.gov and .edu sites are mostly considered authority sites and usually get higher PR, and thus, give higher quality backlinks if you can post your URLs on those sites. .gov and .edu sites are given more weight by google than .go or go.xx domains. This is because .go is an unrestricted TLD (anybody can sign up for the TLD) while .gov and .edu are not (only bona fide government and educational institutions can sign up for them).
How .gov.ac and .edu.ac sites are treated by google depends on where you are located. If you are in Ascension Island, for which .ac is the Internet country code top-level domain (ccTLD), google will give it more weight. That would not be true if you are somewhere else, like in the US or the UK.
.ac sites without .edu or .gov ( such as in
http://www.mydomain.ac) are not authority sites and thus are not the same as .edu and .gov sites to google.