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Sequence for Yourself
Part III: Preparing DNA for Detection
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Clones

Clones to Sequence
We now have billions of identical copies of a stretch of DNA 2,000 to 4,000 base pairs long. From these we need to create copies with special bases that our machines will be able to read. Instead of all being exactly alike, though, these new copies will have varying lengths and will be tagged with colored markers.

Here is one of our many DNA clones, still attached to the vector. We have exposed the vector to heat, which has separated its double strand of DNA into two single strands.

Add Material needed to Build new DNA
We next add some of the material needed to construct the complement of the DNA clone. This includes the "regular" nucleotides (bases) of A's, G's, C's, and T's, an enzyme known as polymerase (which will help to connect the free nucleotides to the single strand of cloned DNA), and DNA primers.

A DNA primer is a single-stranded piece of DNA of known sequence, generally about 20 bases long, that will initiate the building of a new DNA strand that is complementary to the cloned DNA.

Another

Another Ingredient: "Special" Nucleotides
To the mix of regular nucleotides, polymerase, and primers we add a small amount of "special" nucleotides. Because of its molecular structure, whenever one of these special nucleotides adds itself to a growing DNA strand, the growth of that strand stops.

These nucleotides are special in another way, too. They fluoresce when struck by a laser beam. Each type of nucleotide is marked with its own color: A is marked with green, G with yellow, C with blue, and T with red.

Build

Build New Sequences
The primer attaches itself to a complementary sequence on the vector and initiates the building of a new DNA strand.

To separate the new strand from the vector, we expose them to heat. With the new strand separated, the building of yet another strand along the original DNA fragment can begin again. This process is often repeated many times (around 40) to get the most mileage from each of the billions of vectors present.

Renew

Many Copies of Various Lengths
Initiated by the primer and with the help of the polymerase, sequence after sequence is built. Because of the special nucleotides, which get added occasionally at random locations, the lengths of the new copies range in size from one to the total number of bases in the vector.

We end up with billions of DNA strands of various lengths, each ending with a marked nucleotide. Every possible length of the human DNA will have many, many copies.

Continue: Part IV: Detecting the Sequence

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
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