ctrl-a (beginning of line), ctrl-e (end of line), ctrl-k (kill/copy line), ctrl-y (yank/paste line) are standard emacs keybindings. On OS X, they work in any standard NS text field as well. More at http://www.hcs.harvard.edu/~jrus/site/cocoa-text.html
BTW, when using screen, ctrl-a is its default action key, so use ctrl-a ctrl-a to send a real ctrl-a, or change the default screen action key in your .screenrc.