I have a biology project and I need to know what four letters can makeup an entire sentence?

Question

You can reuse all of the four letters

Answer 1

This sounds like the stupid joke we told in kindergarten "Spell icup" then "spell icup and then say 'pretty colors' ... OMG lol

(icup - i see you pee!)

Answer 2

are you working on DNA. there are four letters that make up DNA

they are
Adenine (A), Thymine (T), Cytosine (C), and Guanine (G),
and you can use them as many times as you want in the gene sequence to make us an entire DNA sequence. I guess that would be the sentence.

otherwise I dont know what else your teacher could be referring to