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I used to think that a DNA clamp is a protein. But today I noticed it doesn't appear in this picture. Then I went to it's Wikipedia page, where it was written:

A DNA clamp, also known as a sliding clamp, is a protein fold that serves as a processivity-promoting factor in DNA replication. As a critical component of the DNA polymerase III holoenzyme, the clamp protein binds DNA polymerase and prevents this enzyme from dissociating from the template DNA strand. The clamp-polymerase protein–protein interactions are stronger and more specific than the direct interactions between the polymerase and the template DNA strand

Which I find a little confusing. For now it seems to me like DNA clamp is a subunit of DNA Polymerase and it doesn't have any function by itself and it is not an actual protein but it binds to Polymerase and becomes functional. Can somebody please clarify this matter to me?!

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    $\begingroup$ The reason it doesn't appear in some diagrams is because every diagram needs to achieve a balance between including all known details, and leaving out details to not overload the reader. The clamp isn't essential to understanding the basics of DNA replication, so it gets left out a lot. $\endgroup$ – Armatus Nov 3 '17 at 14:50
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Your partly right both ways. In a sense, the DNA clamp is a protein, in another sense, it's only part of a protein. What it actually is is what we call a protein sub-unit, which is a protein, but which binds with other protein sub-units to form complex proteins.

In order to understand this, you have to remember what a protein is. A protein is, in essence, a chain of amino acids. Those can be short chains, long chains, medium chains: but they're all proteins. Sometimes, a few short proteins (sub-units) will bind together to form one long protein (protein complex). This is what happens in the case of a DNA clamp.

Basically, the DNA clamp is (an) independent protein unit(s) which has (have) the ability to encircle the DNA strand and travel along it. They bind to the polymerase, which is why you don't see them on your diagram. Their function is, very simply, to make it possible for the polymerase to stay closely attached to the DNA stand.


Sources:

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