A Fingerprint of Our Genetic Makeup: DNA Typing

Deoxyribonucleic acid (DNA): A polymeric molecule consisting of deoxyribonucleotide building blocks that in a double-stranded, double helical form is the genetic material of most organisms. (courtesy of iGenetics)

DNA is what sets all organisms apart. The DNA that we have is what makes us humans and the DNA horses have is what makes them horses. Within humans as a species much of the DNA molecule is identical but no two people have the exact same genome and this is where DNA typing or fingerprinting comes into play. DNA typing techniques are used in criminal trials, paternity tests, population genetics studies, proving pedigree status for horses and dogs, and identification of dead bodies. Of course there are numerous other uses and some will be explored later on.

The coding regions of DNA (exons) are very similar between human individuals but there are portions of DNA which seem to be noncoding regions which we know as introns. It is in these regions that repeated sequences of base pairs exist which differ from one person to the next. These sequences are called Variable Number of Tandem Repeats (VNTRs) if they are five to a few tens of base pairs long. Short Tandem Repeats (STRs) are two, three or four base pairs long and also show great variation. Every individual has some VNTRs (and STRs) and their VNTRs come from the genetic information donated by their parents. The VNTRs can be inherited from either that person’s mother or father and can be a combination of both. A very important part of this is that a person’s VNTRs will never have sequences that their parents do not have. Scientists can use these regions (VNTRs) to make a “fingerprint” to determine whether two DNA samples are from the same person, related people, or non-related people.

In the image to the left it is shown that Daughter one and Son one have VNTRs from both their mother and father. Daughter two has her mother’s VNTRs but not her father’s meaning she is from the mom’s other marriage. Son two has neither mom nor dad’s VNTRs because that son was adopted.

The two main processes for DNA analysis are:

1. Using restriction fragment length polymorphisms (RFLPs) detected by restriction enzyme digestion and Southern blotting analysis.

2. Using Length polymorphisms detected by PCR amplification and a dot blot assay.

RFLP Identification

1. Isolate DNA from blood, semen, or any other cell sample.

2. Extract the DNA from the cell.

3. Digest the DNA into different sized fragments by restriction enzymes such as EcoR1.

4. Separate the DNA fragments by gel electrophoresis.

5. Denature the DNA strand into two single strands.

6. Transferring the DNA band pattern to a nylon membrane (also known as Southern Blotting).

7. Hybridization of the single strand of DNA with a radioactive probe which binds to specific sequences of DNA. (The final DNA fingerprint is built by using several probes simultaneously.)

8. Creating a picture of the radioactively marked DNA onto x-ray film.

After this is done you can visualize bands corresponding to the lengths of the fragments of DNA which can then be used to compare to another person’s for criminal trials or paternity tests.

PCR-based Analysis

1. Isolate DNA from blood, semen, or any other cell sample.

2. Extract the DNA from the cell.

3. Amplify the DNA by using the polymerase chain reaction (PCR). This DNA is copied over and over again usually in a machine called a thermal cycler.

4. Treat the amplified DNA with a variety of probes that are bound to a blot (probes on a membrane are dipped into the amplified DNA). Each probe is found in a specific “dot” on the blot strip. A chemical reaction causes the dot to darken and become noticeable when DNA containing a particle variant is present.

5. Infer the genotype by the pattern of dots that indicates which probes the amplified DNA bound to.

A short preparation and analysis time of a few days makes this DNA typing technique more efficient then RFLP which take a few weeks.

There are various other techniques for DNA typing. The technique will be determined depending on the situation and the amount of DNA available. All make use though of the tandemly repeated sequences present in a person’s DNA. There are some issues though with the accuracy of the tests and the possibility that the fingerprint may not be unique to one individual. These problems that arise and how they have affected the history of DNA fingerprinting will be examined.