Story related and cannot be missed.
There are 7 memories in DNA Sequence 5, 5 of which have optional objectives which completing them during your initial run through will help you with another achievement later on.
Memory 1 - The Janissaries
- Distract 2 guards with bombs
Memory 2 - The Arsenal Gates
- Make sure 40 or more rioters survive
Memory 3 - Arsenal Infiltration
- Perform an Air Assassination from a zipline
Memory 4 - Portrait of a Lady
- Steal the painting without the merchant spotting you
Memory 5 - Signs and Symbols, Part II
- No optional objective
Memory 6 - The Forum of the Ox
- Do not take any damage
Memory 7 - A New Regime
- No optional objective
Once you complete Memory 7, the achievement should unlock.