7.3.3 DNA Fingerprinting

Question 1

tandem repeats

Answer 1

• repeating sequences of DNA (such as the sequence

5’ CATCATCATCAT 3’)

Question 2

Variable number tandem repeats (VNTRs)

Answer 2

• are repeating sequences of DNA that vary in their number of repeating units between individuals

Question 3

note

Answer 3

• VNTRs can be used to differentiate between the DNA of individuals in a process known as DNA fingerprinting.
  • In a typical DNA fingerprint, several VNTR markers are compared in order to decrease the odds that more than one person will have identical DNA fingerprints.
• A sample of blood is extracted from a glove at a crime scene and placed in a test tube.
• A specific variable number tandem repeat (VNTR)
  sequence is amplified using the polymerase chain reaction technique.
• A sample of blood is taken from the two suspects (you and George). The same VNTR that was amplified from blood at the crime scene is amplified in each of the suspect’s samples.
• George’s VNTR has 12 repeating units, while your sample has 3 repeating units.
• Each of the amplified VNTR samples is separated using
  gel electrophoresis. Remember: small DNA molecules
  move more quickly in the gel than large DNA molecules; thus, small molecules will travel farther in the gel within a given period of time.
• It looks like George is a match! But George isn’t guilty yet! A single VNTR is not typically sufficient to prove anything because the odds that two people have the same VNTR can be relatively high (for example, one in forty).
• Several VNTR’s must be compared in order to reduce the odds that two people will have the same fingerprint. For example, if the odds of two people having the same VNTR were one in forty and remained constant, the odds of two people having six identical VNTRs would be one in 4,096,000,000 (1/40 x 1/40 x 1/40 x 1/40 x 1/40 x 1/40)!
• If after several VNTRs have been compared and George’s DNA matches, then the chances are remarkably high that it was George’s blood on the glove. See you later George!

Question 4

Variable number tandem repeats can be used for DNA fingerprinting because

Answer 4

• the exact number of repeats within a particular stretch of tandem repeats varies widely in a population.
• variations in the number of repeats can affect the length of restriction fragments.

Question 5

Tandemly repetitive DNA is composed of

Answer 5

• short sequences of DNA that are adjacently repeated

Question 6

Which of the following technologies has made DNA fingerprinting possible even if only a trace amount of blood or other bodily substance is available?

Answer 6

• PCR

Question 7

The chances of an individual having a particular group of variable number tandem repeats (VNTRs) can be calculated. If 3 VNTRs are analyzed, one of which has 40 known variations while the other 2 have thirty known variations each, what is the chance of an individual having any specific banding pattern?

Answer 7

• 1/36,000

Question 8

If all VTNRs have 30 possible lengths, how many different VNTRs would have to be analyzed to identify the DNA to a 1/100,000,000 certainty?

Answer 8

• 6