IMAT 2015 Q32 [Fluorescently-­labelled human DNA]

The picture below shows a DNA microarray. Each spot (labelled 1–4) contains a different DNA probe. A sample of fluorescently-­labelled human DNA, with the sequence TGGTCAAGATTAT, is washed over the array.


Which of the spots would show a positive signal because it fluoresces?

A. 1, 2, 3 and 4
B. 2 and 3 only
C. 1 and 3 only
D. 1 and 4 only
E. 2 and 4 only

The answer to this question is choice B.

It would be useful to be aware of what DNA probes are: Basically, DNA probes are sequences of DNA that are designed to detect the presence of a specific sequence within a DNA sample that is complementary to the probe.

With that being said, what is this question asking for?

  • There are 4 spots on the microarray, each with a specific probe, or a specific sequence. This question is asking you to discern, if there are sequences within the sample that is complementary to any of the probes on the microarray.
  • As a reminder about base pairing: A pairs with T, while G pairs with C.

How do I then tackle this question?

  • First, be aware of the sample DNA sequence: TGGTCAAGATTAT
  • Next, it would be useful to write out the whole sequence that is complementary to the given DNA sample sequence, so that it would be easier for you to see, if there are probes that match with this, since the probes are designed to detect complementary sequences!
  • The sequence that is complementary to the DNA sample is: ACCAGTTCTAATA
  • Now, try to match up each of the DNA probes and see if any sequences are the same!
  • DNA Probe 1: GATTAT - ACCAGTTCTAATA = NO match
  • DNA Probe 2: CTAATA - ACCAGTTCTAATA = will show a positive signal
  • DNA Probe 3: ACCAGT - ACCAGTTCTAATA = will show a positive signal
  • DNA Probe 4: TGGTCA - ACCAGTTCTAATA = NO match

Thus, only choice B is correct, since spots 2 and 3 have probes that have complementary sequences within the DNA sample given.