Putting DNA to Work
  DNA SEQUENCE

Probe The Sequence - Text Only Version

The more letters in a probe, the fewer matches it will find in the genome. The goal in forensics, disease detection, and selecting desirable traits in crops is often to probe for the DNA sequences that make an organism, a specific trait, or an individual person unique. A variety of different techniques are used, but the goal is always the same: probe for specific DNA sequences.

Finding a single, unique sequence of A's, T's, G's, and C's within an organism's genome often reveals something new about that organism.

How can a unique sequence be found in a genome that is three billion letters long? Searching and comparing sequences is the answer. The more letters in a probe, the fewer matches it will find in the genome.

In this activity the user is asked to enter a combination of 3 letters in a black box at the top of the screen and click "Probe". The computer shows the number of matches.

For example the 3-letter sequence "ATC" yields the following results.
On the activity page: 3 matches.
In the 3 billion letter human genome: 38,238,563 matches.
Expected in 3 billion random letters: 44, 767,202 matches.

The 3-letter probe finds an extremely large number of matches in the entire genome. The number of matches found in the genome is different than the number found in a random collection of letters because DNA contains information; its sequence is far from random.

The user is then asked to enter a 6-letter combination. The computer shows the number of matches.

For example the six-letter sequence "AGCAGC" yields the following results.
On the activity page: 1 match.
In the 3 billion letter human genome: 873,107 matches.
Expected in 3 billion random letters: 699,434 matches.

The 6-letter probe finds fewer matches than the 3-letter probe but it is still too short to find a unique match in the human genome. In order to find a unique match the probe would need to contain 16 letters.


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