Genetic fingerprinting

What does a DNA profile represent?

A DNA profile (genetic fingerprint) represents only non-coding portions of DNA.

It is not the same as a DNA sequence which represents all the sequence of bases in a genome.

What are introns?

Non-coding regions of DNA.

What are exons?

Coding regions of DNA (codes for proteins).

What are short tandem repeats (STRs)?

Blocks of repeated base sequences in introns.

The number of times that STRs are repeated is what produces the variation in individuals.

What 2 techniques does producing a genetic fingerprint rely on?

1. The polymerase chain reaction (PCR)

2. Gel electrophoresis.

What is the PCR?

It is the technique used to amplify DNA (make many copies) where only small samples are available in order to make enough for analysis.

Describe the steps in the PCR.

• The original DNA is heated to 95°C to separate the two strands of the DNA molecule by breaking the hydrogen bonds between them.

• The temperature is cooled to 50-60°C to allow primers to anneal (join) to the DNA by complementary base pairing to the start of the DNA sequence.

• The temperature is raised to 70°C and a thermally stable Taq DNA polymerase attaches new nucleotides to their complementary base pairs on each strand. It joins them by catalysing formation of phosphodiester bonds between the sugar and phosphate molecules, making the backbone.

• Steps 1-3 are repeated many times, doubling the quantity of DNA produced each time. After 40 cycles over a billion copies of the target sequence can be produced from just one piece of DNA.

What are primers?

Primers are short fragments of single stranded DNA that attach by complementary bases pairing to the start of the DNA sequence and mark the area to be amplified.

This also prevents the separated DNA strands from re-joining.

What is gel electrophoresis?

Gel electrophoresis is a method of separating DNA fragments according to size / length.

Describe how gel electrophoresis is carried out.

• The DNA is extracted from the sample and cut into small fragments using restriction endonucleases.

• Fragments of DNA are loaded into wells at one end of a trough containing gel.

• The gel is exposed to an electric current.

• Since the fragments are negatively charged (due to the negative phosphate group), they move towards the positive terminal.

• Smaller fragments find it easier to migrate through the pores in the gel and so travel further than large fragments in the same time.

• The DNA becomes separated into bands according to the size of the fragments.

What are restriction endonucleases?

A restriction endonuclease is a bacterial enzyme that cuts DNA at specific nucleotide sequences.