Short tandem repeats: non-coding DNA sequences repeated a different number of times in different people.

List the stages of DNA profiling.

Extract DNA, amplify STRs by PCR, separate by gel electrophoresis, stain or label, compare band patterns.

How does a DNA profile show a parent-child relationship?

Half the child's bands come from each parent, so bands not from the mother must be from the father.

Give two uses of DNA profiling in animals or plants.

Choosing breeding pairs to avoid inbreeding; identifying species or the origin of wildlife products.

Name the components of a PCR mixture.

DNA template, two primers, free nucleotides, Taq polymerase and buffer.

What happens at 95 °C in PCR?

Denaturation: hydrogen bonds break and the DNA strands separate.

What happens at about 55 °C in PCR?

Annealing: primers bind to complementary sequences on the template strands.

What happens at 72 °C in PCR?

Extension: Taq polymerase adds nucleotides to the 3' end of each primer.

Why is Taq polymerase used?

It is heat stable, so it is not denatured during the 95 °C stage.

Why does DNA move to the anode in electrophoresis?

Its phosphate groups are negatively charged.

Why do smaller fragments travel further in the gel?

They meet less resistance as they pass through the agarose mesh.

What are exons and introns?

Exons are retained in mature mRNA and code for protein; introns are non-coding and are removed by splicing.

What is alternative splicing?

Joining different combinations of exons so one gene produces more than one protein.

DNA profiling and PCREdexcel A-Level Biology A: Flashcards

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What is a DNA profile?

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What is a DNA profile?: A pattern of DNA fragments, separated by size, that is characteristic of an individual.
What are STRs?: Short tandem repeats: non-coding DNA sequences repeated a different number of times in different people.
List the stages of DNA profiling.: Extract DNA, amplify STRs by PCR, separate by gel electrophoresis, stain or label, compare band patterns.
How does a DNA profile show a parent-child relationship?: Half the child's bands come from each parent, so bands not from the mother must be from the father.
Give two uses of DNA profiling in animals or plants.: Choosing breeding pairs to avoid inbreeding; identifying species or the origin of wildlife products.
Name the components of a PCR mixture.: DNA template, two primers, free nucleotides, Taq polymerase and buffer.
What happens at 95 °C in PCR?: Denaturation: hydrogen bonds break and the DNA strands separate.
What happens at about 55 °C in PCR?: Annealing: primers bind to complementary sequences on the template strands.
What happens at 72 °C in PCR?: Extension: Taq polymerase adds nucleotides to the 3' end of each primer.
Why is Taq polymerase used?: It is heat stable, so it is not denatured during the 95 °C stage.
How many copies after n PCR cycles from one molecule?: $2^n$ copies.
Why does DNA move to the anode in electrophoresis?: Its phosphate groups are negatively charged.
Why do smaller fragments travel further in the gel?: They meet less resistance as they pass through the agarose mesh.
What are exons and introns?: Exons are retained in mature mRNA and code for protein; introns are non-coding and are removed by splicing.
What is alternative splicing?: Joining different combinations of exons so one gene produces more than one protein.

Exam questions on DNA profiling and PCR

A forensic laboratory has recovered a very small amount of DNA from a single hair left at a crime scene. The DNA must be amplified by the polymerase chain reaction (PCR) before it can be profiled. The thermal cycler repeats a cycle of three temperature stages, using a DNA polymerase taken from the bacterium Thermus aquaticus.
Explain why primers are needed in PCR.2 marks
Short tandem repeat (STR) regions of DNA were amplified by PCR and the fragments separated by gel electrophoresis to compare a child with her mother and with two men, A and B, who might be her father. The fragment sizes in base pairs (bp) were: child 420, 310, 250 and 180; mother 420, 250 and 150; man A 310, 220 and 180; man B 390, 260 and 140.
Deduce which man is more likely to be the child's father. Use the data to justify your answer.2 marks
A student separated DNA fragments by electrophoresis in an agarose gel. A ladder of fragments of known size was loaded alongside the samples. The ladder fragments of 1000, 800, 600, 400 and 200 bp travelled 12, 17, 23, 31 and 45 mm from the wells respectively. Fragment X from a sample travelled 27 mm and fragment Y travelled 10 mm.
Describe how DNA fragments are separated by gel electrophoresis and then made visible.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).