Question

A \(9.2\) kb plasmid DNA shown below has four BamHI sites \((p), (q), (r), (s)\). If a mutation results in the loss of BamHI site \((r)\), assuming the complete digestion by BamHI, the number of DNA fragments seen in agarose gel after electrophoresis will be _ _ _. (in integer)

Hint:

In agarose gel electrophoresis, DNA fragments of the same size migrate together and appear as one band.

Answer 1

Correct Answer: 2

Step 1: Rule for a circular plasmid.
Cutting a circle at $n$ sites gives $n$ pieces. We start with four BamHI sites $p, q, r, s$.

Step 2: Apply the mutation.
Losing site $r$ leaves three sites, $p$, $q$ and $s$, so digestion now makes three physical pieces.

Step 3: Use the fragment sizes.
Removing $r$ merges its two neighbouring pieces, $1.2$ kb and $2.2$ kb, into one $3.4$ kb piece. The piece from $s$ round to $p$ is also $3.4$ kb, and the $p$ to $q$ piece is $2.4$ kb.

Step 4: Count the bands on the gel.
The two $3.4$ kb pieces are the same length, so they travel together and show as one band. Together with the $2.4$ kb band that gives two visible bands.

Step 5: Answer.
\[ \boxed{2} \]