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Microbial Models: The Genetics of Bacteria and Viruses. Chapter 18. Figure 1.  Typical mosaic pattern on flue-cured tobacco leaves infected with Tobacco mosaic virus . (Courtesy H.D. Shew). Virus. LE 18-2. Bacterium. Animal cell. Animal cell nucleus. 0.25 µm. Capsomere of capsid.

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Figure 1.  Typical mosaic pattern on flue-cured tobacco leaves infected with Tobacco mosaic virus. (Courtesy H.D. Shew)

le 18 2

Virus

LE 18-2

Bacterium

Animal

cell

Animal cell nucleus

0.25 µm

le 18 4

Capsomere

of capsid

Membranous

envelope

RNA

Capsomere

Capsid

LE 18-4

DNA

Head

RNA

DNA

Tail

sheath

Tail

fiber

Glycoprotein

Glycoprotein

70–90 nm (diameter)

80–200 nm (diameter)

18  250 mm

80  225 nm

20 nm

50 nm

50 nm

50 nm

Tobacco mosaic virus

Adenoviruses

Influenza viruses

Bacteriophage T4

le 18 5

VIRUS

Entry into cell and

uncoating of DNA

DNA

Capsid

LE 18-5

Transcription

Replication

HOST CELL

Viral DNA

mRNA

Viral DNA

Capsid

proteins

Self-assembly of

new virus particles

and their exit from cell

Activity: Viral Reproductive Cycle

le 18 6

Lytic Cycle

Attachment

LE 18-6

Entry of phage DNA

and degradation of

host DNA

Phage assembly

Release

Head

Tail fibers

Tails

Synthesis of viral

genomes and proteins

Assembly

Activity: Lytic Cycle

le 18 7

Phage

DNA

The phage attaches to a

host cell and injects its DNA.

Daughter cell

with prophage

LE 18-7

Many cell divisions

produce a large

population of

bacteria infected with

the prophage.

Phage DNA

circularizes

Phage

Bacterial

chromosome

Occasionally, a prophage

exits the bacterial chromosome,

initiating a lytic cycle.

Lytic cycle

Lysogenic cycle

The bacterium reproduces

normally, copying the prophage

and transmitting it to daughter cells.

Certain factors

determine whether

The cell lyses, releasing phages.

Lytic cycle

is induced

Lysogenic cycle

is entered

or

Prophage

Phage DNA integrates into the

bacterial chromosomes, becoming a

prophage.

New phage DNA and proteins are

synthesized and assembled into phages.

Activity: Lytic & Lysogenic Cycle

le 18 13

PRIONS = infectious proteins which cause degenerative brain diseases

(like mad cow disease); most likely transmitted in food

LE 18-13

Original

prion

Prion

Many prions

New

prion

Normal

protein

le 18 14

Replication fork

LE 18-14

Origin of

replication

Termination

of replication

le 18 17

LE 18-17

Sex pilus

5 µm

le 18 19

Insertion sequence

LE 18-19

Inverted

repeat

Inverted

repeat

Transposase gene

Transposon

Insertion

sequence

Insertion

sequence

Antibiotic

resistance gene

Transposase gene

Inverted repeat

le 18 20

Regulation of enzyme

production

Regulation of enzyme

activity

Precursor

Feedback

inhibition

LE 18-20

Enzyme 1

Gene 1

Gene 2

Enzyme 2

Regulation

of gene

expression

Gene 3

Enzyme 3

Enzyme 4

Gene 4

Gene 5

Enzyme 5

Tryptophan

le 18 21a

LE 18-21a

trp operon

Promoter

Promoter

Genes of operon

DNA

trpB

trpA

trpE

trpC

trpD

trpR

Operator

Stop codon

RNA

polymerase

Regulatory

gene

Start codon

mRNA 5¢

mRNA

D

B

E

C

A

Inactive

repressor

Protein

Polypeptides that make up

enzymes for tryptophan synthesis

Tryptophan absent, repressor inactive, operon on

le 18 21b 1

DNA

LE 18-21b_1

mRNA

Active

repressor

Protein

Tryptophan

(corepressor)

Tryptophan present, repressor active, operon off

le 18 21b 2

DNA

LE 18-21b_2

No RNA made

mRNA

Active

repressor

Protein

Tryptophan

(corepressor)

Tryptophan present, repressor active, operon off

le 18 22a

Promoter

Regulatory

gene

LE 18-22a

Operator

lacl

lacZ

DNA

No

RNA

made

mRNA

RNA

polymerase

Active

repressor

Protein

Lactose absent, repressor active, operon off

le 18 22b

LE 18-22b

lac operon

DNA

lacl

lacY

lacA

lacZ

RNA

polymerase

mRNA

mRNA 5¢

Transacetylase

Permease

-Galactosidase

Protein

Inactive

repressor

Allolactose

(inducer)

Lactose present, repressor inactive, operon on

Activity: lac Operon

le 18 23

Promoter

DNA

lacl

lacZ

LE 18-23

RNA

polymerase

can bind

and transcribe

Operator

CAP-binding site

Active

CAP

cAMP

Inactive lac

repressor

Inactive

CAP

Lactose present, glucose scarce (cAMP level high): abundant lac

mRNA synthesized

Promoter

DNA

lacl

lacZ

Operator

CAP-binding site

RNA

polymerase

can’t bind

Inactive

CAP

Inactive lac

repressor

Lactose present, glucose present (cAMP level low): little lac

mRNA synthesized

le 18 8

Capsid

Capsid and viral genome

enter cell

RNA

LE 18-8

HOST CELL

Envelope (with

glycoproteins)

Viral genome (RNA)

Template

mRNA

Capsid

proteins

ER

Glyco-

proteins

Copy of

genome (RNA)

New virus

le 18 11

LE 18-11

The SARS-causing agent is a

coronarvirus like this one

(colorized TEM), so named for

the “corona” of glyco-protein

spikes protruding form the

envelope.

Young ballet students in Hong

Kong wear face masks to

protect themselves from the

virus causing SARS.

le 18 9

Viral envelope

Glycoprotein

LE 18-9

Capsid

RNA

(two identical

strands)

Reverse

transcriptase

le 18 10

Membrane of

white blood cell

HIV

LE 18-10

HOST CELL

Reverse

transcription

Viral RNA

RNA-DNA

hybrid

0.25 µm

HIV entering a cell

DNA

NUCLEUS

Provirus

Chromosomal

DNA

RNA genome

for the

next viral

generation

mRNA

Activity: HIV Reproductive Cycle

New HIV leaving a cell