Question

The restriction endonuclease \[EcoRI\]recognise and cleavage DNA sequence as shown below:
\[5' - GAATTC - 3'\]
\[3' - CTTAAG - 5'\]
Which is the probable number of cleavage sites that can occur in a \[10Kb\] long random DNA sequence?
A) 10
B) 2
C) 100
D) 50

Answer 1

Hint: Restriction endonucleases are enzymes that recognize a specific DNA sequence which are called restriction sites and cleave the DNA within or adjacent to that site. It cleaves the DNA between the G and A on each strand producing\[5'\] overhangs of four nucleotides.

Complete answer:
EcoRI is a type of restriction endonuclease enzyme which is isolated from a species of bacteria called Escherichia coli. It's a restriction enzyme that cleaves the double helix of DNA into fragments at specific sites. EcoRI operates on the same recognition sequence in any piece of DNA from any genome means that it always behaves in the same way no matter what organism the DNA comes from. According to this question, the restriction sequence for the enzyme is \[5' - GTTAAC - 3'\] which is a 6 base pair long sequence.

A base pair is a basic unit of double helix nucleic acids consisting of two nucleobases attached to each other by hydrogen bonds. They form the basic building blocks of the DNA double helix and contribute to the folded structure of both DNA and RNA.

If suppose, an arbitrary DNA sequence of 6 pair as”FFFFFF” where F can be any of the nucleotide\[\left( {A/T/C/G} \right).\]
For single F, the possibility of single F to occur in each base pair is four.
So, for 6 base pairs it can occur: \[4 \times 4 \times 4 \times 4 \times 4 \times 4 = 4096\]base pairs of F.
This means the restriction site occurs at every \[4096bp\]
The question says the DNA segment is of\[10Kb\].
So, the possibility of having the given sequence will be \[\dfrac{{10kb}}{{4096}} = 2.45\]
Nearly 2.

Therefore the correct answer is option ‘B’.

Note: A number of restriction enzymes have been isolated from a variety of microbial sources. Recognition sites for specific enzymes range in size from 4 to 13 base pairs, and for most restriction enzymes used in gene cloning are palindromic sequences.