Bacterial cell

Phage head

Tail sheath

Tail fiber

DNA

100

nm

1

EXPERIMENT

Phage

DNA

Bacterial cell

Radioactive protein

Radioactive DNA

Batch 1: radioactive sulfur (35S)

Batch 2: radioactive phosphorus (32P)

Empty protein shell

Phage DNA

Centrifuge

Centrifuge

Pellet

Pellet (bacterial cells and contents)

Radioactivity (phage protein) in liquid

Radioactivity (phage DNA) in pellet

2

Sugar–phosphate backbone

5 end

Nitrogenous

bases

Thymine (T)

Adenine (A)

Cytosine (C)

Guanine (G)

DNA nucleotide

Sugar (deoxyribose)

3 end

Phosphate

3

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

4

(c) Space-filling model

Hydrogen bond 3 end

5 end

3.4 nm

0.34 nm3 end

5 end

(b) Partial chemical structure(a) Key features of DNA structure

1 nm

5

Purine + purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: width consistent with X-ray data

6

Cytosine (C)

Adenine (A) Thymine (T)

Guanine (G)7

A T

GC

T A

TA

G C

(a) Parent molecule

A T

GC

T A

TAG C

(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand

(b) Separation of strands

A T

GC

T A

TA

G C

A T

GC

T A

TAG C

8

Origin of replication Parental (template) strand

Daughter (new) strand

Replication forkReplication bubble

Two daughter DNA molecules

(a) Origins of replication in E. coli

Origin of replication Double-stranded DNA molecule

Parental (template) strandDaughter (new) strand

Bubble Replication fork

Two daughter DNA molecules

(b) Origins of replication in eukaryotes

0.5 µm

0.25 µm

Double-strandedDNA molecule

9

A

C

T

G

G

G

GC

C C

C

C

A

A

AT

T

T

New strand 5 end

Template strand 3 end 5 end 3 end

3 end

5 end5 end

3 end

BaseSugar

Phosphate

Nucleoside triphosphate

Pyrophosphate

DNA polymerase

10

DNA double helix (2 nm in diameter)

Nucleosome(10 nm in diameter)

Histones Histone tailH1

DNA, the double helix Histones Nucleosomes, or “beads on a string” (10-nm fiber)

11

30-nm fiber

Chromatid (700 nm)

Loops Scaffold

300-nm fiber

Replicated chromosome (1,400 nm)

30-nm fiber Looped domains (300-nm fiber)

Metaphase chromosome

12