DNA PROFILING
By Mrs. Priyanka Tyagi
Human DNA
One copy of Homo sapiens DNA contains three billion base pairs
located on 23 chromosomes. It codes for approximately 50,000
genes.
So, only a few percent of our DNA actually codes for protein!
1. DNA that codes for protein. Exons 2-3%
2. Non-coding single copy. Intron DNA 70%
3. Non-coding repetitive DNA. 30%. Of this:2/3 Randomly repetitive (or 20% total
DNA)
1/3 Tandemly repetitive (or 10% total
DNA)
Randomly repetitive An interesting example of
randomly repetitive DNA is the ALU Family. This is a 300bp
sequence that is randomly repeated 500,000 times in our DNA!
(5% of total!)
Tandemly repetitive An example of tandemly repetitive
DNA is the sequence (GGGCAGC)n. This sequence is
tandemly repeated in different locations in our DNA.
This last grouping is called Variable Number
Tandem Repeats or VNTR. This DNA is:
Non-coding
Repetitive
The repeating sequence is repeated next to itself
(tandem)
The number of repeats is variable from person to
person
The VNTR DNA can be used in one method of
obtaining a DNA fingerprint.
This method is called Restriction Fragment Length
Polymorphism and results in an RFLP Fingerprint.
This sequence (GGGCAGC)n is tandemly
repeated in different locations in our DNA.
 However, different individuals have
different numbers of repeats at these locations.
Because the number of repeats is different
from person to person, when we take the DNA
and cleave it using restriction endonucleases,
we wind up with pieces of DNA with different
lengths (Restriction Fragment Length
Polymorphism).
Tandem Repeats :
These area of DNA have repeats of same basic
sequence, for example GATCGG repeated over and
over again.
 These repeats are next to each other in tandem just
like a tandem bicycle built for two.
This is like this: GATCGG GATCGG GATCGG.
So here are three repeats of GATCGG sequence.
•The number of tandem repeats can vary a lot from one
person to another.
•One person could have 15 repeats of the same sequence,
while the another person could have 50 repeats.
•These special segments of repeating tandem DNA that
can vary from one person to another are called Variable
Number Tandem Repeats or VNTR.
In gel electrophoresis, small sized pieces of DNA will
move through the gel faster than larger pieces.
A piece of DNA with a large number of VNTRs is
larger than a piece with small number of VNTRs.
In this case, the piece with 4 VNTRs will move
quickly than the piece with 6 VNTRs .
Now look at the different pictures from gel electrophoresis
below and pick one which matches the number of VNTRs
in person B above.
Basic outline to perform a RFLP fingerprint is as follows:
1. Break open the cells and isolate the DNA.
2. Digest the DNA with a restriction enzyme.
3. Make the DNA single stranded by heating.
4. Separate the DNA fragments according to size using gel
electrophoresis.
5. Transferring of separated DNA fragments to synthetic
membranes, such as nylon or nitrocellulose.
6. Soak the membrane in a solution containing a radioactive
single strand DNA probe to the VNTR sequence.
7. Wash the gel to remove any free probe.
8. Expose the gel to a photographic plate to locate where the
probe bound (Autoradiography).