Chapter 18: The Genetics of Viruses and Bacteria

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

Chapter 18:The Genetics of Viruses

and Bacteria


Figure 18.1 T4 bacteriophage infecting an E. coli cell

0.5 m


Figure 18.2 Comparing the size of a virus, a bacterium, and an animal cell

0.25 m

Virus

Animalcell

Bacterium

Animal cell nucleus


Figure 18.3 Infection by tobacco mosaic virus (TMV)


Figure 18.4 Viral structure

18 250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80 225 nm

20 nm 50 nm 50 nm 50 nm

(a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

RNA

RNACapsomereof capsid

DNACapsomere

Glycoprotein Glycoprotein

Membranousenvelope

CapsidDNA

Head

Tail fiber

Tail sheath


Figure 18.5 A simplified viral reproductive cycleVIRUS

Capsid proteins

mRNA

Viral DNA

HOST CELL

Viral DNA

Entry into cell anduncoating of DNA

Replication Transcription

DNA

Capsid

Self-assembly of new virus particles and their exit from cell


Figure 18.6 The lytic cycle of phage T4, a virulent phage

Attachment. The T4 phage usesits tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell.

Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,injecting the phage DNA intothe cell and leaving an emptycapsid outside. The cell’sDNA is hydrolyzed.

Synthesis of viral genomes and proteins. The phage DNAdirects production of phageproteins and copies of the phagegenome by host enzymes, usingcomponents within the cell.

Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,and tail fibers. The phage genome ispackaged inside the capsid as the head forms.

Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100 to 200 phage particles.

12

4 3

5

Phage assembly

Head Tails Tail fibers


Figure 18.7 The lytic and lysogenic cycles of phage , a temperate phage

Many cell divisions produce a large population of bacteria infected with the prophage.

The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.

Phage DNA integrates into the bacterial chromosome,becoming a prophage.

New phage DNA and proteins are synthesized and assembled into phages.

Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.

Certain factorsdetermine whether

The phage attaches to ahost cell and injects its DNA.

Phage DNAcircularizes

The cell lyses, releasing phages.

Lytic cycleis induced

Lysogenic cycleis entered

Lysogenic cycleLytic cycle

or Prophage

Bacterialchromosome

Phage

PhageDNA


Table 18.1 Classes of Animal Viruses


Figure 18.9 The structure of HIV, the retrovirus that causes AIDS

Reversetranscriptase

Viral envelope

Capsid

Glycoprotein

RNA(two identicalstrands)


Figure 18.10 The reproductive cycle of HIV, a retrovirus

Vesicles transport theglycoproteins from the ER tothe cell’s plasma membrane.

7

The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER).

6

The double-stranded DNA is incorporatedas a provirus into the cell’s DNA.

4

Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins.

5

Reverse transcriptasecatalyzes the synthesis ofa second DNA strandcomplementary to the first.

3

Reverse transcriptasecatalyzes the synthesis of aDNA strand complementaryto the viral RNA.

2

New viruses budoff from the host cell.9

Capsids areassembled aroundviral genomes and reverse transcriptase molecules.

8

mRNA

RNA genomefor the nextviral generation

Viral RNA

RNA-DNAhybrid

DNA

ChromosomalDNA

NUCLEUSProvirus

HOST CELL

Reverse transcriptase

New HIV leaving a cell

HIV entering a cell

0.25 µm

HIV Membrane of white blood cell

The virus fuses with thecell’s plasma membrane.The capsid proteins areremoved, releasing the viral proteins and RNA.

1


Figure 18.11 SARS (severe acute respiratory syndrome), a recently emerging viral disease

(a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.

(b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.


Figure 18.13 Model for how prions propagate

Prion

Normalprotein

Originalprion

Newprion

Many prions


Figure 18.14 Replication of a bacterial chromosome

Replicationfork

Origin of replication

Termination of replication


Figure 18.15 Can a bacterial cell acquire genes from another bacterial cell?

Only the samples from the mixed culture, contained cells that gave rise to colonies on minimal medium, which lacks amino acids.

RESULTS

EXPERIMENT Researchers had two mutant strains, one that could make arginine but not tryptophan (arg+ trp–) and one that could make tryptophan but not arginine (arg– trp+). Each mutant strain and a mixture of both strains were grown in a liquid medium containing all the required amino acids. Samples from each liquid culture were spread on plates containing a solution of glucose and inorganic salts (minimal medium), solidified with agar.

Mutantstrain

arg+ trp–

Mutantstrain

arg trp+

Mixture


Mutantstrain

arg+ trp–

Mutantstrain

arg– trp+

No colonies(control)

No colonies(control)

Mixture

Coloniesgrew

Because only cells that can make both arginine and tryptophan (arg+ trp+ cells) can grow into colonies on minimal medium, the lack of colonies on the two control plates showed that no further mutations had occurred restoring this ability to cells of the mutant strains. Thus, each cell from the mixture that formed a colony on the minimal medium must have acquired one or more genes from a cell of the other strain by genetic recombination.

CONCLUSION


Figure 18.16 Generalized transduction

Phage DNA

Donorcell

A+ B+

A+ B+

A+

Phage infects bacterial cell that has alleles A+ and B+

Host DNA (brown) is fragmented, and phage DNA and proteins are made. This is the donor cell.

A bacterial DNA fragment (in this case a fragment withthe A+ allele) may be packaged in a phage capsid.

1

2

3


Figure 18.16 Generalized transduction

Phage DNA

Donorcell

Recipientcell

A+ B+

A+ B+

A+

A+ B–

A– B–

A+

Recombinant cell

Crossingover

Phage infects bacterial cell that has alleles A+ and B+

Host DNA (brown) is fragmented, and phage DNA and proteins are made. This is the donor cell.

A bacterial DNA fragment (in this case a fragment withthe A+ allele) may be packaged in a phage capsid.

Phage with the A+ allele from the donor cell infects a recipient A–B– cell, and crossing over (recombination)between donor DNA (brown) and recipient DNA(green) occurs at two places (dotted lines).

The genotype of the resulting recombinant cell (A+B–) differs from the genotypes of both the donor (A+B+) and the recipient (A–B–).

1

2

3

4

5


Figure 18.17 Bacterial conjugation

Sex pilus 1 m


Figure 18.18 Conjugation and recombination in E. coli (layer 1)

1 A cell carrying an F plasmid(an F+ cell) can form amating bridge with an F– celland transfer its F plasmid.

A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins tomove into the recipient cell. As transfer continues, the donor plasmid rotates(red arrow).

2 DNA replication occurs inboth donor and recipientcells, using the single parental strands of the F plasmid as templates to synthesize complementary strands.

3 The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+.

4

F Plasmid Bacterial chromosome

Bacterial chromosomeF– cell

F+ cell

F+ cell

F+ cell Hfr cell

F factorThe circular F plasmid in an F+ cellcan be integrated into the circularchromosome by a single crossoverevent (dotted line).

1The resulting cell is called an Hfr cell (for High frequency of recombination).

2

Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F– cell and transfer DNA.

3 A single strand of the F factorbreaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA

4 The location and orientation of the F factor in the donor chromosome determine the sequence of gene transfer during conjugation. In this example, the transfer sequence for four genes is A-B-C-D.

5 The mating bridgeusually breaks well before the entire chromosome and the rest of the F factor are transferred.

6

Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green).

7 The piece of DNA ending up outside thebacterial chromosome will eventually be degraded by the cell’s enzymes. The recipient cell now contains a new combination of genes but no F factor; it is a recombinant F– cell.

8

Temporarypartialdiploid

Recombinant F–

bacterium

Conjugation and transfer of an F plasmid from an F+ donor to an F– recipient

(a)

Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombination

(b)

A+B+ C+

D+

F– cell A–B–

C–

D–

A–B–

C–

D– D–

A–

C–B– D–

A–

C–

B–

A+

B+C+D+A+

B+C+D+A+B+

D+C+

A+

A+

B+

A–B–

C–

D–

A–B+

C–

D–

A+

B+ B–

A+

A+

B+

F+ cell

Mating bridge







4



F+ cell

F+ cell

F+ cell Hfr cell



2





6



8


Recombinant F–

bacterium


(a)


(b)

A+B+ C+

D+

F– cell A–B–

C–

D–

A–B–

C–

D– D–

A–

C–B– D–

A–

C–

B–

A+

B+C+D+A+

B+C+D+A+B+

D+C+

A+

A+

B+

A–B–

C–

D–

A–B+

C–

D–

A+

B+ B–

A+

A+

B+

F+ cell

Mating bridge







4



F+ cell

F+ cell

F+ cell Hfr cell



2





6



8


Recombinant F–

bacterium


(a)


(b)

A+B+ C+

D+

F– cell A–B–

C–

D–

A–B–

C–

D– D–

A–

C–B– D–

A–

C–

B–

A+

B+C+D+A+

B+C+D+A+B+

D+C+

A+

A+

B+

A–B–

C–

D–

A–B+

C–

D–

A+

B+ B–

A+

A+

B+

F+ cell

Mating bridge

Hfr cell







4



F+ cell

F+ cell

F+ cell Hfr cell



2





6



8


Recombinant F–

bacterium


(a)


(b)

A+B+ C+

D+

F– cell A–B–

C–

D–

A–B–

C–

D– D–

A–

C–B– D–

A–

C–

B–

A+

B+C+D+A+

B+C+D+A+B+

D+C+

A+

A+

B+

A–B–

C–

D–

A–B+

C–

D–

A+

B+ B–

A+

A+

B+

F+ cell

Mating bridge

Hfr cell

Documents

Chapter 18: The Genetics of Viruses and Bacteria