SNAB topic 6: spec point 4

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  • Created on: 24-02-16 19:02
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4 Describe how DNA profiling is used for identification and determining genetic relationships
between organisms (plants and animals).
DNA profiling relies on the fact that apart from identical twins, every person's DNA is unique
A large amount of the DNA does not code for proteins. The non coding blocks are called= introns
Genes with the coding regions= exons
Within introns, short DNA sequences are repeated many times
The sequence of repeated bases are known as the short tandem repeats (STRs) or satellites
An STR can contain 250 base pairs and can be repeated from 5 to 700 times
The same STRs occur at the same place (locus) on both chromosomes of a homologous pair
However the number of times they are repeated on each of the homologous chromosomes can be
different. The number of repeats at a locus also varies between individuals
How is a DNA profile made?
A tissue sample is obtained and the DNA is extracted
Sufficient DNA to manipulate in the lab must be available
Fragments of different lengths are then created by cutting up the DNA . The fragments are then
separated and visualised in some way
Finally the profile created is then compared with another
A DNA profile on its own is useless there has to be a reference profile for comparison
The reference profile might come from a suspect in a murder investigation (for example)
Obtaining the DNA
A DNA sample can be obtained from almost all biological tissue , animal or plant
DNA extraction the tissue sample is physically broken down into a buffer solution that includes salt and
a detergent to disrupt the cell membranes
The small suspended particles including the DNA are separated from the rest of the cell debris by
filtering or centrifuging
Protease enzymes are incubated with the suspension to remove proteins , and then cold ethanol is
added to precipitate out the DNA
Creating the fragments
The DNA sample is treated with restriction enzymes ( endonucleases) which are found naturally in
bacteria, where their function is to cut up invading viral DNA
The enzymes will only cut DNA at specific base sequences this is why they're valuable
If the restriction sites are at either side of a short tandem repeat sequence= that fragment will remain
intact but it will only be cut away from the rest of the genome the repeated sequences remain intact
Bacteria protect their own DNA sequence from restriction enzymes by changing the bases in the
sequences that are targeted by their own restriction enzymes
Restriction enzymes cut a DNA sample into fragments only where their specific restriction sequence
occurs
If the same restriction enzyme is used to cut 2 identical DNA samples= identical STR fragments are
produced

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