Let's Talk DNA

We've all read the book, or watched the show, where a main piece of evidence linking a suspect with the crime is a single strand of hair.  But do you know the facts?

Did you know that hair contains no nuclear DNA, although it can contain small amounts of Mitochondrial DNA.  What does this mean?  First of all, let's get an understanding as to the difference.

Nuclear DNA is considered to be the "blueprint or thread of life" which is genetic coding used in the development and functioning of all living organisms.  It contains 15 different markers or locations along the DNA molecule to determine a full nDNA profile.  Nuclear DNA is found in living cells.  Now because hair is composed of dead cellular debris, unless the hair has been yanked from the head and contains a follicular bulb (skin tag as when it's pulled out), it cannot provide nuclear DNA.

However, Mitochondrial DNA is found within the cell cytoplasm, and are the energy producers of the cell and body.  MtDNA has several characteristics that make it unique.  First of all it's passed from one generation to the next from the maternal linage.  For example, your mtDNA is inherited unchanged from your mother, and only your mother.  And since it only undergoes a significant mutation once every 6500 years, your mtDNA is virtually identical to your mother's, your great-great-grandmother's and your maternal ancestors of a thousand years ago.  Now because hair is composed of dead cellular debris, forensic scientists can extract this type of DNA from the shaft of a hair.  This could be used to help identify the person who shed the hair, at least the family to which it belonged.  (If more than one family member was suspect this type of DNA wouldn't clarify who was at the scene.)

So, while Nuclear DNA can confirm identity to an individual, MtDNA can confine identity to a family.  MtDNA makes it possible to profile more degraded and deteriorated samples which can be used to identify skeletal remains.

From where else can DNA be obtained?

Now the most common we all know about is blood, but often times semen, tears, urine, bone, teeth and skin are found at the scene.  Each of these can yield enough usable DNA when using modern technologies.

Below are some interesting facts.  Maybe these can be incorporated in your novel(s) to add more realism to your forensic results.

Male ejaculate:  As noted, DNA can be pulled from sperm, however if the man has had a vasectomy, there will be no sperm or DNA.  However, the epithelial cells that line the urethra contain DNA.  The urethra is the channel that connects the bladder to the outside and as the ejaculate moves along the urethra, it collects some of these cells.  It is possible a forensic scientist could pull enough DNA to make an identity.

Saliva:  It doesn't contain any cells but it collects the DNA-containing epithelial cells of the salivary ducts as it passes through the glands to the mouth.

Tears:  Like saliva, tears pick up epithelial cells that line the tear ducts, carrying out with them DNA.

Teeth:  The enamel has contains no cells, but the pulp does.  This pulp can survive for a very long time under some adverse conditions.

How long later can DNA be extracted and useful?

Situations vary and it depends on the quality of the sample.  But as far as quantity, very little is required.  For example, according to one source, human DNA could be extracted from maggots found on a decaying corpse up to four months after death.

__

Information for this post obtained from Howdunit Forensics A Guide for Writers by DP Lyle MD