This all about plasmids: I am totally lost, and my notes/textbook/internet doesn't give me the answer to this. Thanks for the help!
Q: How is the number of fragments produced associated with the number of cut-sites available to a restriction enzymes?
Q: Why do all the fragment sizes for each restriction enzyme add up to the same total?
THANKS THANKS THANKS!
You're welcome! I'm here to help you understand plasmids and answer your questions. Let's start with your first question:
Q1: How is the number of fragments produced associated with the number of cut-sites available to a restriction enzyme?
To understand the relationship between the number of fragments produced and the number of cut-sites available to a restriction enzyme, we need to know how restriction enzymes work.
Restriction enzymes are proteins that can recognize specific DNA sequences, and once they find these target sequences, they bind to them and cut the DNA molecule at specific points, resulting in fragments. The DNA sequences that restriction enzymes recognize are called recognition sites or cut-sites.
Each restriction enzyme has a specific recognition sequence that it can bind to and cut. The more recognition sequences there are in a DNA molecule, the more sites the restriction enzyme can potentially cut.
Here's how the number of fragments produced is associated with the number of cut-sites available to a restriction enzyme:
1. If the DNA molecule has only one recognition sequence, the restriction enzyme can cut the DNA only once, resulting in two fragments - one on each side of the cut-site.
2. If the DNA molecule has multiple recognition sequences, the restriction enzyme can cut the DNA at each of these sites, resulting in more fragments. The number of fragments will be equal to the number of cut-sites plus one.
In summary, the more recognition sites available to a restriction enzyme, the more cut-sites there are, leading to a higher number of fragments produced.
Now let's move on to your second question:
Q2: Why do all the fragment sizes for each restriction enzyme add up to the same total?
The sum of all fragment sizes produced by a particular restriction enzyme will always add up to the same total. This is because the total length of the DNA molecule doesn't change; it is divided into fragments by the restriction enzyme's cuts, but the total amount of DNA remains the same.
Let's consider an example:
Suppose you have a DNA molecule that is 1000 base pairs (bp) long. If you use a restriction enzyme that cuts at a specific recognition sequence, it will produce fragments of various sizes. For simplicity, let's assume that the recognition sequence exists 4 times in the DNA molecule.
When the enzyme cuts at each recognition sequence, it will create four fragments of different sizes: Fragment A (100 bp), Fragment B (300 bp), Fragment C (400 bp), and Fragment D (200 bp).
If you add up the sizes of all the fragments (100 + 300 + 400 + 200), the total is 1000 bp, which is the original length of the DNA molecule.
This principle holds true for any restriction enzyme and any DNA molecule. Regardless of the number of fragments generated by a specific enzyme, their sizes will always add up to the total length of the original DNA molecule.
I hope this explanation helps you understand the concepts better! Let me know if you have any further questions.