Viruses

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4

Phages that use both the lytic and lysogenic cycles are called temperate phages

Slide 28

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

Lytic cycle

is induced

or

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.

Slide 29

Reproductive Cycles of Animal Viruses

There are two key variables used to classify viruses that infect animals:

DNA or RNA?

Single-stranded or double-stranded?

Slide 30

Table 19-1

Slide 31

Table 19-1a

Slide 32

Table 19-1b

Slide 33

Viral Envelopes

Many viruses that infect animals have a membranous envelope

Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell

Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit

Slide 34

Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane

Slide 35

Fig. 19-7

Capsid

RNA

Envelope (with

glycoproteins)

Capsid and viral genome

enter the cell

HOST CELL

Viral genome (RNA)

Template

mRNA

ER

Glyco-

proteins

Capsid

proteins

Copy of

genome (RNA)

New virus

Slide 36

RNA as Viral Genetic Material

The broadest variety of RNA genomes is found in viruses that infect animals

Retroviruses use reverse transcriptase to copy their RNA genome into DNA

HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)