Question

In an experiment, the genomic DNA was isolated from three different eukaryotic sources, A, B and C. The gene of interest from all three sources was amplified. These were subjected to digestion with one restriction enzyme and then electrophoresed in agarose gel. Provided that the restriction endonuclease used cleaved the DNA within the recognition sequence, study the figure given below carefully and match the options in column I with that of column II.


$\begin{array}{cccc}& \text{Column I}& & \text{Column II}\\ \text{I}& \text{DNA of sample A}& \text{p}& \text{Contains two recognition sites}\\ \text{II}& \text{DNA of sample B}& \text{q}& \text{Does not contain any recognition site}\\ \text{III}& \text{DNA of sample C}& \text{r}& \text{Contains one recognition site}\end{array}$

• A. I - r, II - q, III - p
• B. I - q, II - r, III - p
• C. I - q, II - p, III - r
• D. I - q, II - p, III - r

Answer 1

The correct option is B I - q, II - r, III - p

A restriction enzyme is an enzyme that recognises a particular DNA sequence, called recognition site, and cleaves the DNA molecule within or near to the sequence to generate fragments. The fragments can be separated according to their size by utilising the technique of agarose gel electrophoresis. It is a technique that separates charged molecules of different sizes under the influence of an electric field. The separated fragments yield bands when the gel is stained with ethidium bromide and observed under ultraviolet radiation.

• The gene of interest obtained from source A has been found to form one band in the lane corresponding to the well where it was loaded. Hence, it is intact even after treatment with the restriction enzyme. Hence, DNA of sample A did not contain any recognition site.
• The gene of interest isolated from source B has been found to form two bands in the lane corresponding to the well where it was loaded. Hence, it can be assumed to have one recognition site. Treatment with the restriction enzyme must have cleaved it into two pieces.
• The gene of interest from source C, after restriction digestion, has been found to form three bands in the lane corresponding to the well where it was loaded. Hence, it can be assumed to have two recognition sites. Treatment with the restriction enzyme must have cleaved it into three pieces.