Start with a decent tour of the basics, from Learn.Genetics.
Class presentation is here (click shadowed images for animations and movies):
Essential Biology: 3.3 (SL Only) DNA Structure
Essential Biology: 3.3 & 7.1 (HL) DNA Structure
Here’s a decent video from BBC AS Guru with David Suzuki:
The story of the discovery of the double-helix structure is a good example international collaboration and competition, and led to the Nobel prize for Crick, Watson and Wilson (who we never hear about). You’ve got to feel for Rosalind Franklin – her work was key in their discovery and she wasn’t cited for it until after her death.
Here’s a great video, though the presenter sound like he has a mouth full of marbles: