT was amplified 50 times using PCR.

Calculate the total number of base pairs in all the STR molecules after 50 cycles of PCR.

Show your working.

During genetic fingerprinting, the different STRs are separated by gel electrophoresis.

Give two features of STRs which enable them to be separated by gel electrophoresis.

In Europe, viruses have infected a large number of frogs of different species. The viruses are closely related and all belong to the Ranavirus group.

Previously, the viruses infected only one species of frog.

Suggest and explain how the viruses became able to infect other species of frog.

Name two techniques the scientists may have used when analysing viral DNA to determine that the viruses were closely related.

Determining the genome of the viruses could allow scientists to develop a vaccine.

Explain how.

Describe how the B lymphocytes of a frog would respond to vaccination against Ranavirus.

You can assume that the B lymphocytes of a frog respond in the same way as B lymphocytes of a human.

Do not include details of the cellular response in your answer.

What is a DNA probe?

DNA probes are used to detect specific base sequences of DNA.

The process is shown in Figure 6.

Describe how the DNA is broken down into smaller fragments.

The DNA on the nylon membrane is treated to form single strands. Explain why.

A scientist used DNA probes and electrophoresis to screen four volunteers for five different viral DNA fragments.

Figure 7 shows the results the scientist obtained. The lanes numbered 2 to 5 represent the four volunteers.

Lane 1 of Figure 7 enabled the size of the different viral fragments to be determined.

Suggest and explain how.

The lengths of the viral DNA fragments were:

• 600 base pairs

• 250 base pairs

• 535 base pairs

• 300 base pairs

• 500 base pairs.

Which volunteers had at least one of the viral DNA fragments with 250 base pairs or 535 base pairs?