Ch 13: Viruses, Viral Replication

What is the first step of lytic replication in bacteriophages?

Attachment: Virions attach to the host cell through chemical attraction and a precise fit between tail fibers and host receptors.

How does a bacteriophage enter a host cell?

Entry occurs when the phage releases lysozyme to dissolve the cell wall, followed by contracting the tail sheath to force a tube into the bacterial cell.

What happens during the synthesis phase of lytic replication?

The bacterial chromosomes degrade, stopping their own molecular production while the viral genome directs synthesis of new viruses.

What occurs during the assembly stage of lytic replication?

Capsid proteins and tails assemble inside the host cell, and capsids form around viral genomes.

What is the role of lysozymes in the release stage of bacteriophage replication?

Lysozymes weaken the bacterial cell wall, allowing new virions to burst free from the cell.

What does 'burst time' refer to in viral replication?

Burst time is the duration it takes to complete the entire process of lytic replication.

What is meant by 'burst size' in the context of viral replication?

Burst size refers to the number of new virions released from the host cell after replication.

What happens during the attachment phase of the lysogenic cycle?

Attachment occurs similarly to the lytic cycle, but the host cell's DNA is not destroyed.

What is the role of viral DNA during entry in the lysogenic cycle?

Viral DNA remains silent as a prophage after entering the host cell.

What is the significance of prophage entry?

The prophage incorporates itself into the host cell's DNA.

What occurs during lysogeny?

Every time the bacterial chromosome replicates, the viral DNA is copied along with it, allowing all daughter cells to carry the prophage.

How can lysogenic phages affect the phenotype of bacteria?

They can cause the bacterium's phenotype to change from harmless to pathogenic, which is significant because it can lead to disease.

What does induction refer to in the lysogenic cycle?

Induction occurs when the prophage is excised from the host DNA and reenters the lytic phase.

What can trigger the induction phase in the lysogenic cycle?

Inductive agents include physical and chemical agents that damage bacterial DNA.

How do the synthesis, assembly, and release phases of the lysogenic cycle compare to the lytic cycle

The synthesis, assembly, and release phases occur in the lytic cycle after induction, while in the lysogenic cycle, they are absent until the prophage enters the lytic phase.

What are the general stages of replication that animal viruses share with lytic bacteria?

Animal viruses share the same 5 stages as lytic bacteria, but there are some distinct differences due to the presence of envelopes.

How do animal viruses attach to host cells?

Animal viruses use glycoprotein spikes instead of tail fibers to attach to host cells.

What are the three methods for viruses to enter animal cells?

1. Direct penetration (some naked viruses).
2. Membrane fusion (viral envelope fuses with host cell).
3. Endocytosis (the virus triggers receptors to engulf the virion).

How do dsDNA viruses replicate in animal cells?

The replication of dsDNA viruses is similar to normal cellular DNA replication and occurs in the nucleus.

What process do ssDNA viruses undergo upon entering a cell?

When an ssDNA virus enters a cell, the host synthesizes a complementary strand of DNA, after which replication and protein synthesis proceed.

How does +ssRNA virus replication work?

Ribosomes of the host cell translate proteins directly using the codons of +ssRNA viruses, essentially treating them as mRNA.

What unique mechanism do retroviruses use for replication?

Retroviruses utilize reverse transcriptase to create cDNA from their RNA genome, which serves as a template for making more virus and its genome.

What key challenge does -ssRNA virus replication present?

Host ribosomes cannot translate a -ssRNA strand, necessitating the use of RNA-dependent RNA transcriptase to generate +RNA strands for protein synthesis and new viral genomes.

Where are DNA viruses assembled and released?

DNA viruses are assembled in the nucleus and released into the cytoplasm before their final release.

How are most RNA viruses assembled and released?

Most RNA viruses are assembled and released directly from the cytoplasm.

What factors influence the number of viruses produced?

The number of viruses produced depends on the type of virus and the health of the host cell.

How does the replication time of animal viruses compare to bacteriophages?

Animal viruses take more time to replicate than bacteriophages.

What is the process by which enveloped viruses are released from host cells?

Enveloped viruses are released by budding, incorporating phospholipid membranes from their host cells.

How do the host cells typically respond during the release of enveloped viruses?

Often, the host cells remain alive for some time during the process and do not lyse as bacteriophages do.

What is meant by a Persistent infection?

A long steady release of virions from host cells is called a Persistent infection.

How are naked viruses expelled from host cells?

Naked viruses are either expelled in vesicles via exocytosis or through cell lysis.

What are latent viruses?

Latent viruses are viruses that can remain inactive in the host for years before becoming active.

What term is used to describe viruses that can remain inactive in the host?

Viruses that are capable of remaining inactive in the host are known as proviruses.

How do proviruses differ from lysogenic bacteriophages?

In many cases, proviruses never become part of the host cell's DNA, unlike lysogenic bacteriophages.

What happens if viral DNA is incorporated into the host cell's DNA?

If incorporated, the viral DNA is permanent and will be passed on to all descendants of that cell.

What is the role of viruses in some cancers?

Viruses are thought to cause about 20-25% of human cancers.

What is a well-known virus associated with cervical cancer?

The Human Papilloma Virus (HPV) is well-known for causing cervical cancer.

What are protooncogenes?

Protooncogenes are genes in a host cell involved in normal cell division.

What is the Two Hit hypothesis in cancer causation by viruses?

The Two Hit hypothesis suggests that a virus inserts a promoter that converts a protooncogene into an oncogene; the first hit typically does not cause cancer, but a second hit damaging a downstream repressor gene allows the oncogene to disrupt cell division and cause cancer.

What are prions?

Prions are proteinaceous infectious particles that can cause disease.

How were prion diseases perceived before the discovery of prions in 1982?

They were thought to be caused by very slow-acting viruses, with a delay of 50-60 years between infection and disease.

What is the effect of altered PrP proteins in human prion diseases?

The alteration of a normal PrP protein's structure affects other PrP proteins, leading to their aggregation, causing neurons to die and leaving holes in the brain.

What are the diseases caused by prions called?

Diseases caused by prions are known as spongiform encephalopathies.

What is BSE and how does it relate to prion disease?

BSE, also known as Mad Cow disease, is a prion disease.

What is CWD and in which animals is it found?

CWD (Chronic Wasting Disease) is a prion disease found in deer and elk.

What is vCJD and in which population does it occur?

vCJD (variant Creutzfeldt-Jakob Disease) occurs in humans.

How can prion diseases be transmitted?

Prion diseases can be transmitted through ingestion of infected tissue, transplants of infected tissue, or contact between infected tissues and mucous membranes