Sunday, 19 June 2011

DNA Testing: An Introduction For Non-Scientists An Illustrated Explanation

 The explanation of DNA testing that follows is intended as an introduction to the subject for those who may have limited backgrounds in biological science.  While basically accurate, this explanation involves liberal use of illustration and, in some cases, over-simplification.  Although intended to be informative, this is brief and incomplete explanation of a complex subject.  The author suggests consulting the scientific literature for more rigorous details and alternative views. 
DNA is material that governs inheritance of eye color, hair color, stature, bone density and many other human and animal traits.  DNA is a long, but narrow string-like object.  A one foot long string or strand of DNA is normally packed into a space roughly equal to a cube 1/millionth of an inch on a side.  This is possible only because DNA is a very thin string.
Our body's cells each contain a complete sample of our DNA.  One cell is roughly equal in size to the cube described in the previous paragraph.  There are muscle cells, brain cells, liver cells, blood cells, sperm cells and others.  Basically, every part of the body is made up of these tiny cells and each contains a sample or complement of DNA identical to that of every other cell within a given person.  There are a few exceptions.  For example, our red blood cells lack DNA.  Blood itself can be typed because of the DNA contained in our white blood cells.  
Not only does the human body rely on DNA but so do most living things including plants, animals and bacteria.  
A strand of DNA is made up of tiny building-blocks.  There are only four, different basic building-blocks.  Scientists usually refer to these using four letters, A,  T,  G,  and C.  These four letters are short nicknames for more complicated building-block chemical names, but actually the letters (A,T, G and C) are used much more commonly than the chemical names so the latter will not be mentioned here.  Another term for DNA's building blocks is the term, "bases."  A, T, G and C are bases. 
For example, to refer to a particular piece of DNA, we might write:  AATTGCCTTTTAAAAA.  This is a perfectly acceptable way of describing a piece of DNA. Someone with a machine called a DNA synthesizer could actually synthesize the same piece of DNA from the information AATTGCCTTTTAAAAA alone.  
The sequence of bases (letters) can code for many properties of the body's cells.  The cells can read this code.  Some DNA sequences encode important information for the cell.  Such DNA is called, not surprisingly, "coding DNA."  Our cells also contain much DNA that doesn't encode anything that we know about.  If the DNA doesn't encode anything, it is called non-coding DNA or sometimes, "junk DNA."[1]  
The DNA code, or genetic code as it is called, is passed through the sperm and egg to the offspring.  A single sperm cell contains about three billion bases consisting of A, T, G and C that follow each other in a well defined sequence along the strand of DNA.  Each egg cell also contains three billion bases arranged in a well-defined sequence very similar, but not identical to the sperm. 
Both coding and non-coding DNAs may vary from one individual to another.  These DNA variations can be used to identify people or at least distinguish one person from another. 
What is a Locus?
A locus (with a hard "c", LOW-KUS)  is simply a location in the DNA.  The plural of locus is, loci ( with a soft "c", pronounced LOW-S-EYE).  Again, the DNA is a long string like object as illustrated below.  A locus is simply a location in the DNA.  Such locations, or loci, reside at specific places on chromosomes. 
What is a Chromosome?
When a cell is getting ready to divide creating two daughter cells, it packs its DNA into bundles called chromosomes.  Chromosomes are just bundles of DNA.  For humans, there are consistently 23 pairs of chromosomes, each with a consistent size and shape.  Chromosomes are numbered.  Chromosome number 1 is the largest chromosome; chromosome number 2 a little smaller and so on.  Among the 23 pairs of chromosomes there is a pair called the sex chromosomes.  This is something of a misnomer, since there are many functions on the "sex" chromosomes that have nothing to do with sex.  In females, the sex-chromosome pair consists of two similar size chromosomes called X chromosomes.   Males have one X and one small Y chromosome.

Unless it has been purified, our DNA is actually not a loosely tangled string as illustrated but rather is well organized and packaged into what are called chromosomes.  A chromosome is a tightly folded bundle of DNA.  Chromosomes are most visible when cells divide.  In a microscope, chromosomes look something like this without the numbers and letters:

The illustration shows a pair of chromosomes named chromosome number 4, one pair among 23 pairs of chromosomes.  The illustration also shows the position of a locus that happens to be called "GYPA."   In this example, the chromosome on the left has the variation called the B allele while the chromosome on the right has the variation called the A allele. 
What are alleles?
Alleles (ALL-EELS') are just variations at a particular site on a chromosome.  Since each chromosome has a similar chromosome partner (except for males with their X and Y chromosomes) each locus is duplicated.  Loci can vary a bit.  If a person has two identical versions of the locus, they are said to be homozygous (HOMO-Z-EYE'-GUS).  If there is a difference, they are said to be heterozygous (HETERO-Z-EYE'-GUS).

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