DNA is a large molecule – too large to map while a complete strand. A single cell’s DNA can stretch six feet when it is not condensed into chromosomes. It seems logical, then, to break the DNA into smaller pieces. Once these are sequenced, the sequences can be put back together like a puzzle. Also like a puzzle, you have to be careful about lining up the DNA fragments correctly, rather than forcing pieces to fit together.
There are several ways to break apart DNA. One of them is to break the DNA at specific locations such as Variable Number Tandem Repeats (VNTRs). These are segments of DNA that are repeated a variable number of times. As seen in the image below, the VNTRs can have the same sequence repeated a different number of times. Some are repeated 2, 3, or 4 times, for example.
An example of breaking DNA up based on Variable Number Tandem Repeats.
The Human Genome Project and Cancer 
