Chapter 6

Viruses are known to infect all organisms. Viruses are microscopic infectious agents that can infect a wide range of living organisms, including plants, bacteria, fungi, animals, and even other viruses. They are obligate intracellular parasites, meaning they can only replicate inside host cells. Viruses have the ability to infect all types of organisms due to their ability to hijack the cellular machinery of their hosts and use it to replicate and spread. This broad host range is one of the defining characteristics of viruses.

The nucleic acid of a virus can be either DNA or RNA because viruses can have either one of these nucleic acids as their genetic material. Some viruses have DNA as their genetic material, while others have RNA. This variation in nucleic acid composition allows viruses to infect and replicate in different types of host cells. Therefore, the correct answer is that the nucleic acid of a virus can be either DNA or RNA.

Enveloped viruses have surface receptors called spikes. These spikes are glycoproteins that protrude from the viral envelope and play a crucial role in the attachment and entry of the virus into host cells. They bind to specific receptors on the surface of host cells, allowing the virus to gain entry and initiate infection. The spikes also play a role in the immune response by eliciting the production of antibodies that can neutralize the virus.

RNA viruses multiply in the cell cytoplasm because they do not require a nucleus to replicate their genetic material. On the other hand, DNA viruses multiply in the cell nucleus because they need to access the host cell's DNA replication machinery to replicate their genetic material.

The capsid is the outer protein coat of a virus, and it is composed of repeating subunits called capsomers. These capsomers come together to form the structure of the capsid. Spikes, protomers, and virions are not the correct terms to describe the protein subunits of the capsid. Spikes are protein structures that protrude from the surface of the capsid, protomers are individual protein molecules, and virions are complete infectious virus particles. Therefore, the correct answer is capsomers.

The correct answer is "particle" because a virus is a small infectious agent that cannot be classified as a living thing. It lacks the characteristics of living organisms such as cells and the ability to reproduce on its own. Instead, a virus is composed of genetic material, either DNA or RNA, enclosed in a protein coat. It requires a host cell to replicate and cause infection, making it more accurately described as a particle rather than a living organism.

The envelope of an animal virus is derived from the membrane of its host cell. This is because when a virus infects a host cell, it takes over the cellular machinery to replicate itself. During this process, the virus acquires a portion of the host cell's membrane, which becomes the envelope of the virus. The envelope is important for the virus to enter and exit host cells, as well as for protection and recognition by the immune system.

The correct answer is adsorption, penetration, synthesis, assembly, and release. This sequence of steps describes the general process of viral multiplication. Adsorption refers to the attachment of the virus to the host cell. Penetration involves the entry of the viral genetic material into the host cell. Synthesis refers to the replication and production of viral components within the host cell. Assembly involves the assembly of new viral particles using the replicated components. Finally, release refers to the exit of the newly formed viruses from the host cell, allowing them to infect other cells and continue the cycle of viral multiplication.

Blood agar is a type of culture medium that is commonly used to grow bacteria, but it is not suitable for cultivating viruses. Viruses require living host cells in order to replicate and multiply, and blood agar does not provide the necessary environment for their growth. Cell culture, bird embryos, and live mammals are all commonly used methods for cultivating viruses because they provide the necessary host cells for viral replication.

Animal viruses can enter the host cell through two different mechanisms: injection and fusion, as well as endocytosis. Injection occurs when the viral nucleic acid is directly injected into the host cell by the virus. Fusion, on the other hand, involves the fusion of the viral envelope with the host cell membrane, allowing the viral nucleic acid to enter the cell. Endocytosis is a process in which the virus is engulfed by the host cell, forming a vesicle that then fuses with a lysosome, releasing the viral nucleic acid into the cytoplasm. Therefore, the correct answer is b and c, as both fusion and endocytosis are valid mechanisms for the entry of animal virus nucleic acid into the host cell.

A prophage is a latent stage in the cycle of bacterial viruses. During this stage, the viral DNA is integrated into the host bacterium's DNA and remains dormant until it is triggered to enter the lytic cycle, where it replicates and produces new viral particles. This latent stage allows the virus to persist within the host cell without causing immediate harm, allowing it to evade the host's immune system and ensuring its survival.

Plaques are clear patches in cell cultures that indicate sites of virus infection. When a virus infects a cell, it replicates and spreads, ultimately causing the death of the infected cell. This leads to the formation of a clear area, or plaque, where the virus has killed the cells. Plaques are commonly used in virology to quantify the number of virus particles present in a sample and to study the characteristics of different viruses.

Viruses that persist in the cell and cause recurrent disease are considered "latent". This term refers to viruses that are able to establish a lifelong infection in the host cell without causing immediate symptoms. These viruses can remain dormant or inactive for long periods of time, but can reactivate and cause recurrent disease episodes later on.

The correct answer is "complex helical." This is because enveloped, helical, naked icosahedral, and enveloped icosahedral are all general patterns of virus morphology. Complex helical, on the other hand, is not a general pattern and refers to viruses that have additional structural components or features beyond the typical helical structure.