How many types of restriction enzymes (REs) are there? Can all REs be used in rDNA technology? Give justification.
Restriction enzymes are mainly categorised into three groups: Type I, Type II, and Type III. This classification is based on their co-factor requirement and the position of their DNA cleavage site relative to the target sequence.
No, all restriction enzymes cannot be used in rDNA technology. Only Type II restriction enzymes find application in this field.
The justification is that Type II enzymes cleave DNA within the recognition site, which allows for precise cutting at the specific sequence. In contrast, Type I enzymes cleave DNA at a site about 1000 bp away from the recognition site, and Type III enzymes cleave at sites about 24 to 26 bp away. This specific cleavage activity of Type II enzymes is essential for generating sticky or blunt ends required for recombination.
Explanation
The textbook context explicitly states that restriction enzymes are categorised mainly into three groups (Type I, II, and III). It further clarifies that only Type II REs find application in rDNA technology. The justification lies in the cleavage mechanism: Type II enzymes cleave within the recognition site, providing precision, whereas Type I and III enzymes cleave at variable distances away from the recognition site (1000 bp and 24-26 bp respectively), making them unsuitable for the precise cutting required in genetic engineering.