Question

The restriction endonuclease EcoRI recognises and cleaves DNA sequence as shown below :
5' - GAATTC - 3'
3' - CTTAAG - 5'
What is the probable number of cleavage sites that can occur in a 10 kb long random DNA sequence?

• A. 10
• B. 2
• C. 100
• D. 50

Answer 1

Answer: (B)

2

According to the question, the restriction sequence for the enzyme is 5’GAATTC3’ which is a 6 base pair long sequence. If suppose, an arbitrary DNA sequence of 6 base pair as XXXXXX where “X” can be any of four nucleotides (A/T/G/C). There are 4 possibilities for one “X” and the total sequence is 6 base pairs; for all 6 bases; the probability of having the given sequence is (4 x 4 x 4 x 4 x 4 x 4= 4096 base pairs) which means that the given restriction site occur every 4096 base pairs. For a DNA segment of 10 kb; the possibility of having the given sequence = 10000/4096= 2.45, i.e., 2. The correct answer is B.