MadSci Network: Genetics

Re: How can DNA from a single strand of hair is extracted?

Date: Wed Sep 13 14:27:31 2000
Posted By: Mark Sullivan, Medical Student
Area of science: Genetics
ID: 968646932.Ge

Hi Malik,
     Extraction of DNA from a cell is actually very easy.  I woon't go 
through the process but it is done by using chemical steps that degrade 
and remove the components of a cell which protect DNA.  Now, if you were 
to take the DNA from one cell, which exists as a set or 46 chromosomes, 
and attached each one end to end like a tying small strings to form a 
larger one, it would extend for several meters yet only be 1/20,000,000 of 
a meter wide.  So, one cell packs in quite a bit of information.  Now 
within this DNA is found somewhere around 100,000 genes, but the exact 
number has not yet been determined.  Humans share all the genes as a 
species.  They are what give us our characteristic features as humans.  It 
is the small differences that make each of us unique among all the other 
human beings on earth.  And we can use techniques of molecular biology to 
pick out small genetic differences to identify the uniqueness of one 
person versus everyone else.  
     It is possible to genetically fingerprint someone with just one hair 
sample, if the hair contains the bulb which is where the cells are 
located.  In this bulb there are hundreds to thousands of cells.  When DNA 
is extracted that gives us just enough to work with for the next step.  In 
order to genetically fingerprint a person we have to use another chemical 
reaction called Polymerase Chain Reaction or PCR for short.  This is a 
means of selectively amplifying portions of DNA so that you have enough to 
utilize in your tests.  Think of PCR as a photocopy machine, and your DNA 
as a book that is unique to you.  Everyone else has a book like yours with 
the same amount of pages and chapters.  The books contain recipes for 
making you, and your recipes though similar to everyone elses are slightly 
different.  Now I get a copy of your book and look at page 16, and make 
500 photocopies, then I make some copies of my book at page 16.  If we 
compare them side by side there are small differences in what is written.  
When a forensic scientist is analyzing a DNA sample from hair of blood or 
anything, this is what they are doing.  The reason they use PCR is to make 
enough copies of the DNA in question first because you have to have enough 
to see, and second so that there is a lot more that one small original 
sample to work with.  
     Now remember that PCR only makes copies of small portions of DNA, not 
the entire set of chromosomes.  The portions that they make copies of are 
called microsatellites.  These are common sequences of DNA that differ in 
length from one person to another.  And by comparing lots of different 
microsatellites you can make sure that the sample you are analyzing is 
unique to one person.  The more variables you look at, the better you are 
able to determine uniqueness.  Think of it in terms of comparing two 
people with brown hair.  Not very discriminatory yet.  What is the 
texture?  Well one might have very fine hair, and the other more coarse.  
What is the length?  Are there any highlights?  Is it straight or wavy or 
curly?  It is parted on one side?  All of these questions help narrow down 
each person to having unique characteristics.  Genetic fingerprinting is 
just a very good way of doing this cause though you can change your hair 
style, or surgically alter your face, you can't change your DNA.  Hope 
this helped you.  The best way to learn more about this is to take some 
courses in something like Biology or Biochemistry.  Good luck,

Mark Sullivan

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