Discussion
A circular plasmid was digested with restriction enzymes to generate a map. Digestion with Bam H1 enzyme gave a single band of 11kb, while digestion with Not I gave two bands corresponding to 7kb and 8 kb. What is the minimal size of the circular plasmid that will be consistent with above observation ?
Answer: D
The minimal size consistent with both digests is 22 kb.
A circular plasmid cut at n sites gives n fragments whose sizes add up to the plasmid. Fragments of the same size run together as one band, so a single band can hide several copies.
\[ \begin{aligned} \text{BamHI: } N &= 11n \\ \text{NotI: } N &= 7a + 8b,\ a, b \ge 1 \end{aligned} \]
For n = 1, N = 11 kb, but the NotI fragments alone add up to at least 7 + 8 = 15 kb, so that fails. For n = 2, N = 22 kb, and 22 = 7 + 7 + 8, so NotI cuts three times, giving two 7 kb fragments in one band and one 8 kb fragment. BamHI cuts twice, giving two 11 kb fragments in one band.
15 kb fits NotI but cannot give a single 11 kb band, 11 kb is too small for NotI, and 26 kb is not a multiple of 11.
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