Bio Factsheet: Using DNA

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B io Factsheet Number 250
Using DNA to solve crimes
This Factsheet:
· Summarises the key techniques of the Polymerase Chain Reaction (PCR) and gel electrophoresis
· Describes how these techniques are used to solve crimes
· Reviews the many exam questions on this topic
Polymerase Chain Reaction (PCR)
The PCR enables the detection and identification of extremely small samples of DNA. It amplifies sections of DNA, massively increasing
the size of the sample. NOTE: In the exam don't write " PCR makes millions of copies of the DNA" It doesn't, it increases the amount of a
DNA sample by copying small sections only - giving forensic scientists enough to carry out genetic profiling. Fig.1 shows the key steps:
Fig.1 The polymerase chain reaction (PCR)
target DNAsequence
orginal DNA
1 1. The double-stranded DNA is heated to 95 °C to break the hydrogen bonds holding the two
DNA strands together
two single strands of DNA
DNA Primer
2. DNA nucleotides (A,T,C,G) and excess DNA primers (short, single
DNA Primer strand fragments of DNA made up of about 15-30 bases) are added
Encoding direction and the mixture is slowly cooled to 55 °C. The primers mark the
beginning and ends of the part of DNA that will be copied. Cooling
Encoding direction
2 the mixture allows the primers to bind (via hydrogen bonds) to their
DNA sections by complementary base-pair matching (A to T and C
to G etc). The primers contain fluorescent labels so that they can be
located in the amplified DNA
Taq Polymerase 3. The temperature of the mixture is increased to 72ºC and a heat-stable form
of the enzyme DNA polymerase (Taq polymerase) is added. The enzyme
binds to the double-stranded sections and extends them by catalyzing the
formation of new hydrogen bonds with the free nucleotides
3 primer
4. The cycle is repeated. Each
primer time all of the DNA molecules
are amplified. So after one cycle
we would have two DNA
molecules; 2 cycles = 4; 3 cycles
= 8; 4 cycles = 16; 5 cycles =
32; 6 cycles = 64. Each cycle
may take only 5 minutes. After
30 cycles, taking 2.5 hours the
PCR can increase the amount of
the target DNA by a factor of

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Using DNA to solve crimes Bio Factsheet
PCR can sufficiently amplify the DNA from a single cell (6pg) to · The samples are loaded into wells on a plate of agarose gel that
give a genetic profile. This enables very old crime scene evidence is attached to electrodes so that an electric current can be passed
to be re-examined.…read more

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Using DNA to solve crimes Bio Factsheet
matched or eliminated from the enquiry;
Typical Exam Question pattern is unique to every individual so samples can be
DNA tests were used to confirm the identity of a suspected war radioactive/ single stranded DNA detected on film;
criminal. DNA had previously been recovered from the suspect's probe applied;
toothbrush and now the authorities needed to confirm that they Ref to Southern blotting;
had captured the right individual.…read more

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Using DNA to solve crimes Bio Factsheet
Is the body male or female?
Forensic scientists can identify the sex of even very old corpses by A B C E
analysing the DNA of its bone. PCR is used to copy part of the gene Origin
that codes for amelogenin, which is found on the sex chromosomes.…read more



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