General Features of the Virus Replication Cycle Flashcards
What do viruses alternate between?
Viruses alternate between an extracellular stage that is inert and an intracellular stage in which new viruses are made.
The entire process from attachment to release from the cell is called the virus replication cycle.
Why is it termed virus replication cycle and not virus reproduction cycle?
Since viruses are not able to reproduce without a host cell, the term replication is used instead of reproduction when describing the production of new virus particles.
The entire virus replication cycle from attachment to progeny virus release can be completed within 8–24 hours for many animal viruses - describe what happens in this cycle.
- The virus particle must bind to a structure on the cell surface and the viral genome must enter into the cell.
- Once inside the cell, the viral genome is replicated, viral mRNA is synthesized, viral proteins are made, and new virus particles are assembled and released.
- Viruses use the cellular ribosomes, ATP, tRNA, amino acids, etc., to replicate their genome and synthesize their proteins.
- Depending on the particular virus example, it may use the cellular DNA and RNA polymerases, or it may have its own.
- The protein components self-assemble and associated with a copy of the replicated genome.
- The virus particle then egresses from the cell by budding or lysis the plasma membrane.

What is the first step of viral attachment to the host cell?
The first step is the adsorption of the virus particle to the host cell; this is a reversible step that results from random collisions between the virus particles and the host cells.
It does not require energy from the host cell.
What is the second step of viral attachment to a host cell?
The second step is the specific binding of proteins on the outside of the virus particle to a cell surface molecule of the host cell.
This structure may be a protein, a lipid or the carbohydrate portion of a glycoprotein or glycolipid involved in some normal function of the cell.
It is often called the “virus receptor” because it is a structure that is exploited by the virus for infecting a host cell

What is the host range?
The cell types, and even which species a virus can infect
What determines the host range?
Determined in part, by whether the cells express the virus receptor.
Host range is also determined by whether the host cell has proper intracellular factors (e.g., polymerases) to allow the replication and expression of the virus genome.
Give an example of a virus with a restricted host range and contrast with a virus with a wider host range.
Some viruses have a restricted host range in which the virus receptor is found only on a few cell types.
For example, HIV binds to the CD4 protein that is primarily found on helper T cells and perhaps on some macrophages.
The normal role of CD4 is to function as a co-stimulatory molecule of T cells.
In contrast, most cells express the CD46 molecule, which is the receptor for the measles virus.
So whereas HIV can only infect certain cells in a human, the measles virus can infect most human cells.
What is a susceptible cell?
Cell types that a virus can infect are called susceptible.
What is a permissive cell?
Cells in which a virus can replicate are called permissive.
Describe how SARS-CoV-2 coronavirus binds to a cell surface protein?
The SARS-CoV-2 coronavirus binds to the cell surface protein Angiotensin-Converting Enzyme 2 (ACE-2, a metallopeptidase) which is abundantly expresses on the epithelia of the lung and small intestine.
ACE-2 is referred to as the cell’s virus receptor.
The spike protein (S) on the virus, is the virus’ anti-receptor, a structure that is complementary to the cell’s virus receptor.

What is a major strategy for preventing viral diseases?
Give an example.
Give an example where this strategy would not be useful.
Preventing virus attachment is a major strategy for preventing viral diseases.
For example, vaccines have been developed that induce the production of neutralizing antibodies in the host. The antibodies would bind to the “anti-receptor” on the virus surface (the structure of the virus that binds to host cell surface receptor) and block the binding of the virus to the viral receptor on the host’s cell. However, if the viral anti-receptor mutates at a high rate (e.g., Influenza virus) the antibodies that had been induced by vaccination would not be useful against this modified virus.
There are many different mechanisms for a virus (or only its genome) to enter the host cell. For any given virus, the same method is always used. Give some examples of different viruses attaching to cells and the entry of the genome.
A) Receptor-mediated endocytosis of virus particle followed by coating of the genome when the capsid breaks apart.
B) Receptor-mediated endocytosis of virus particle followed by genome release by the formation of a pore in the endosome membrane.
C) Receptor-mediated endocytosis of virus particle followed by fusion of the endosome membrane and virus envelope.
D) Fusion of the virus envelope with the plasma membrane.

When does virus entry occur?
Virus entry is an energy dependent process that occurs almost immediately after attachment.
The viral genome and any essential viral proteins must enter the cytoplasm of a cell in order for the virus to replicate.
The nucleic acid of the viral genome can be DNA or RNA, and both types of nucleic acid are sometimes found inside the same virus particle.
True or false?
False
The nucleic acid of the viral genome can be either DNA or RNA, but both types of nucleic acid are never found inside the same virus particle.
Some RNA viruses have segmented genomes - that is, their entire genome is contained in several molecules of RNA.
What are the two functions that the viral genome that enters the cell has?
- it is used as a template for the synthesis of viral genomes for the progeny virus, and
- it is used to synthesize viral mRNA so that viral proteins can be synthesized.
What are the (+) and (-) strand designations?
A double-stranded nucleic acid has one strand designated as the plus (+) strand and the other as the minus (–) strand. The two strands are complementary.
When one of the strands is used as a template to synthesize a complementary strand, the new strand has the opposite sign.
By a molecular biology convention, mRNA is defined as a positive (+) strand because it contains immediately translatable information.
Is mRNA a (+) or (-) strand?
By a molecular biology convention, mRNA is defined as a positive (+) strand because it contains immediately translatable information.
Are single stranded RNA viruses (+) or (-) strand?
Single stranded RNA viruses are either (+) strand or (–) strand depending on whether or not the RNA can be directly translated by the ribosomes.
When virus RNA can be translated directly by the ribosome, it is designated as the (+) strand and contains the coding sequence that specifies the amino acids of the protein.
If the RNA sequence cannot be used directly as the mRNA it is designated as the (–) strand. The complementary sequence of the (–) strand must be made to serve as the mRNA.
The process of transcription and translation are the same for DNA and RNA viruses.
True or false?
False.
The process of transcription and translation differs greatly between DNA and RNA viruses, and RNA viruses with opposite nucleic acid polarity.
The steps and the enzymes involved in viral replication depend on the type of nucleic acid the virus carries.
How do DNA viruses replicate their DNA?
Using a DNA polymerase.
Some viruses will use the host cell DNA polymerase; other viruses will supply their own (pre-made) DNA polymerase.
How do RNA viruses replicate their genome?
RNA viruses (except retroviruses) encode RNA-dependent RNA polymerases (RDRP).
The host cells do not have an enzyme that can read RNA as a template.
RDRP will synthesize the complementary strand of RNA using the virus RNA as a template.
How do retroviruses replicate their genome?
Retroviruses are RNA viruses that use reverse transcriptase (RT) to synthesize a single strand of complementary DNA (cDNA) using the viral RNA genome as a template.
The RT enzyme then synthesizes the second strand of DNA.
What are the five distinct stages of the virus replication cycle?
- attachment
- entry
- replication and synthesis of viral proteins
- assembly of new virions
- egress.
























