Story related and cannot be missed.
There are 8 memories in DNA Sequence 2, 7 of which have optional objectives which completing them during your initial run through will help you with another achievement later on.
Memory 1 - As Good as New
- Do not drop below 1 health square
Memory 2 - Well Executed
- Kill Your Target With the Hidden Blade
Memory 3 - New Man in Town
- Throw the Borgia Captain into the scaffold to kill him
Memory 4 - Easy Come, Easy Go
- Don't lose any health
Memory 5 - Who's Got Mail?
- Catch the Borgia courier in under 1 minute
Memory 6 - Crepi il Lupo
- Don't lose any health
Memory 7 - The Halls of Nero
- Complete the memory within 8 minutes
Memory 8 - Roman Underground
- No optional objective
Once you complete memory 8, the achievement should unlock for you.