Chapter 21.1 - Viruses

Viruses vs. Prokaryotes are noncellular; cannot metabolize cannot respond to stimuli; multiply only within living cells by parasitizing the synthetic machinery of the infected cell evolve by mutation and natural selection Prokaryotes are unicellular; metabolize; respond to stimuli; Reproduce independently evolve as a result of mutation and natural selection

Viruses Viruses are nonliving with varied appearance. All viruses are infectious. In 1884, Pasteur suspected something smaller than bacteria caused rabies; he chose Latin term for “poison.”

Viral Structure Viruses are obligate intracellular parasites = cannot multiply outside a living cell.

Viral Reproduction Virus relies on host enzymes, ribosomes, transfer RNA (tRNA), and ATP for its own replication. A virus takes over the metabolic machinery of the host cell when it reproduces.

Viral Reproduction Lytic Cycle Lysogenic Cycle

Replication of Bacteriophages Bacteriophages (phages) are viruses that parasitize a bacterial cell. Lytic cycle is a bacteriophage “life” cycle of five stages where a virus takes over operation of the bacterium immediately upon entering it and then destroys the bacterium. Attachment -capsid bind with receptors Penetration - viral DNA enters host Biosynthesis – viral components are synthesized Maturation – assembly of viral components Release – new viruses leave host cell

Retroviruses Retrovirus is an RNA animal virus with a DNA stage. Retroviruses contain reverse transcriptase that carries out reverse transcription producing cDNA. Viral cDNA is integrated into host DNA and is replicated as host DNA replicates. Viral DNA is transcribed; new viruses are produced by biosynthesis, maturation and release by budding. Retroviruses include the AIDS viruses (e.g., HIV) and also cause certain forms of cancer.