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Question

# Eco RI and Rsa I restriction endonucleases require 6 and 4 bp sequences respectively for cleavage. In a 10kb DNA fragment how many probable cleavage sites are present for these enzymes

A
0 Eco RI and 10 Rsa I
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B
1 Eco RI and 29 Rsa I
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C
4 Eco RI and 69 Rsa I
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D
2 Eco RI and 39 Rsa I
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Solution

## The correct option is D 2 Eco RI and 39 Rsa IThere are four different bases, so probability of finding a particular base at one position will be 1/4 and frequency of cutting will be 4n where n is number of base pairs in recognition sequence. Eco RI recognizes the 6 bp sequence. Hence, there are 46 = 4096 different cleavage site possibilities for given 6 bp sequence. Similarly, Rsa I recognizes 4 bp sequence. Hence, there are 44 = 256 different cleavage site possibilities for given 4 bp sequence. So, EcoRI will cut 1 in 4096 6-base-pair long sites and Rsa I will cut 1 in 256 4-base-pair long sites. In 10,000 bp DNA fragment, Rsa I will have 10,000/256 = 39 cleavage sites and Eco RI will have 10,000/4096 = 2 cleavage sites.Thus, the correct answer is option D.

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