Signing

[takethebus at gmx.de]

Hi everybody,

In an former email (Subject: "Is self-signing necessary? Basic questions.") I asked:

> When signing another key, what I do is to ONLY sign the other person's
>  public master signing key with my own private master signing key. I don't
>  sign a certain user ID or something. Is that right? 

Hauke Laging thankfully answered:

>>[No], you always sign the key together with a UID. gpg --list-sigs shows >>this to you: 
>>The root entry is pub, the uids are the next level 
>>("connected" to pub) and the signatures refer to UIDs.

How is this "connection" done? It's it possible to extract a signed user ID from the public master signing key and place it in another public master signing key? 

I'm grateful for answers.
Take care,
Sansibar