forensic science
Your body is made up of ~ 60 trillion cells.  These cells are little factories that perform the work your
body requires such as storing energy, making hormones, making enzymes to help digest your food,
and more.
DNA (a.k.a. deoxyribonucleic acid) is the recipe for how to make an organism and the instructions for
each cell to know what to do.  So you have tiny recipe cards in most of the cells of your body that give
the exact formula for how to make one of you!   DNA can also be used as an identifier as no two
individuals have the same DNA makeup - well, that is, unless you are identical twins!  

In every living thing (plant or animal) are these recipes or sets of instructions made up of shorter
segments called genes. The genes provide the instructions on what is the plant or animal, what it looks
like, how it behaves, how it is to survive, and how it will interact with its surrounding environment.

The genes are strung together in long stands of DNA and these strands are called chromosomes. Most
living organisms inherited a copy of DNA from each parent - one from dad and one from mom.  These
strands come together and 'fight it out' as to which genes will win and guide your looks, functions and
behaviors!   Therefore, most organisms have pairs of chromosomes in most of their cells with
reproductive cells and red blood cells being the exceptions). In humans, we have 23 pairs of
chromosomes and a lowly fruit fly has only 4 pairs!  Poor fruit fly!  

What if mom gave you the gene for brown eyes and dad gave you the gene for green eyes?  Well,
these different versions of genes are called alleles.   You have a pair of alleles for each trait and one of
them will typically 'win the fight' and be expressed in the organism.  Brown eyes is the dominant allele
that always wins the fight!  The green eyed allele is recessive and if there is a dominant allele present,
the green eyes won't be expressed - they are 'silenced!'  They're still there in the DNA code but nobody
will ever know...that is, until you have a green eyed child!  (This is a simplified example - eye color is
governed by more than one gene.)

Just like an alphabet makes up sentences, each gene is made up of combinations of four different
types of chemicals called nucleotides.  Just like words, it is the combinations of the nucleotide bases
that determine everything about a living organism!

The four nucleotides are called:
  • adenine(A)
  • cytosine (C)
  • guanine (G)
  • thymine (T)
Let's see an example:

The gene for brown eyes might have this nucleotide sequence.

AAAGTTCCTTT

The gene for blue eyes might have this nucleotide sequence.

AAAGTTCCAAA

Notice how the nucleotide sequences below are very similar. The only difference being the last three
bases. Both of these codes are giving information on how to make the eye but the last three tell what
color to make the iris of the eye.   Now this was only an example - if you were really looking at the DNA
sequence for eye color, it would be several thousand nitrogenous bases long!)
Here is a video for setting up and running a Gel
Electrophoresis
Here is a hilarious video about PCR (Polymerase
Chain Reaction)
Here is an informative video about Polymerase
Chain Reaction (PCR)
APPLYING YOUR DNA KNOWLEDGE TO THE CRIME SCENE...         
                 
DNA is found in nearly every cell of
the body.  Hair follicles, skin,
saliva (saliva picks up dead cells
from the mouth and glands), and
blood (the white blood cells) are
common sources for evidence at
the crime scene.
A single hair follicle, a licked
stamp or a single drop of blood is
all you need to perform an
analysis and catch your criminal!
Once you find DNA evidence at the
crime scene,  great care needs to
be taken to keep it from degrading!
DNA samples need to be placed in
adequately dried or frozen
containers until the analysis can be
performed.  
The information in the non-coded
region of DNA:  the total amount of
DNA in a cell is called the genome.
Only about 5% of this contains vital
instructions on how to make and
run the factory of 'you.'  The other
95% is like 'junk mail.'  But is it really
junk in a criminal investigator's
eyes?  NOPE!  There are variable
regions (polymorphisms) that vary
in length as well as base sequence.
 We use these variations to make a
DNA fingerprint.  
Polymorphisms is a fancy word
indeed!   Alec Jeffreys, in 1985,
discovered that there are segments
in a DNA molecule  that are unique
to each person . If you analyze these
segments, you can distinguish
between one person and another.  
We call this 'DNA Fingerprinting' or
'DNA Typing' and this is what is
used to make a match between a
suspect and a crime scene.  
Variable Number Tandem Repeats
(VNTRs):  the same base sequence that
repeats throughout a specific region in a
strand is called a VNTR.  These can be
hundreds of base pairs long and repeat a
variable number of times.  

Short Tandem Repeats (STRs): Shorter
than VNTRs, STRs repeat as well but are
only 3 to 7 base pairs in length.  STRs
repeat over segments of DNA that are as
long as 350 or more bases.   
LET'S PREPARE A SAMPLE OF DNA:


  • Extract the DNA from the sample: this is using chemical agents to separate the
    DNA (without harming or altering it) from the sample.

  • Cutting or Amplifying the DNA: you want to determine the VNTR  or STR
    sequences in any given locus (segment) of the DNA strand.  Several methods
    are available but the most common is 'restriction fragment length polymorphism'
    or RFLP) which is using enzymes to chop it up into specific fragments and then
    PCR (polymerase chain reaction) might be necessary  which is simply
    amplifying a small sample when there is not enough.

  • Now you have fragments, you need to separate these fragments by separating
    them in a viscous gel with an electrical current passed through and the
    fragments will separate based upon their charge (DNA is negatively charged)
    and their size.  The larger fragments will be slower to move through the gel and
    will separate from the quicker moving short fragments!)  

  • Now you've got separated fragments, you'll need to transfer them to a nylon
    membrane.  This is called a Southern Blot - named for the researcher who
    discovered it - not because it was only done in the South!  So you'll put a nylon
    membrane on top of the gel and kind of like soaking up liquid with a sponge -
    the bands of DNA will transfer onto the nylon membrane.  

  • Allright, you've got the DNA fragments, separated, and stuck on a membrane.  
    Now what?  You want to look at them and take a picture!  But first, you have to
    tag them with something you can look at!  You can view the bands of DNA by  
    tagging them with radioisotopes (radioactive chemicals) that attach to the
    fragments.

  • Now, look at your tagged fragments!  Using an autoradiograph, or autorad, you
    can view and make a picture of these awesome DNA bands and compare to a
    known sample of known weight as well as your unknown sample from the crime
    scene!  
CODIS:  Combined DNA Index
System is a database of DNA
fingerprints from criminals and
victims of assaults, homicides
and rapes.  Like the AFIS system
for Fingerprints, the CODIS
database gives investigators a
great tool to match up their
suspect with a crime scene and
other crimes around the world!  
Here is an informative video about Microarray
Techniques
NOW AVAILABLE
Volume I in the
Fiona Frost
Murder Mystery
novel series by
Dr. Bon Blossman.
9581602-20144005