Put everything in
.bashrc and then source
From the bash man page (on OS X 10.9):
When an interactive shell that is not a login shell is started, bash reads and executes commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile file option will force bash to read and execute commands from file instead of ~/.bashrc
The above text is why everything is put in
.bashrc. However, there's a bit different behavior when you're dealing with a login shell. Again, quoting from the man page:
When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.
.profile is read for login shells, but
.bashrc is not. Duplicating all that stuff in
.bashrc is bad™ so we need to source it in
.profile in order for the behavior to remain consistent.
However, you don't want to source
.profile unconditionally. Please see the comments and other answers for additional details.