Restriction enzymes and their properties

Introduction:
Restriction enzymes are enzymes that recognize specific DNA sequences, known as recognition sites, and cut the DNA at or near these sites. They are essential tools in molecular biology and genetic engineering, often used to manipulate DNA for various purposes.

Properties of restriction enzymes:
1. Recognition sites and cleavage:
- Recognition sites: These are specific DNA sequences that the restriction enzymes recognize and bind to. These recognition sites are usually palindromic, meaning they read the same forward and backward on both DNA strands. For example, the recognition site for the restriction enzyme EcoRI is 5'-GAATTC-3', which is the same on both strands.
- Cleavage: Once the restriction enzyme binds to its recognition site, it cleaves the DNA at specific positions. The cleavage can occur within the recognition site or at specific positions adjacent to it.

2. Types of restriction enzymes:
- Type I: These enzymes recognize specific DNA sequences but cleave the DNA at random positions away from the recognition site.
- Type II: These enzymes recognize specific DNA sequences and cleave the DNA at or near the recognition site. Type II restriction enzymes are commonly used in molecular biology.

Explanation of the correct answer:
The correct answer is option 'A,B', which means both statements A and B are not correct.

a) All restriction enzymes do not cut within recognition sites:
- This statement is incorrect. Type II restriction enzymes, which are widely used in molecular biology, cleave the DNA at or near their recognition sites. The cleavage can occur within the recognition site or at specific positions adjacent to it. Therefore, all restriction enzymes can cut within recognition sites.

b) Restriction sites are always palindromic:
- This statement is incorrect. Although many restriction sites are palindromic, not all of them follow this pattern. Some restriction sites are asymmetric and not palindromic. However, palindromic recognition sites are more common among restriction enzymes.

c) Restriction enzymes are synthesized in mammalian cells:
- This statement is correct. Restriction enzymes are not synthesized in mammalian cells. They are produced by bacteria as a defense mechanism against foreign DNA, such as bacteriophages. These enzymes recognize and cleave the DNA of the invading organisms, protecting the bacteria from infection.

d) Two restriction enzymes cannot cut at the same sites:
- This statement is incorrect. It is possible for two different restriction enzymes to recognize and cut at the same DNA sequence. This can be useful in molecular biology experiments where multiple DNA fragments need to be generated or when combining different DNA fragments.

In summary, the correct answers are option 'A,B' because not all restriction enzymes cut within their recognition sites, and not all restriction sites are palindromic. The other statements are incorrect as restriction enzymes are not synthesized in mammalian cells, and two different restriction enzymes can cut at the same DNA sequence.