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PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Chapter 19

Viruses

Structure of Viruses

Viruses:

not cells

very small infectious particles:

nucleic acid (DNA or RNA)

protein coat (caspid)

(in some cases) membranous envelope

Capsids and Envelopes

Capsid- protein shell that encloses the viral genome

can have various structures

Fig. 19-3

RNA

Capsomere

of capsid

DNA

Glycoprotein

18  250 nm

70–90 nm (diameter)

Glycoproteins

80–200 nm (diameter)

80  225 nm

Membranous

envelope

RNA

Capsid

Head

DNA

Tail

sheath

Tail

fiber

50 nm

20 nm

(a) Tobacco mosaic

virus

-Helical capsid

(b) Adenoviruses

-Icoshedral caspid

-8 RNA molecules

Wrapped in Helical

Caspid

(d) Bacteriophage T4

-Icosahedral head and

Tail apparatus

Figure 19.3 Viral structure

Fig. 19-3d

Head

DNA

Tail

sheath

Tail

fiber

80  225 nm

50 nm

(d) Bacteriophage T4

Figure 19.3 Viral structure

Fig. 19-1

0.5 µm

Figure 19.1 Are the tiny viruses infecting this E. coli cell alive?

Viruses that infect bacteria
Most complex capsids found among viruses

Bacteriophages (phages)-

Elongated capsid head that encloses their DNA

Protein tail piece attaches the phage to the host and injects the phage DNA inside

Reproductive Cycles of Phages

Phages- best understood of all viruses

Ex: Phage 

Phages have two reproductive mechanisms:

Lytic cycle

Lysogenic cycle

The Lytic Cycle

Lytic cycle-
phage reproductive cycle that culminates in the death of the host cell
produces new phages and digests the host’s cell wall, releasing the progeny viruses

Animation: Phage T4 Lytic Cycle

Lytic cycle

1. The phage injects its DNA
2. Phage DNA circularizes

Lytic Cycle

Environmental cues = LYTIC cycle is induced
3. New phage DNA and proteins are synthesized and assembled into phages.

4. The cell lyses, releasing phages

LYTIC

The Lysogenic Cycle

Lysogenic cycle replicates the phage genome without destroying the host
Viral DNA molecule incorporated into the host cell’s chromosome

Integrated viral DNA = prophage

Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

Animation: Phage Lambda Lysogenic and Lytic Cycles

1. The phage injects its DNA
2. Phage DNA circularizes

Environmental cue = Lysogenic cycle induced

3. Phage DNA integrates into the bacterial chromosome, becoming a Prophage

Prophage

Lysogenic

Environmental cue = Lysogenic cycle induced

4. The bacterium reproduces, copying the prophage and transmitting it to daughter cells

Prophage

Lysogenic

Daughter cell

with prophage

Environmental cue = Lysogenic cycle induced

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

Prophage

Lysogenic

Daughter cell

with prophage

Fig. 19-6

Phage

DNA

Phage

The phage injects its DNA.

Bacterial

chromosome

Phage DNA

circularizes.

Daughter cell

with prophage

Occasionally, a prophage

exits the bacterial

chromosome,

initiating a lytic cycle.

Cell divisions

produce

population of

bacteria infected

with the prophage.

The cell lyses, releasing phages.

Lytic cycle

is induced

Lysogenic cycle

is entered

Lysogenic cycle

Prophage

The bacterium reproduces,

copying the prophage and

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.

Figure 19.6 The lytic and lysogenic cycles of phage λ, a temperate phage

Reproductive Cycles of Animal Viruses

Many viruses that infect animals have:
RNA genome
Membranous envelope
Glycoproteins

Reproductive Cycles of Animal Viruses

1. Glycoproteins on viral envelope bind to specific receptors on the host cellentry
2. Caspid and viral genome enter the cell, caspid digested by host cell enzymes
3. Viral genome serves as template for complementary RNA strands by a viral enzyme
4. New copies of viral RNA genome are made using the complementary strands of RNA by host enzyme
5. Complementary RNA strands also serve as mRNA which is translated into caspid proteins and glycoproteins for the new viral envelopes.
6. Vesicles transport glycoproteins to the plasma membrane of host cell
7. A caspid assembles around viral genome molecule
8. Each new virus buds from the host cell with viral glycoproteins embedded in membrane derived from the host cell.

Fig. 19-7

Capsid

RNA

Envelope (with

glycoproteins)

Capsid and viral genome

enter the cell

HOST CELL

Viral genome (RNA)

Template

mRNA

Glyco-

proteins

Capsid

proteins

Copy of

genome (RNA)

New virus

Figure 19.7 The reproductive cycle of an enveloped RNA virus

Fig. 19-8b

HIV

Membrane of

white blood cell

HIV entering a cell

0.25 µm

New HIV leaving a cell

Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS

You should now be able to:

Describe virus structure

Describe and Distinguish between the lytic and lysogenic reproductive cycles

Explain why viruses are obligate intracellular parasites

Describe the generalized reproductive cycle of an animal virus with a membranous envelope

Phage replication: Lytic vs. Lysogenic