Steps In Dna Identification

figure 13-1

In the polymerase chain reaction (PCR), a scientist chooses a DNA fragment to copy and designs primers that will bind to both ends of the fragment. DNA polymerase copies the segment between the two primers. Repeating the procedure through about 30 cycles generates millions of copies of a single piece of DNA fragment.

Add DNA polymerase, free nucleotides, and primers. Heat.

Add DNA polymerase, free nucleotides, and primers. Heat.

Original DNA sample to be copied

The main steps involved in DNA identification are (1) isolate the DNA in a sample and, if needed, make copies, (2) cut the DNA into shorter fragments that contain known VNTR areas, (3) sort the DNA by size, and (4) compare the size fragments in the unknown sample of DNA to those of known samples of DNA. If a match occurs between the unknown sample and a known sample, then a person's identity can be confirmed.

Copying DNA: Polymerase Chain Reaction

Often, DNA that is recovered from a crime scene or from human tissues is present in very small amounts. In these cases, scientists need to make copies in order to have enough DNA to use for DNA identification. The polymerase chain reaction (PCR) is a technique that quickly produces many copies of a DNA fragment.

As shown in step Q of Figure 13-1, PCR requires a template, a DNA fragment containing the sequence the scientist wants to copy. PCR also requires a supply of the four DNA nucleotides, heat-tolerant DNA polymerase, and primers. Primers are artificially made pieces of single-stranded DNA that are 20 to 30 nucleotides long that must be present for DNA polymerase to initiate replication. Primers are complementary to the ends of the DNA fragment that is to be copied.

When all the ingredients are combined, copying can begin. Primers bind to DNA, and DNA polymerase makes a copy of both DNA strands. Heating breaks the bonds holding the template DNA to the newly made strands. After cooling, as shown in step ©, primers can once more bind to the DNA. Then, in step ©, DNA polymerase can copy again. In step ©, the cycle is repeated. With each new cycle, the DNA between the two primers doubles.

Primer

Cool to allow primers to bind to original strands.

Original DNA sample to be copied

DNA strands separate

Primer

Cool to allow primers to bind to original strands.

DNA polymerase adds free nucleotides to complete the strands.

DNA polymerase adds free nucleotides to complete the strands.

Heat again to repeat the process.

Each double-stranded DNA is identical to original DNA.

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