Question

What happens when 2 different DNA samples are snipped with a similar sticky end restriction enzyme?

Answer 1

Hint: Restriction enzymes belong to a larger class of enzymes called nucleases, they cut DNA at specific locations. A restriction enzyme recognizes a specific base pair sequence in DNA called a restriction site and cleaves the DNA(hydrolyzes the phosphodiester backbones) within the sequence. There are two types- 1. Exonucleases: they remove nucleotides from the end of DNA. 2. Endonucleases: They make cuts at specific positions within DNA.

Complete answer:
Restriction endonucleases are enzymes that produce internal cuts called cleavage in DNA molecules at random sites. A class of endonucleases cleaves DNA only within or near those sites with specific base sequence called restriction endonucleases.

Sites recognised by them are recognition sites for recognition sequences. These sites differ for different restriction enzymes.

Many restriction enzymes like Small isolated from Serratia marcescens cleave both the strands of DNA at exactly the same nucleotide position almost in the centre of the recognition site resulting in blunt or flush end.

Some other restriction enzymes cleave the recognition sequence asymmetrically. Thus due to cleavage, they produce short, single stranded hanging structures. Such ends are called sticky or cohesive ends because base pairing between them can stick the DNA molecule again. Example- A 6 nucleotides palindromic nucleotide sequence recognised by EcoR1 cleave both strands at different points.

So when 2 different DNA samples are snipped with a sticky end restriction enzyme then in both the DNA different lengths of fragments can be observed because in both DNA the recognition site would be at different locations. We can observe it by doing gel electrophoresis.

Note:
Restriction enzymes are widely found in prokaryotes and provide protection to host cell by destroying foreign DNA that makes entry into it.Here they act as a part of defense mechanism called Restriction Modification System.It bears two components: (a) First component is a restriction enzyme that selectively identifies a specific DNA sequence and degrades any DNA bearing that sequence. (b) The second component is a modification enzyme. It adds a methyl group to one or two bases within the sequence identified by restriction enzymes.If a base in DNA is modified due to addition of methyl group, restriction enzyme cannot identify and cleave that DNA. By this method bacteria are able to protect their chromosomal DNA from cleavage by restriction enzymes.