Signing an OpenPGP key adds a signature packet, calculated on the fingerprint of the signee's key and one of his user IDs. You can easily observe this running
gpg --export [signee's key id] | gpg --list-packets (the output can be rather massive for keys with a larger number of signatures, consider using
less or a similar tool to be able to search and scroll the output), which displays rather technical details on the key. An example (a signature issued by CAcert on my own OpenPGP key):
:public key packet:
version 4, algo 1, created 1356475387, expires 0
pkey: [8192 bits]
pkey: [17 bits]
:user ID packet: "Jens Erat (born 1988-01-19 in Stuttgart, Germany)"
[snip, self-signatures omitted as irrelevant in this example]
:signature packet: algo 17, keyid D2BB0D0165D0FD58
version 4, created 1356531044, md5len 0, sigclass 0x10
digest algo 2, begin of digest 0b cc
hashed subpkt 2 len 4 (sig created 2012-12-26)
critical hashed subpkt 3 len 4 (sig expires after 1y1d0h0m)
hashed subpkt 26 len 29 (policy: http://www.cacert.org/cps.php)
subpkt 16 len 8 (issuer key ID D2BB0D0165D0FD58)
data: [160 bits]
data: [157 bits]
With other words: the actual public key is not modified, but a new packet is added containing the signature (or possibly multiple packets, if you signed multiple user IDs).
The last line did consist of five to six characters before as well as after signing it, however the characters have been altered. So my signature of my friends key is obviously contained in this last line.
The last few characters contain a CRC-24 checksum. If this is changed, something more must have changed. Try to inspect both exports before and after adding the signature using