Question

The restriction enzymes that cut the bacteriophage DNA cannot cut the bacterial chromosomal DNA.
Explain this statement.

Answer 1

We need to talk a little about a few things here.

1. RESTRICTION ENZYMES are proteins, MADE BY BACTERIA, that are used to chop up DNA into little pieces.

2. BACTERIOPHAGES are viruses that infect bacteria.

3. Bacteria use restriction enzymes, to protect themselves from viruses.
When a virus invades bacteria,
the bacteria's restriction enzymes can chop up the virus's DNA.

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Okay, so this is basically what the statement is telling us:

Restriction enzymes can chop up the DNA of viruses.
But the enzymes CANNOT chop up the DNA of bacteria.


That makes sense to you, right?

Restriction enzymes are MADE BY BACTERIA.
The bacteria use the enzymes to defend themselves from VIRUS invaders.

It would be a bad thing if the restriction enzymes could chop up the bacteria's DNA too!

Do you get it?

Answer 2

If I may add a minor comment to this I understand if LitOneForPresident17 might also find it strange that the restriction endonucleases (restriction enzymes) dosen't cut up the bacterial DNA and this I perfectly understand as we can ask our self what is the chance that there are not a sequence that can be cuted by the restriction endonucleases (which there usualy is).

A way it can prevent this form happening is to methylate the bacterial chromosomal DNA. By doing so, the sequence that can be recognized by the restriction endonucleases will no longer be able to catalyze the reaction, preventing the bacterial chromosomal DNA from being destroyed. The methylation is catalyzed by a methyltransferase enzyme.

Answer 3

So what you're saying is that the restriction enzyme gives the bacterial DNA a methyl group. (CH3).

And that protects the bacteria's DNA from getting cut by the restriction enzyme.

Yes?

Answer 4

I'm saying that the enzyme type methyltransferase add methyl groups (CH3) on the DNA so that the restriction enzyme no longer is able to catalyze the cutting of DNA.

The mechanism for this can be thought of like the way we think about the active site of an enzyme. As the enzymes are substrate specific they can only allow un-methylated DNA to fit inside the active site, so when the methylated DNA try to fit inside the active site nothing will happen.

Answer 5

Alright. Yes, I understand.

The restriction enzymes "methylate" the bacterial DNA.
That means that they lend methyl groups to the bacterial DNA.

And this protects the bacterial DNA. Prevents it from getting cut by the restriction enzymes.

Answer 6

Almost. I talk about two enzymes. Might be I misunderstand:
*The methyltransferase: The one that adds methyl groups (not a restriction enzyme).
*The restriction enzyme (restriction endonucleases): the enzyme that cut the DNA

Answer 7

Okay, sorry, I misread that.

Restriction enzymes come with methyltransferase.