The DNA Engineering Life… This crash course on DNA is meant to provide non-scientists a basic knowledge of molecular biology concepts, in order to better understand the applications of DNA sequencing in forensic medicine and the interpretations of its results

What is it? We can think of the DNA as a huge library containing the information necessary to the fabrication of proteins, the building blocks of life. The DNA has the structure of a very long double helix, formed by chemical compound, the bases.

Where is it? The DNA is in the nucleus of every cell, except for red blood cells that don’t have one. DNA is also found in cytoplasmic bodies called mithocondria. It has a double helix structure, the result of bonds between the bases, that are complementary chemical structures. DNA is highly compacted to fit the nucleus. Cells are made up of a nucleus separated by the cytoplasm by a membrane, called nuclear membrane. The cytoplasm is enclosed by the cytoplasmic membrane, is a selectively permeable membrane that determines what goes in and out of the organism. All cells must take in and retain all the various chemicals needed for metabolism. Water, dissolved gases such as carbon dioxide and oxygen, and lipid-soluble molecules simply diffuse across the membrane. Water-soluble ions generally pass through small pores - less than 0.8 nm in diameter - in the membrane . All other molecules require carrier molecules to transport them through the membrane.

The double helix is coiled and compacted within the nucleus The double helix is coiled and compacted within the nucleus. It is kept together by the bases –the building bricks.

For the chemistry nuts  , the above shows the chemical structure of the bricks or bases, how they’re called. They are coupled, so we always find a bond A-T (Adenine-Thymine) and G-C (Guanine-Cytosine)

3D structure of different shapes of DNA 3D structure of different shapes of DNA. The different color beads represent the bases A-T, G-C. They are “glued” together by chemical bonding

Here we see better the bonds between the bases

Now that we know what a DNA molecule is, it’s much easier to understand how DNA profiling can be very useful in paternity profiling, disease diagnostics, or even to identify someone who left a few cells behind, for example on a murder scene. As we’ll see later, only a few cells are needed to perform a DNA analysis yielding meaningful results

Let’s take another look at our DNA molecule Let’s take another look at our DNA molecule. We can see that the bases follow a sequence eg. CAAT CGT TCA etc. Some sequences on the DNA are the “recipes” encoding for the synthesis of specific proteins. Other portions are called non-coding DNA, and they contain repeated base-pair sequences arranged in tandem which, while having no known function, are inherited by individuals from their parents. These tandem repeats make up a DNA "fingerprint" that is unique for each individual (apart from identical twins) because the number of repeated sequences is highly variable from person to person. These non-coding base-pair repeated sequences are called Variable Number Tandem Repeats, or VNTR.

The DNA fragments chosen for forensic use generally have a tandem repeat unit of only three to four base-pairs, which may be repeated in the DNA molecule from a few to dozens of times. They are called Short Tandem Repeats (STR) These units consist only of three to four base pairs and are very small, so only small amounts of even badly degraded DNA may be sufficient for forensic use. In order to do so, we need to increase the number of STR’s by use of  polymerase chain reaction (PCR) a technology that has been widely used in different fields of medicine as well as forensics

By use of the PCR technique, we can make millions or billions of copies of a selected, or target, DNA sequence. Just think of PCR as a “molecular DNA photocopier”.

The PCR/STR system is seen as producing profiles that are highly individual and random matches are judged to be very unlikely.

Sources 1) Watson, J.D., Crick, F.H.C.(1953) Molecular Structure of Nucleic Acids. Nature,171, 737-738 2)DNA Testing: An Introduction For Non-Scientists-An Illustrated Explanation www.scientific.org/tutorials/articles/riley/riley.html