Question

1. you have performed PCR and run your samples on a gel. When you look at the gel, you see very bright bands of DNA- what is true about this experiment

A) there is ethidium bromide in the gel

B) the PCR machine was not working

C) you "ran to red" (samples run towards the positive electrode)

D) you "ran to black" and ran your samples off the gel

E)all of the above are reasons for the bright bands

2. you have performed PCR and run your samples on a gel. when you look at the gel, you dont see anything- what is the most likely reason(s)

A) there is ethidium bromide in the gel

B) you "ran to red" (samples run towards the positive electrode)

C) you "ran to black" and ran your samples off the gel

D)all of the above

E) both A and B are possible answers

please help me, not enough notes about this and cant find the answer.

Answer 1

In a PCR reaction, a sample of DNA is amplified manifold in a tube by providing the appropriate DNA polymerase enzyme, dNTPs, primers, template in a suitable buffer. The end product is multiple copies of the DNA of interest. Now, to observe the PCR product, it should be run on an agarose gel. It's a porous gel made up of a polysachharide. The DNA is negatively charged. So, when an electric field is applied, the DNA moves through the gel from the negative electrode end (black) to the positive electrode end. Depending on the size of the DNA, it moves slow or fast through the gel pores. Under the same electric field, a smaller DNA will move faster than a larger DNA from the black to the red ends. If the electrodes are fixed wrongly, then the DNA will run in the opposite direction and move out of the gel and we can't see a DNA band.

Now, to visualize a DNA band on the gel, we need a dye such as Ethidium bromide (EtBr). EtBr can enter into the double stranded DNA when it runs through an agarose gel pre-mixed with EtBr. After the run is complete, the entire gel is exposed to UV radiation which excites EtBr. EtBr then gives an orange to red colour. Only the EtBr bound to the DNA can give the color upon UV exposure and shows up the position of DNA in the gel in the form of a thick band.

Now, the answer to question 1 are (A) and (C).

The answer to question 1 is (C)