Story related and cannot be missed.
There are only 2 memories in DNA Sequence 9, 1 of which has an optional objective which completing them during your initial run through will help you with another achievement later on.
Memory 1 - Pax Romana
- Do not drop below 5 health squares
Memory 2 - A Seed
- No optional objective
Once you complete memory 2, the achievement should unlock for you.