Replicative Cycles of Animal Viruses

Replicative Cycles of Animal Viruses

• Viruses that infect animals can be classified based on their genetic material (DNA or RNA) and strandedness (single-stranded or double-stranded)
• Many viruses that infect animals have a membranous envelope, which is formed from the host cell's plasma membrane or the Golgi apparatus membrane
• The viral envelope has glycoproteins that bind to specific receptor molecules on the host cell surface

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 a retrovirus that causes AIDS (acquired immunodeficiency syndrome)

HIV Reproductive Cycle

• The viral DNA that is integrated into the host genome is called a provirus
• A provirus remains a permanent resident of the host cell
• The host's RNA polymerase transcribes the proviral DNA into RNA molecules, which function as mRNA for synthesizing viral proteins and as genomes for new virus particles
• HIV enters a host cell, and its RNA genome is reverse-transcribed into DNA, which is then integrated into the host genome
• The host cell's RNA polymerase transcribes the proviral DNA into RNA molecules, which are used to synthesize new viral proteins and genomes
• New HIV particles are assembled and released from the host cell

Evolution of Viruses

• Viruses do not fit the definition of living organisms
• Viruses probably evolved as bits of cellular nucleic acid
• Candidates for the source of viral genomes are plasmids, transposons, and other mobile genetic elements

Viroids and Prions

• Viroids are small circular RNA molecules that infect plants and disrupt their growth
• Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
• Prions propagate by converting normal proteins into the prion version

Genetics of Viruses

• Viruses called bacteriophages can infect and take over the genetic machinery of bacteria
• Viruses lead "a kind of borrowed life" between life-forms and chemicals
• The origins of molecular biology lie in early studies of viruses that infect bacteria

The Discovery of Viruses

• Tobacco mosaic disease stunts the growth of tobacco plants and gives their leaves a mosaic coloration
• In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease
• In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

Structure of Viruses

• Viruses are not cells
• A virus is a very small infectious particle consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope
• Viral genomes may consist of either double- or single-stranded DNA or RNA

Capsid and Envelope

• A capsid is the protein shell that encloses the viral genome
• Capsids are built from protein subunits called capsomeres
• Some viruses have membranous envelopes that help them infect hosts

Bacteriophages

• Bacteriophages, also called phages, are viruses that infect bacteria
• They have the most complex capsids found among viruses
• Phages have an elongated capsid head that encloses their DNA
• A protein tail piece attaches the phage to the host and injects the phage DNA inside

Viral Replication

• Viruses replicate only in host cells
• Each virus has a host range, a limited number of host cells that it can infect
• Viruses use host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules to replicate

Replicative Cycles of Phages

• Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle
• The lytic cycle is a phage replicative cycle that culminates in the death of the host cell
• The lysogenic cycle is a phage replicative cycle that does not kill the host cell immediately