Forensics

Gel electrophoresis is used to separate DNA fragments of different lengths.

The rate at which the DNA fragments move through the gel depends on several factors including:

• • Molecular size of the DNA fragment
  • Shape of the DNA fragment
  • Concentration of the gel.

(1)

(3)

Fragments of double-stranded DNA move through the gel at a relative rate (M_r) that is inversely proportional to the log of their molecular weight (MW).

(i)

Complete the table using the equation:

M_{r =} 

(2)

The diagram shows the position of a DNA fragment with a MW of 10 000, after gel electrophoresis.

Complete the diagram to show the position of a DNA fragment with a MW of 100 000.

Use the information in the question.

The fragments move more slowly through a higher concentration of gel.

Suggest why the fragments move more slowly through a higher concentration of gel.

Circular DNA moves at a faster rate through the gel than linear DNA.

(i)

Give two examples of circular DNA found in cells.

(2)

(ii)

Give two differences between the structure of circular DNA and that of linear DNA, other than their shapes.

The time of death of a person can be estimated in a number of ways.

The time of death can be estimated using the length of insect larvae.

What is the name of the method that uses insect larvae to estimate the time of death?

The larvae of one species of blowfly can be used to estimate the time of death.

The graph shows the mean length of larvae from this species incubated at 10.62 °C.

Calculate the mean growth rate of these larvae from 25 to 120 hours.

Include the units with your answer.

(2)

(ii)

Comment on the suitability of using this data to estimate the time of death.

Use the information in the graph to support your answer.

(3)

(iii)

Describe how the data shown in this graph could have been collected.

(3)

The time of death can be estimated using the body temperature of the corpse.

Evaluate the use of the body temperature of a corpse to estimate the time of death.

The photographs show a Northwestern crow and an American crow.

These two crows look very similar and are therefore difficult to distinguish as two separate species.

Scientists have studied the nuclear DNA and the mitochondrial DNA (mtDNA) of these two species of birds.

Give two differences between the structure of nuclear DNA and mtDNA.

The mtDNA was isolated from these two species of crow and amplified using the polymerase chain reaction (PCR).

The diagram shows details of the process used.

Name three molecules, other than the mtDNA and water, that would be needed in this process.

(2)

(ii)

Calculate the total length of time, in hours, that this process would take.

Give your answer to two decimal places.

(2)

(iii)

Explain how this process amplifies the DNA.

Use information in the diagram to support your answer.

(3)

Explain how the amplified mtDNA could be used to determine the genetic relationships between these two species of crow.

Forensic entomology is a method for estimating the time of death of a mammal.

Seventy‐two hours or more after death, forensic entomology is the most accurate method for estimating the time of death.

Forensic entomology can also provide information about the place of death and indicate if the body has been moved.

(i)

Explain why forensic entomology is the most accurate method for estimating the time of death, if this is greater than 72 hours.

(2)

(ii)

Explain how forensic entomology can indicate if a body has been moved from the place of death.

(2)

A study looked at the succession of insects associated with the decomposition of a dead mammal in the Andean Coffee region.

In this study, a mammal was killed, placed inside a metal cage in this region and left until it had completely decomposed.

The body of this mammal was monitored regularly.

The stages of decomposition were identified and various measurements were taken and recorded. Insects in the different stages of their lifecycle were collected and identified.

The graph shows the changes in mass of this mammal during the decomposition period.

Suggest why the mammal was placed inside a metal cage to decompose.

(1)

(ii)

Calculate the rate of change in mass between day 5 and day 15 of decomposition.

Express your answer in kg hr^-1.

Give your answer to two decimal places.

(2)

(iii)

Explain the changes in mass of this mammal during decomposition.

(4)

(iv)

Suggest why some of the insect eggs, collected from the decomposing mammal, were taken back to the laboratory and kept for a few days.

(1)

(v)

The table shows data on some of the insects collected from this decomposing mammal.

Explain how these results illustrate succession.

Use the information in the table to support your answer.

(3)