Viruses

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5

Slide 37

Fig. 19-8

Glycoprotein

Viral envelope

Capsid

RNA (two

identical

strands)

Reverse

transcriptase

HIV

HIV

Membrane of

white blood cell

HIV entering a cell

0.25 µm

Viral RNA

RNA-DNA

hybrid

HOST CELL

Reverse

transcriptase

DNA

NUCLEUS

Provirus

Chromosomal

DNA

RNA genome

for the

next viral

generation

mRNA

New virus

New HIV leaving a cell

Slide 38

Fig. 19-8a

Glycoprotein

Reverse

transcriptase

HIV

RNA (two

identical

strands)

Capsid

Viral envelope

HOST CELL

Reverse

transcriptase

Viral RNA

RNA-DNA

hybrid

DNA

NUCLEUS

Provirus

Chromosomal

DNA

RNA genome

for the

next viral

generation

mRNA

New virus

Slide 39

Fig. 19-8b

HIV

Membrane of

white blood cell

HIV entering a cell

0.25 µm

New HIV leaving a cell

Slide 40

The viral DNA that is integrated into the host genome is called a provirus

Unlike a prophage, a provirus remains a permanent resident of the host cell

The host’s RNA polymerase transcribes the proviral DNA into RNA molecules

The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell

Animation: HIV Reproductive Cycle

Slide 41

Evolution of Viruses

Viruses do not fit our definition of living organisms

Since viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acid

Candidates for the source of viral genomes are plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segments

Plasmids, transposons, and viruses are all mobile genetic elements

Slide 42

Mimivirus, a double-stranded DNA virus, is the largest virus yet discovered

There is controversy about whether this virus evolved before or after cells

Slide 43

Concept 19.3: Viruses, viroids, and prions are formidable pathogens in animals and plants

Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide

Smaller, less complex entities called viroids and prions also cause disease in plants and animals, respectively

Slide 44

Viral Diseases in Animals