chapter 16: the molecular basis of inheritance

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Chapter 16:The Molecular Basis of Inheritance


Figure 16.1 Watson and Crick with their DNA model


Figure 16.2 Can the genetic trait of pathogenicity be transferred between bacteria?

Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below:

Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by anunknown, heritable substance from the dead S cells.

EXPERIMENT

RESULTS

CONCLUSION

Living S(control) cells

Living R(control) cells

Heat-killed(control) S cells

Mixture of heat-killed S cellsand living R cells

Mouse dies Mouse healthy Mouse healthy Mouse dies

Living S cellsare found inblood sample.


Figure 16.3 Viruses infecting a bacterial cell

Phagehead

Tail

Tail fiber

DNA

Bacterialcell

100

nm


Figure 16.4 Is DNA or protein the genetic material of phage T2?

In their famous 1952 experiment, Alfred Hershey and Martha Chase used radioactive sulfur and phosphorus to trace the fates of the protein and DNA, respectively, of T2 phages that infected bacterial cells.

EXPERIMENT

Radioactivity(phage protein)in liquid

Phage

Bacterial cell

Radioactiveprotein

Emptyprotein shell

PhageDNA

DNA

Centrifuge

Pellet (bacterialcells and contents)

RadioactiveDNA

Centrifuge

PelletRadioactivity(phage DNA)in pellet

Batch 1: Phages weregrown with radioactivesulfur (35S), which wasincorporated into phageprotein (pink).

Batch 2: Phages weregrown with radioactivephosphorus (32P), which was incorporated into phage DNA (blue).

1 2 3 4Agitated in a blender toseparate phages outsidethe bacteria from thebacterial cells.

Mixed radioactivelylabeled phages withbacteria. The phagesinfected the bacterial cells.

Centrifuged the mixtureso that bacteria formeda pellet at the bottom ofthe test tube.

Measured theradioactivity inthe pellet and the liquid


Phage proteins remained outside the bacterial cells during infection, while phage DNA entered the cells. When cultured, bacterial cells with radioactive phage DNA released new phages with some radioactive phosphorus.

Hershey and Chase concluded that DNA, not protein, functions as the T2 phage’s genetic material.

RESULTS

CONCLUSION


Figure 16.5 The structure of a DNA strandSugar-phosphate

backboneNitrogenous

bases

5 endO–

O P O CH2

5

4O–

HH

OH

H

H3

1H O

CH3

N

O

NH

Thymine (T)

O

O P O

O–

CH2

HH

OH

HH

HN

N

N

H

NH

H

Adenine (A)

O

O P O

O–

CH2

HH

OH

HH

HH H

HN

NN

OCytosine (C)

O

O P O CH2

5

4O–

H

O

H

H3

1

OH2

H

N

NN H

ON

N HH

H H

Sugar (deoxyribose)3 end

Phosphate

Guanine (G)

DNA nucleotide

2

N


Figure 16.6 Rosalind Franklin and her X-ray diffraction photo of DNA

(a) Rosalind Franklin Franklin’s X-ray diffractionPhotograph of DNA

(b)


Figure 16.7 The double helix

O

–O O

OHP

H2C

O

–O O

OP

H2C

O

–O O

OP

H2C

O

–OO

OP

O

OH

O

O

OT A

C

GC

A T

O

O

O

O

OH

CH2

O O–

OOP

CH2

O O–

OOP

CH2

O O–

OO

P

CH2

OO–

OOP

5 end

Hydrogen bond3 end

5 end

3 end

C

T

A

A

T

CG

GC

A T

C G

AT

AT

A T

TA

C

TA0.34 nm

3.4 nm

(a) Key features of DNA structure (b) Partial chemical structure (c) Space-filling model

G

1 nm

G

H2C

G


Unnumbered Figure p. 298

Purine + Purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: widthConsistent with X-ray data


Figure 16.8 Base pairing in DNAH

N H O CH3

N

N

O

N

N

N

N H

Sugar

Sugar

Adenine (A) Thymine (T)

N

N

N

N

Sugar

O H N

H

NH

N OH

H

N

Sugar

Guanine (G) Cytosine (C)


Figure 16.9 A model for DNA replication: the basic concept (layer 1)

(a) 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.

ACTAG

TGATC




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

ACTAG

ACTAG

TGATC

TGATC





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

ACTAG

ACTAG

ACTAG

TGATC

TGATC

AC

T

A

G

TGATC

T

G

A

TC





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

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

ACTAG

ACTAG

ACTAG

ACTAG

TGATC

TGATC

ACTAG

AC

T

A

G

TGATC

TGATC

TGATC

T

G

A

TC


Figure 16.10 Three alternative models of DNA replication

Conservativemodel. The twoparental strandsreassociate after acting astemplates fornew strands,thus restoringthe parentaldouble helix.

(a)

Semiconserva-tive model.The two strandsof the parentalmolecule separate, and each functionsas a templatefor synthesis ofa new, comple-mentary strand.

(b)

Dispersivemodel. Eachstrand of bothdaughter mol-ecules containsa mixture ofold and newlysynthesizedDNA.

(c)

Parent cellFirstreplication

Secondreplication


Figure 16.11 Does DNA replication follow the conservative, semiconservative, or dispersive model?

Matthew Meselson and Franklin Stahl cultured E. coli bacteria for several generations on a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, 15N. The bacteria incorporated the heavy nitrogen into their DNA. The scientists then transferred the bacteria to a medium with only 14N, the lighter, more common isotope of nitrogen. Any new DNA that the bacteria synthesized would be lighter than the parental DNA made in the 15N medium. Meselson and Stahl could distinguish DNA of different densities by centrifuging DNA extracted from the bacteria.

EXPERIMENT

The bands in these two centrifuge tubes represent the results of centrifuging two DNA samples from the flask in step 2, one sample taken after 20 minutes and one after 40 minutes.

RESULTS

Bacteriacultured inmediumcontaining15N

Bacteriatransferred tomediumcontaining14N

21

DNA samplecentrifugedafter 20 min(after firstreplication)

3 DNA samplecentrifugedafter 40 min(after secondreplication)

4Lessdense

Moredense


CONCLUSION Meselson and Stahl concluded that DNA replication follows the semiconservative model by comparing their result to the results predicted by each of the three models in Figure 16.10. The first replication in the 14N medium produced a band of hybrid (15N–14N) DNA. This result eliminated the conservative model. A second replication produced both light and hybrid DNA, a result that eliminated the dispersive model and supported the semiconservative model.

First replication Second replication

Conservativemodel

Semiconservativemodel

Dispersivemodel


Figure 16.12 Origins of replication in eukaryotes

Replication begins at specific siteswhere the two parental strandsseparate and form replicationbubbles.

The bubbles expand laterally, asDNA replication proceeds in bothdirections.

Eventually, the replicationbubbles fuse, and synthesis ofthe daughter strands iscomplete.

1

2

3

Origin of replication

Bubble

Parental (template) strandDaughter (new) strand

Replication fork

Two daughter DNA molecules

In eukaryotes, DNA replication begins at many sites along the giantDNA molecule of each chromosome.

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

(b)(a)

0.25 µm


Parental DNA

DNA pol Ill elongatesDNA strands only in the5 3 direction. 3

5

5

3

35

21

Okazakifragments

DNA pol III

Templatestrand

Leading strandLagging strand

32 1

Templatestrand DNA ligase

Overall direction of replication

One new strand, the leading strand,can elongate continuously 5 3as the replication fork progresses.

The other new strand, thelagging strand must grow in an overall3 5 direction by addition of shortsegments, Okazaki fragments, that grow5 3 (numbered here in the orderthey were made).

DNA ligase joins Okazakifragments by forming a bond betweentheir free ends. This results in a continuous strand.

2

3

1

4

Figure 16.14 Synthesis of leading and lagging strands during DNA replication


Figure 16.15 Synthesis of the lagging strand

Overall direction of replication

3

3

3

35

35

35

35

35

35

35

3 5

5

1

1

21

12

5

5

12

35

DNA ligase forms a bond between the newest DNAand the adjacent DNA of fragment 1.

6 The lagging strand in this region is nowcomplete.

7

DNA pol 1 replaces the RNA with DNA, adding to the 3 end of fragment 2.

5

After the second fragment is primed. DNA pol III adds DNAnucleotides until it reaches the first primer and falls off.

4

After reaching the next RNA primer (not shown), DNA pol III falls off.

3

DNA pol III adds DNA nucleotides to the primer, forming an Okazaki fragment.

2

Primase joins RNA nucleotides into a primer.

1

Templatestrand

RNA primer

Okazakifragment


Figure 16.17 Nucleotide excision repair of DNA damage

Nuclease

DNApolymerase

DNAligase

A thymine dimerdistorts the DNA molecule.1

Repair synthesis bya DNA polymerasefills in the missingnucleotides.

3

DNA ligase seals theFree end of the new DNATo the old DNA, making thestrand complete.

4

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

2

chapter 16: the molecular basis of inheritance

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