Story related and cannot be missed.
There are 7 memories in DNA Sequence 2, 5 of which have optional objectives which completing them during your initial run through will help you with another achievement later on.
Memory 1 - A Warm Welcome
- Do not wander far from Yusuf
Memory 2 - Upgrade and Explore
- No optional objective
Memory 3 - The Hookblade
- Perform every hook blade move successfully at least once
Memory 4 - A View from Galata
- Climb the tower in under 60 seconds
Memory 5 - Advanced Tactics
- Perform a zipline assassination
Memory 6 - On the Defense
- No optional objective
Memory 7 - On the Attack
- Do not create any conflict in the Imperial North District