Question

A colony derived from flask # 2 (with compound B) is used to examine the histidine gene. By PCR, a double stranded piece of DNA is made. The PCR product is cleaved with either EcoRI alone, BamHI alone, or with both restriction enzymes simultaneously. Following electrophoresis of the three DNA samples, the following fragment sizes are found:
EcoRI: fragments of lengths 3 kb, 5 kb and 14 kb.
BamHI: fragments of lengths 7 kb and 15 kb.
EcoRI + BamHI: fragments of lengths 1 kb, 2 kb, 5 kb and 14 kb.

vii) How big (in bp) was the PCR product before cleavage?
viii) How many EcoRI sites did the DNA contain?
ix) How many BamHI sites did the DNA contain?
x) What is the shortest distance between an EcoRI site and a BamHI site?
xi) What is the longest distance between an EcoRI site and a BamHI site?

Answer 1

First you have to see the uploaded picture then read the answers.BACTERIAL DNA is circular DNA.In picture you can ask that why I got understand that the Bam HI recognition site will be in between the cut site of Eco RI.The answer is when both endonuclease has acted upon DNA and produces 1 kB 2 kb bands that only would be possible if the cut site of BamHI is present between the cut site of EcoRI.

Vii) 3+5+14 or 7+15 or 1+2+5+14 =22 kb because all the fragments have come from digestion of this DNA that's why total fragments sum will be the intact length of DNA.

VIII) 3 EcoRI sites are present. There fragments can produced only when 3 cut site are present because this was circular DNA.

IX)SIMILARLY, TWO BamHI site are present.

X)shortest distance between EcoRI and BamHI recognition site is 1 kb.

XI)the longest distance between EcoRI and BamHI site is 14 kB.

Thank you