Chapter 16: The Molecular Basis of Inheritance (DNA)

Zooming in on DNA

LE 16-2

Living S cells(control)

Living R cells(control)

Heat-killedS cells (control)

Mixture of heat-killedS cells and livingR cells

Mouse dies

Living S cellsare found in blood sample

Mouse healthy Mouse healthy Mouse dies

RESULTS

LE 16-3

Bacterialcell

Phagehead

Tail

Tail fiber

DNA

100

nm

LE 16-4

Bacterial cell

Phage

DNA

Radioactiveprotein

Emptyprotein shell

PhageDNA

Radioactivity(phage protein)in liquid

Batch 1:Sulfur (35S)

RadioactiveDNA

Centrifuge

Pellet (bacterialcells and contents)

PelletRadioactivity(phage DNA)in pellet

Centrifuge

Batch 2:Phosphorus (32P)

Activity: Hersey Chase Experiment

LE 16-5Sugar–phosphate

backbone

5 end

Nitrogenousbases

Thymine (T)

Adenine (A)

Cytosine (C)

DNA nucleotidePhosphate

3 endGuanine (G)

Sugar (deoxyribose)

Chargaff

DNAi.org

LE 16-6

Franklin’s X-ray diffractionphotograph of DNA

Rosalind Franklin

DNAi.org

Figure 16-01

DNAi.org

LE 16-UN298

Purine + purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: widthconsistent with X-ray data

DNAi.org

LE 16-8

Adenine (A) Thymine (T)

Guanine (G) Cytosine (C)

Sugar

Sugar

Sugar

Sugar

LE 16-7

5 end

3 end

5 end

3 end

Space-filling modelPartial chemical structure

Hydrogen bond

Key features of DNA structure

0.34 nm

3.4 nm

1 nm

Activity: DNA & RNA Structure

Activity: DNA Double HelixDNAi.org

LE 16-9_4

The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

The first step in replication is separation of the two DNA strands.

Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

The nucleotides are connected to form the sugar-phosphate back-bones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand.

LE 16-10

Conservative model. The two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix.

Semiconservative model. The two strands of the parental moleculeseparate, and each functions as a template for synthesis of a new, comple-mentary strand.

Dispersive model. Each strand of both daughter molecules contains a mixture of old and newly synthesized DNA.

Parent cellFirstreplication

Secondreplication

LE 16-11

Bacteriacultured in mediumcontaining15N

DNA samplecentrifugedafter 20 min(after firstreplication)

DNA samplecentrifugedafter 40 min(after secondreplication)

Bacteriatransferred tomediumcontaining14N

Lessdense

Moredense

Conservativemodel

First replication

Semiconservativemodel

Second replication

Dispersivemodel

Meselson & Stahl

LE 16-12

In eukaryotes, DNA replication begins at may sitesalong the giant DNA molecule of each chromosome.

Two daughter DNA molecules

Parental (template) strand

Daughter (new) strand0.25 µm

Replication fork

Origin of replication

Bubble

In this micrograph, three replicationbubbles are visible along the DNAof a cultured Chinese hamster cell(TEM).

Activity: DNA Replication: An Overview

LE 16-13

New strand

5 end

Phosphate Base

Sugar

Template strand

3 end 5 end 3 end

5 end

3 end

5 end

3 end

Nucleosidetriphosphate

DNA polymerase

Pyrophosphate

Activity: DNA Replication: A Closer Look

LE 16-14

Parental DNA

5

3

Leading strand

35

3

5

Okazakifragments

Lagging strand

DNA pol III

Templatestrand

Leading strand

Lagging strand

DNA ligase Templatestrand

Overall direction of replication

LE 16-15_6

53

Primase joins RNAnucleotides into a primer.

Templatestrand

5 3

Overall direction of replication

RNA primer3

5

35

DNA pol III addsDNA nucleotides to the primer, formingan Okazaki fragment.

Okazakifragment

3

5

5

3

After reaching thenext RNA primer (not

shown), DNA pol IIIfalls off.

33

5

5

After the second fragment isprimed, DNA pol III adds DNAnucleotides until it reaches thefirst primer and falls off.

33

5

5

DNA pol I replaces the RNA with DNA,adding to the 3 endof fragment 2.

33

5

5

DNA ligase forms abond between the newestDNA and the adjacent DNAof fragment 1.

The lagging strand in the regionis now complete.

LE 16-16

5

3Parental DNA

3

5

Overall direction of replication

DNA pol III

Replication fork

Leadingstrand

DNA ligase

Primase

OVERVIEW

PrimerDNA pol III

DNA pol I

Laggingstrand

Laggingstrand

Leadingstrand

Leadingstrand

LaggingstrandOrigin of replication

Activity: DNA Replication: A ReviewDNAi.org

LE 16-17

DNA ligase

DNA polymerase

DNA ligase seals thefree end of the new DNAto the old DNA, making thestrand complete.

Repair synthesis bya DNA polymerasefills in the missingnucleotides.

A nuclease enzyme cutsthe damaged DNA strandat two points and the damaged section isremoved.Nuclease

A thymine dimerdistorts the DNA molecule.

Proofreading and Repair

LE 16-18

End of parentalDNA strands

5

3

Lagging strand 5

3

Last fragment

RNA primer

Leading strandLagging strand

Previous fragment

Primer removed butcannot be replacedwith DNA becauseno 3 end available

for DNA polymerase5

3

Removal of primers andreplacement with DNAwhere a 3 end is available

Second roundof replication

5

3

5

3Further roundsof replication

New leading strand

New leading strand

Shorter and shorterdaughter molecules

Telomeres