Nucleotides & Nucleic Acids Watson & Crick 1953 (Franklin & Wilkins too) Franklin & Wilkins Heavy bands - indicates recurring bases X - indicates helical structure

Nucleotides & Nucleic Acids Watson & Crick 1953 (Franklin & Wilkins too) Watson & Crick Used x-ray crystallography data Used Chargaff’s rules 3D structure of DNA, strands antiparallel

Nucleotides & Nucleic Acids DNA structure Complementarity Strands not identical in base sequence or composition DNA replication

Nucleotides & Nucleic Acids DNA structure - different forms

A-DNA B-DNA Z-DNA

Nucleotides & Nucleic Acids DNA structure - different forms Structure type A B Z Axial rise (h) per nt (Å) 2.56 3.38 3.7 Groove width (Å) minor major 11 2.7 5.7 11.7 8.8 2 Groove depth (Å) minor major 2.8 13.5 7.5 8.5 3.7 13.8 Pitch (Å) 28.2 33.8 45 Z-DNA virtually no major groove due to syn-G, pulls C into major grv Pitch = axial rise x repeat (bp/turn) distance for complete turn of helix So for A-DNA  2.56 Å x 11 bp/turn = 28.2 Å axial rise (h) distance between bp

purine rotated into major groove

A-DNA B-DNA A-DNA + bulge 32˚ tilted upward

Intrastrand base pairing Triple Helix Intrastrand base pairing

Triple Helix using a third strand of DNA Interstrand base pairing Restriction endonuclease