Living S cells (control) Living R cells (control) Heat-killed S cells (control) Mixture of heat-killed S cells and living R cells Mouse dies Mouse healthy Living S cells RESULTS EXPERIMENT 1

Bacterial cell Phage head Tail sheath Tail fiber DNA 100 nm 2

EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA Centrifuge Pellet Pellet (bacterial cells and contents) Radioactivity (phage protein) in liquid Radioactivity (phage DNA) in pellet 3

Sugar–phosphate backbone 5 end Nitrogenous bases Thymine (T) Adenine (A) Cytosine (C) Guanine (G) DNA nucleotide Sugar (deoxyribose) 3 end Phosphate 4

(a) Rosalind Franklin (b) Franklin’s X-ray diffraction photograph of DNA 5

(c) Space-filling model Hydrogen bond 3 end 5 end 3.4 nm 0.34 nm 3 end 5 end (b) Partial chemical structure(a) Key features of DNA structure 1 nm 6

Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width consistent with X-ray data 7

Cytosine (C) Adenine (A)Thymine (T) Guanine (G) 8

A T G C TA TA G C (a) Parent molecule AT GC T A T A GC (c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand (b) Separation of strands A T G C TA TA G C A T G C T A T A G C 9

Parent cell First replication Second replication (a) Conservative model (b) Semiconserva- tive model (c) Dispersive model 10

EXPERIMENT RESULTS CONCLUSION Conservative model Semiconservative model Dispersive model Bacteria cultured in medium containing 15 N Bacteria transferred to medium containing 14 N DNA sample centrifuged after 20 min (after first application) DNA sample centrifuged after 40 min (after second replication) More dense Less dense Second replicationFirst replication 11

Origin of replication Parental (template) strand Daughter (new) strand Replication fork Replication bubble Two daughter DNA molecules (a) Origins of replication in E. coli Origin of replicationDouble-stranded DNA molecule Parental (template) strand Daughter (new) strand Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes 0.5 µm 0.25 µm Double- stranded DNA molecule 12

Topoisomerase Helicase Primase Single-strand binding proteins RNA primer

A C T G G G GC CC C C A A A T T T New strand 5 end Template strand 3 end 5 end 3 end 5 end 3 end Base Sugar Phosphate Nucleoside triphosphate Pyrophosphate DNA polymerase 14

Leading strand Overview Origin of replication Lagging strand Leading strandLagging strand Primer Overall directions of replication Origin of replication RNA primer “Sliding clamp” DNA poll III Parental DNA

Overview Origin of replication Leading strand Lagging strand Overall directions of replication

Template strand RNA primer Okazaki fragment Overall direction of replication 17

Overview Origin of replication Leading strand Lagging strand Overall directions of replication Leading strand Lagging strand Helicase Parental DNA DNA pol III PrimerPrimase DNA ligase DNA pol III DNA pol I Single-strand binding protein

Nuclease DNA polymerase DNA ligase 19

Ends of parental DNA strands Leading strand Lagging strand Last fragment Previous fragment Parental strand RNA primer Removal of primers and replacement with DNA where a 3 end is available Second round of replication New leading strand New lagging strand Further rounds of replication Shorter and shorter daughter molecules

DNA double helix (2 nm in diameter) Nucleosome (10 nm in diameter) Histones Histone tail H1 DNA, the double helixHistones Nucleosomes, or “beads on a string” (10-nm fiber) 21

30-nm fiber Chromatid (700 nm) LoopsScaffold 300-nm fiber Replicated chromosome (1,400 nm) 30-nm fiber Looped domains (300-nm fiber) Metaphase chromosome 22