NOVA Online | Cracking the Code of Life | Fragmentation of Chromosomes

Sequence for Yourself
Part I: Fragmentation of Chromosomes
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One Set of Chromosomes
Displayed here are 23 pairs of chromosomes -- one copy of the human genome. How is the genome sequenced? Let's do a little sequencing ourselves to find out...

Current technology lets us read only 500 bases at a time, which means that we can't start reading the A's, G's, C's, and T's at one end of a chromosome and continue until we get to its other end. So the first thing we need to do is to cut these chromosomes into small fragments.

Start

Start with Many Sets of Chromosomes
To help us in piecing the fragments back together later on (explained in Part V), the fragments need to overlap. To create the overlap, we'll have to cut up many copies of the chromosomes, but at different locations.

We start with numerous cells from one person to obtain many sets of the chromosomes.

Add

Add Restriction Enzymes
Restriction enzymes will cut up, or cleave, DNA molecules wherever they encounter a specific sequence of bases.

For example, one type of enzyme will cleave DNA wherever it encounters the sequence AAGCTT.

Potential Cleavage Sites
Shown here are three identical chromosomes. The lines show all of the locations that one type of restriction enzyme might target.

However, we don't want the enzymes to cut at every possible location because then there would be no overlap.

Limiting Cleavage
To limit the cutting, we need to use a low concentration of the enzyme. With a low concentration, there just isn't enough of the enzyme available to cut the DNA at every possible site.

What we end up with are many overlapping fragments. At this point each fragment is about 150,000 base pairs long.

Continue: Part II: Cloning

Watch the Program Here | Our Genetic Future (A Survey)
Manipulating Genes: How Much is Too Much? | Understanding Heredity
Explore a Stretch of Code | Nature vs Nurture Revisited
Sequence for Yourself | Journey into DNA | Meet the Decoders
Resources | Update to Program | Teacher's Guide | Transcript
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