This is an entry in the Fortnightly Topic Challenge #41: Short and Sweet.
1 letter only could be mean anything.
With an additional letter in front, could be used as medicine component and something you definitely learn or at least heard once.
Add another letter in front, you would possibly hear that many times.
Add an additional letter in front, you would found an independent country.
With an additional letter, in the end, you would found something related to apple.
Add another letter in the end, you would found something bad.
Each letter is unique.