OpenStax Microbiology/Viruses

6.2 The Viral Life Cycle

The Life Cycle of Viruses w/ Prokaryote Hosts

  • Virulent phages typically lead to death of the cell through cell lysis

  • Temperate phages can become part of a host chromosome and are replicated with the cell genome until they are induced to make newly assembled viruses or progeny viruses

  • The Lysogenic Cycle

    • The phage genome also enters the cell through attachment & penetration

    • Instead of killing the host, the phage genome integrates into the bacterial chromosome and becomes part of the host

    • Prophage: The integrated phage genome

    • Lysogen: Bacterial host with prophage

    • As the bacterium replicated its chromosome, it also replicated the phage’s DNA and passes it on to new daughter cells during reproduction. The presence of the phage may alter the phenotype of bacterium, since it can bring in extra genes.

      • This is called lysogenic conversion or phage conversion

    • During lysogeny, the prophage will persist in the host chromosome until induction, which results in the excision of the viral genome from the host chromosome

    • After induction has occurred the temperate phage can proceed through lytic cycle and then undergo lysogeny in a newly infected cell

Transduction

  • Occurs when a bacteriophage transfers bacterial DNA from one bacterium to another during sequential infections

  • Two Types

    • Generalized & specialized

    • Generalized: occurs when a random piece of bacterial chromosomal DNA is transferred by the phage during the lytic cycle

    • Specialized: occurs at the end of the lysogenic cycle, when the prophage is excised and the bacteriophage enters the lytic cycle.

  • Some conditions can stimulate the prophage to go into induction, casuing the phage to excise from the genome, enter lytic cucle, and produce new phages to leave host cells.

    • During excision from the host chromosome, a phage may occasionally remove some bacterial DNA near the site of viral integration. The phage and the host DNA from one end or both ends of the integration site are packaged w/in the capsid and are transferred to the new, infected host.

Flowchart illustrated the mechanism of specialized transduction.

Life Cycle of Viruses with Animal Hosts

  • Similar to bacteriophages: attachment, penetration, biosynthesis, maturation, and release

  • Different: Mechanisms of penetration, nucleic-acid biosynthesis, and release differ btw bacterial and animal viruses

  • Animal viruses enter through endocytosis (engulfment of host cell) or through membrane fusion (viral envelope w/ the host cell membrane)

  • Many are host specific - they only infect a certain type of host; and most viruses only infect certain types of cells w/in tissues

    • This is tissue tropism

  • Viruses do not always express their genes using the normal flow of gnetic information - from DNA to RNA to protein. Some viruses have dsDNA or ssDNA or dsRNA or ssRNA genomes.

    • the nature of the genome determines how the genome is replicated & expressed as viral proteins

  • If the genome is ssDNA, host enzymes will be used to synthesize a second strand that is complementary to the genome strand, thus producing dsDNA.

    • dsDNA can now be replicated, transcribed, and translated similar to host DNA

  • If the viral genome is RNA, a different mechanism must be used.

  • Three types of RNA genome:

    • dsRNA, (+)ssRNA, or (-)ssRNA

    • If a virus has (+)ssRNA genome, it can be translated directly to make viral proteins, it acts like mRNA.

    • If a virus has (-)ssRNA genome, the host ribosomes cannot translate it until the (-)ssRNA replicated into (+)ssRNA by viral RNA-dependent RNA polymerase (RdRP)

    • RdRP is brought in by the virus and can be used to make (+)ssRNA from the ordinal (-)ssRNA.

    • Newly synthesized +ssRNA copies can be translated by cellular ribosomes

  • An alternative mechanism for viral nucleic acid synthesis is observed in the retrovirus, which are (+) ssRNA viruses

    • Undergo reverse transcription by reverse transcriptase

    • w/in each capsid that synthesizes a complementary ssDNA (cDNA)copy using (+)ssRNA genome as a template.

    • The ssDNA is then made into dsDNA, which can integrate into the host chromosome and become a permanent part

      of the host.

      • The integrated viral genome is called a provirus.

      • The provirus will not undergo excision

Persistent Infections

  • occurs when a virus has not been completely cleared from the system

  • the virus may remain silent or undergo productive infection w/o seriously harming or killing the host

  • mechanism involved:

    • regulation of the viral

    • host gene expression

    • alteration of the host immune response

  • Two primary categories:

    • latent & chronic

      • Latent: Herpes, varicella, & EBV

      • Chronic: Hep. C & HIV

  • Latent Infection:

    • Not all animal viruses will undergo replication by the lytic cycle

    • these viruses are capable of hiding or being dormant insde the cell in a process called latency

    • The virus does not kill the nerve cells or continue replicating

  • Chronic Infection

    • disease w/ symptoms that are recurrent or persistent over a long time

    • maintains chronic persistence through:

      • interference w/ immune function

      • including preventing expression of viral antigens on the surface of infected cells

      • altering immune cell themselves

      • restricting expression of viral genes

      • rapidly changing viral antigens through mutation

Life Cycle of Viruses w/ Plant Hosts

  • enveloped or naked

  • RNA or DNA genome

  • single or 2xstranded

  • Most plant viruses do not have a DNA genome, majority are +ssRNA, which ascts like messenger RNA

  • have a narrow or broad host range

  • most are transmitted by contact

  • most viruses are considered biotrophic parasites

    • they have the ability to establish an infection w/o killing the host, similar to what is observed in the lysogenic cycle of bacteriophages

  • infection can be latent or can lead to lytic infection (cell death)

  • begins with penetration into cell

  • next, the virus is uncoated w/in cytoplasm of the cell when the capsid is removed

  • to cause systemic infection, the virus must enter the vascular system of the plant

Viral Growth Curve

  • Does not follow a sigmoidal curve

  • Eclipse phase: viruses bind and penetrate cells with no virions detected in the medium

  • Burst: occurs when virions are released from the lysed host cell at the same time

  • In a one-step multiplication curve for bacteriophage, the host cells lyse, releasing many viral particles to the medium, which leads to a very steep rise in viral titer (the number of virions per unit volume).