DNA Fingerprints

Tandem repeats are short regions of DNA that differ substantially among people. There are many sites in the genome where tandem repeats occur. Each person carries a unique combination of repeat numbers.

RFLPs are Restriction Fragment Length Polymorphisms. DNA from areas with tandem repeats is cut with restriction enzymes. Because of the variation in the amount of repeated DNA, the restriction fragments vary in size. The variation is detected by gel electrophoresis.

Gel Electrophoresis is used to separate different sized fragments of DNA. DNA is placed at one end of a gel and a current is applied to the gel. DNA molecules are negatively charged and move towards the positive end of the gel. Smaller molecules move faster than larger ones

To analyze DNA fingerprints , DNA is stained or made visible by use of a radioactive probe. The pattern of bands is used to identify or rule out criminal suspects and to determine paternity. In the photograph, DNA "fingerprints" from the mother (M) of a child (C) and two possible fathers (F1, F2) are shown. The pointers on the left side indicate DNA bands shared by the child and the mother. The pointers on the right side indicate DNA bands shared by the child and possible father F1. Possible father F2 appears to share only one band (at the top) with the child and is less likely to have been the actual father of the child than is F1. photograph by Cellmark Diagnostics, Abingdon, Oxfordshire, England

This animation (Audio - Important) describes the role of electrophoresis in DNA fingerprinting.

In 1995, the sequence of the bacterium Haemophilus influenzae was determined. Since then, genome sequencing has been used to determine the 3.2 billion nucleotides in the human genome.

Current laboratories use automated DNA sequencing to determine the unknown sequence of bases in a DNA sample.

The machine builds DNA molecules but uses eight kinds of bases: four normal (A, T, C, and G) and four that are modified to fluoresce in laser light. They are structurally different so that they stop DNA synthesis when they are added to a strand. The reaction mixtue is made up of copies of DNA to be sequenced, a primer, DNA polymerase, standard nucleotides, and modified nucleotides. When a modified base is incorporated, DNA synthesis is halted producing tagged fragments of different lengths. The automated DNA sequencer separates the sets of fragments by gel electrophoresis.

The "tag" base at the end of each fragment in the set is identified by the laser beam. The computer program in the machine assembles the information from all the nucleotides in the sample to reveal the entire DNA sequence.

This animation (Audio - Important) describes the use of an automated DNA sequencer.

REVIEW: Automated DNA sequencing relies on _____ .
a. supplies of standard and labeled nucleotides
b. primers and DNA polymerases
c. gel electrophoresis and a laser beam
d. all of the above
Answer d