In DNA of uniform composition, a specific hexanucleotide occurs by chance once i
ID: 257693 • Letter: I
Question
In DNA of uniform composition, a specific hexanucleotide occurs by chance once in every 4096 base pairs. The recognition sequence for EcoRI is a hexanucleotide. How many restriction sites for EcoRI would you expect to exist in lambda DNA if the phage DNA were of uniform composition? There are few restriction sites for a number of enzymes including EcoR1 and HpaI in the left 20 kilobase of lambda DNA. Explain. (Hint: The left region of lambda DNA is rich in G+C base pairs.) Can evolution of E. coli and /or the lambda phage explain this discrepancy? Explain. (3 points)
Explanation / Answer
Lambda phage genome size = 48502 bp
The probability of a six cutter to be present in a given DNA sequence = 1/4^6 = 1/4096
Recognition site of Eco RI = GAATTC
The probability of Eco RI recognition site in the lambda genome is given by
1 in 4096 bp.
? in 48502 bp
= 48502/4096
= 11.841 sites
Lambda phage genome contains 5 recognition sites for Eco RI.
Eco RI is produced as a defence system by E coli. The restriction system in bacteria has co-evolved with the genome sequences of the phages so that they can show resistance to phage infection. The presence of Eco RI recognition sites in the lambda genome supports this concept.