Deoxyribonucleic Acid (DNA) is found in almost all
living cells and carries the coded information that determines a
persons characteristics. This code is inherited from a person's
parents, so it can be used to examine genetic relationships.
DNA is made up of four main chemicals (bases) which are like the
teeth of a zipper and arranged in a spiral called a double helix.
The four chemicals consist of Adenine (A), Guanine (G), Cytosine
(C) and Thymine (T); where A always pairs with T and G always pairs
with C. DNA is composed of millions of these bases and their combinations
are unique to each person, with the exception of identical twins.
Nuclear vs. mitochondrial DNA...
What do you think of when someone mentions DNA? Do you think of
the 46 chromosomes that inhabit the nucleus of almost every cell
that comprises your body? These chromosomes hold the vast bulk of
genetic information that you've inherited from your parents.
Outside the nucleus, but still within the cell, lie mitochondria.
Mitochondria are tiny structures that help cells in a number of
ways, including producing the energy that cells need. Each mitochondrion
-- there are any where from 500 to 2000 in every human cell -- includes an identical
loop of DNA about 16,000 base pairs long containing 37 genes. In
contrast, nuclear DNA consists of three billion base pairs and an
estimated 25,000 genes
Inheriting mitochondrial DNA (mtDNA)...
During fertilization, nuclear chromosomes from a sperm cell enter
the egg and combine with the egg's nuclear DNA, producing a mixture
of each parents' genetic code. The mtDNA from the sperm cell, however,
is not transferred to the egg. As a result, the fertilized egg contains
a mixture of the father and mother's nuclear DNA and an exact copy
of the mother's mtDNA, but none of the father's mtDNA.
Therefore, mtDNA is passed on only along the maternal line. This
means that all of the mtDNA in the cells of a person's body are
copies of his/her mother's mtDNA, and all of the mother's mtDNA
is a copy of her mother's, and so on. No matter how far back you
go, mtDNA is always inherited only from the mother
What is DNA testing?
DNA testing is a valuable method of comparing and/or identifying
individuals. By analyzing DNA, we are able to pinpoint individuals
involved in crimes, identify genetic relationships (as in questionable
paternity cases and estate claims), show lineage among animal populations,
and even identify disease in both modern and ancient populations.
Profiles of different samples/individuals can then be compared to
see if they match. Whether the testing is conducted for forensic
analysis or the analysis of genetic relationships, DNA testing is
a powerful tool for identifying a match or excluding any genetic
How is DNA Tested?
1. Collection of biological materials - typically blood, hair
or skin cells from a buccal (cheek) swab.
2. Extract and isolate DNA by mixing the sample with chemicals
that break down other cellular materials and separates the DNA
from other unwanted cellular components.
3. Amplify the DNA by separating the DNA helix ("unzipping
the zipper") by mixing them with short fragments called primers.
When a primer locks onto a particular site on a sample DNA molecule,
it triggers the production of a longer fragment that matches a
piece of the sample. Multiple cycles of this amplification step
are carried out and result in the production of millions of copies
4. Segregate the resulting DNA strands to generate a DNA profile
for each extracted sample.
Why is mtDNA used most for ancient DNA analysis?
MtDNA is particularly well suited for ancient DNA analyses because
of its high cytoplasmic copy number, mode of maternal inheritance,
and accelerated mutation rate over that of nuclear DNA. Ancient
DNA, like any artifact, suffers damage over time making useful interpretation
difficult; however, the high copy number of mitochondrial genomes
per cell (1,000's), as opposed to generally two copies of a given
nuclear gene, ensures that short informative sequences survive through
time. Informative sequences can be recovered, copied, and analyzed
for maternal relationships within, and between groups.