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Bio 15 Microbiology Ch 13 Viruses Study Guide Key Terms: Plaques Capsid Capsomeres nucleocapsid Icosahedral Virus bacteriophage envelope retrovirus lytic Lysogenic reverse transcriptase provirus syncytia cytopathic oncogenic prions scrapie Creutzfeldt-Jakob disease chronic wasting disease
- Describe the general structure of viruses a. What is a capsid, and what is its function b. How are the two types of capsids similar, how are they different? c. What types of coverings can a nucleocapsid have? d. What are spikes? i. how are they formed ii. what is their function
1. General Structure of Viruses
Viruses have a simple structure but are highly diverse in their composition. They consist mainly of two or three essential components: Genetic Material (Nucleic Acid): This can be either DNA or RNA, and it carries the genetic instructions for making new virus particles. It can be single-stranded or double-stranded and can have various forms (linear, circular, segmented). Capsid: This is a protein shell that encloses and protects the viral genetic material. The capsid is made up of repeating protein subunits called capsomers. In some viruses, the capsid is surrounded by an additional lipid layer or membrane, which comes from the host cell. Envelope (Optional): Some viruses have an additional outer covering called an envelope , which is made of lipids and proteins derived from the host cell membrane. This envelope is only found in certain viruses.
a. What is a Capsid, and What is Its Function?
A capsid is the protein shell that encases the viral genome. Its primary functions include: Protection: It protects the viral nucleic acid from physical damage and from enzymes that may degrade the genome. Facilitating Attachment: The capsid has specific proteins that interact with host cell receptors, allowing the virus to attach and enter the host cell. Facilitating Entry: The capsid plays a role in helping the virus gain entry into the host cell by either fusing with the host membrane or undergoing conformational changes.
b. How are the Two Types of Capsids Similar, and How Are They Different?
There are two main types of capsids :
- Helical Capsid: In this type, the protein subunits form a spiral (helical) structure around the nucleic acid, resembling a coiled spring or a cylinder. These are commonly seen in RNA viruses like the tobacco mosaic virus.
- Icosahedral Capsid: This type has a symmetrical, 20-sided shape, which is a geometric arrangement of protein subunits forming a protective shell. It's common in both DNA and RNA viruses like herpesvirus and poliovirus. Similarities: Both types serve the same basic function: to protect the viral genome and assist in the virus's attachment and entry into host cells. Both are composed of protein subunits called capsomers that come together to form the capsid. Differences: Shape: The helical capsid is cylindrical, while the icosahedral capsid is spherical or polyhedral. Structure Complexity: Icosahedral capsids are more symmetrical and rigid, while helical capsids are more flexible. Genome Packaging: Helical capsids are more often associated with RNA viruses, while icosahedral capsids can be found in both RNA and DNA viruses.
c. What Types of Coverings Can a Nucleocapsid Have?
The nucleocapsid is the combination of the viral genome (nucleic acid) and its capsid. Some viruses may have additional coverings around their nucleocapsid, such as: Envelope: This is a lipid bilayer membrane derived from the host cell’s membrane. It surrounds the nucleocapsid and is often studded with viral glycoproteins. No Envelope (Naked Viruses): Some viruses lack an envelope and consist solely of a nucleocapsid. These are typically more resistant to environmental conditions (heat, drying, acids) because they lack the fragile lipid bilayer.
d. What Are Spikes?
Spikes are specialized protein structures that protrude from the surface of some viruses, particularly enveloped viruses. i. How Are Spikes Formed? Spikes are formed by viral glycoproteins that are embedded in the viral envelope. These glycoproteins are synthesized during viral replication in the host cell and are then incorporated into the viral envelope. These proteins are often critical for the virus’s ability to recognize and bind to host cell receptors. ii. What Is Their Function? Spikes have a crucial role in the virus’s ability to infect host cells. Their functions include: Attachment to Host Cells: Spikes allow the virus to recognize and bind to specific receptors on the surface of the host cell. This is often the first step in viral entry. Facilitating Entry: In some cases, the spikes are involved in the fusion process, where the viral envelope merges with the host cell membrane, allowing the viral genome to enter the host cell. Overall, spikes are key to the virus’s infectivity and host specificity, as they determine which types of cells the virus can infect.
- Inoculation with Bacteriophage: o A diluted sample of the bacteriophage suspension is mixed with the bacterial culture (often using a small volume of phage solution), and this mixture is then poured onto the surface of the agar plate.
- Incubation: o The agar plate is incubated, usually at an optimal temperature for the growth of the bacteria. During incubation, the bacteriophages in the sample infect the bacterial cells. Each bacteriophage attaches to a bacterial cell, injects its genetic material, and begins to replicate, eventually causing the bacterial cell to lyse (burst open).
- Plaque Formation: o When a bacterium is infected by a phage, it dies and is destroyed. This causes a clear zone or plaque to form on the bacterial lawn. Each plaque represents an area where a single bacteriophage has infected and killed a bacterial cell, and the surrounding bacteria are also destroyed as the virus spreads. o The plaques appear as clear, round zones in the bacterial lawn. The size and number of plaques will depend on the concentration of phages and the efficiency of infection.
- Counting the Plaques: o After the incubation period, the number of plaques on the agar plate is counted. The plaques represent the number of infectious bacteriophage particles in the sample.
Plaque Forming Unit (PFU):
Each plaque corresponds to a plaque-forming unit (PFU) , which is a measure of the number of viable bacteriophages capable of infecting and lysing bacteria. The number of PFUs is directly related to the concentration of bacteriophages in the sample. The plaque count gives the concentration of the phage in the original sample, typically expressed as PFUs per milliliter (PFU/mL).
Steps in Summary:
- Prepare a bacterial lawn on an agar plate.
- Add a diluted bacteriophage sample to the surface.
- Incubate the plate, allowing phages to infect and lyse bacteria.
- Count the number of clear plaques formed.
- Calculate the number of phages based on the number of plaques.
Importance of Dilution:
The bacteriophage sample is often diluted to ensure that the number of plaques formed is countable (usually between 30-300 plaques per plate). If the phage concentration is too high, the plaques will overlap, making it difficult to count. If the concentration is too low, there may be too few plaques to measure accurately.
Advantages of the Plaque Method:
It provides a quantitative measure of the number of infectious bacteriophages. It is relatively simple and inexpensive. It also allows the detection of bacteriophages that may be difficult to detect using other methods. In summary, the plaque method detects and counts bacteriophages based on the formation of clear plaques on a bacterial lawn, with each plaque representing the successful infection and lysis of bacteria by a single phage particle. The number of plaques is used to estimate the concentration of bacteriophages in the sample.
- What dictates the host range of animal viruses? a. What are two ways that animal viruses penetrate the host cell? b. Describe the two ways that animal viruses leave their host cell
Host Range of Animal Viruses:
The host range of an animal virus refers to the range of host cells or species that a virus can infect and replicate in. The host range is determined by several factors:
- Specific Receptors on Host Cells: o Viruses can only infect cells that have specific receptors on their surface that the virus can recognize and bind to. These receptors are typically proteins or glycoproteins on the host cell membrane. The virus’s surface proteins (often located in the capsid or spike proteins) interact with these receptors, allowing the virus to attach to the cell.
- Cellular Machinery Compatibility: o Once the virus has attached to a host cell, it must be able to penetrate the cell membrane and utilize the host cell's machinery (such as ribosomes, enzymes, and energy production systems) for replication. The compatibility of the host cell's machinery with the virus’s requirements dictates whether the virus can replicate within that cell.
- Tissue Tropism: o Some viruses have a preference for infecting specific tissues within a host. For example, hepatitis viruses infect liver cells, while HIV primarily infects CD4+ T cells. This tissue- specific infection is also dictated by the presence of specific receptors on the surface of the cells in that tissue.
a. Two Ways Animal Viruses Penetrate the Host Cell:
- Direct Fusion with the Host Cell Membrane (for Enveloped Viruses): o How it works: Enveloped viruses (those with a lipid bilayer membrane) can fuse directly with the host cell membrane. The viral envelope merges with the host cell's membrane, releasing the viral genome into the host cell’s cytoplasm. o Example: HIV, Herpesvirus. o Process: The virus's envelope proteins interact with the host cell membrane, leading to fusion. Once fusion occurs, the virus's nucleocapsid (capsid + genome) is released into the host cell.
- Endocytosis (for Both Enveloped and Naked Viruses): o How it works: In endocytosis, the host cell engulfs the virus into a vesicle by invaginating its membrane, forming an endosome. The virus is then internalized into the host cell. o Example: Influenza virus, Adenovirus (a naked virus). o Process: The virus binds to the host cell's surface receptors, triggering the cell to engulf the virus into a vesicle. After the virus is inside the cell, the acidic environment inside the vesicle triggers
hijack the host's cellular machinery (such as ribosomes, enzymes, and energy resources) to replicate their genetic material and produce new virus particles. This is in stark contrast to bacteria and fungi, which can reproduce and grow independently in their environment.
3. Replication via the Host’s Machinery
No independent replication : Viruses do not replicate by division like bacteria or fungi. Instead, they must infect a host cell and use the host's biological machinery for their replication. This includes making copies of their genome and synthesizing viral proteins using the host's ribosomes and enzymes. The virus’s genetic material directs the host cell to assemble new viral components, which are then packaged into new virus particles. This replication cycle results in the production of progeny viruses, which are released into the environment to infect other cells.
4. Genetic Material Can Be DNA or RNA, But Not Both
Diverse genetic material : Unlike other pathogens, which typically have DNA genomes, viruses can have either DNA or RNA as their genetic material. Their genomes can be single-stranded or double- stranded, and they can be linear, circular, or segmented. Some viruses, like retroviruses (e.g., HIV), even use reverse transcription to convert their RNA genome into DNA once inside a host cell. This is a process not seen in other types of pathogens.
5. Cannot Be Treated with Antibiotics
No response to antibiotics : Because viruses do not have the cellular structures and metabolic processes that antibiotics target (such as cell walls, ribosomes, or metabolic pathways), they cannot be treated with antibiotics. This sets viruses apart from bacterial pathogens, for which antibiotics are often effective. Treatment of viral infections typically involves antiviral medications that target specific stages of the viral life cycle (e.g., blocking viral entry, replication, or assembly), but vaccines and other preventative measures are often more effective.
Summary:
Viruses are distinct from other pathogens due to their acellular nature , their reliance on host cells for replication , the variety of their genetic material , their ability to infect using only genetic instructions , and the fact that they cannot be treated with antibiotics. These characteristics make viruses fundamentally different from bacteria, fungi, and other pathogens.
- Describe how a virus is cultured in the lab. What special conditions of viruses have to be considered when cultivating them. Cultivating viruses in the laboratory is fundamentally different from growing bacteria or fungi because viruses are obligate intracellular parasites , meaning they require a host cell for replication. Here’s how viruses are cultured in the lab and the special conditions that must be considered:
1. Selection of Host Cells for Viral Culture
Since viruses need living cells to replicate, researchers must first choose an appropriate host cell line. The choice depends on the type of virus being studied:
Primary cells : These are cells taken directly from living organisms (e.g., animal tissues or plants). They have a limited lifespan in culture. Cell lines : These are cultured cells that can be maintained in the lab for extended periods and are often derived from tumors (e.g., HeLa cells , derived from human cervical cancer). They are commonly used to culture viruses. Embryonated eggs : Some viruses, especially influenza , can be cultured in fertilized chicken eggs, where the virus can infect the tissues and be harvested.
2. Methods of Virus Culturing
There are several methods for growing viruses in the lab: Monolayer culture method : o How it works : Cells are grown in a single layer on a petri dish or flask. The virus is introduced to the monolayer, and the virus infects the host cells. After a period of incubation, researchers examine the cells for cytopathic effects (CPE), such as cell lysis or other damage caused by the virus. o Why it’s used : This method is especially useful for detecting the presence of viruses and observing the effects of infection. Tissue culture and suspension culture : o How it works : Some viruses are cultured in suspension , where cells are grown in liquid media rather than as a monolayer. This is often used for viruses that prefer to infect suspended cells, such as some retroviruses. o Why it’s used : It allows for large-scale production of viruses, particularly for vaccines or research. Embryonated egg culture method : o How it works : Fertilized chicken eggs are injected with the virus, and the virus infects the tissues inside the egg. The virus can then be harvested from the fluid or tissue in the egg (e.g., influenza virus). o Why it’s used : This method is used for viruses that are difficult to grow in cell culture and remains a key technique for vaccine production. Organ culture : o How it works : Certain viruses, such as polio and hepatitis viruses , can be grown in small pieces of tissue (like slices of liver, kidney, or other organs) kept in culture. o Why it’s used : This is often done when specific tissues are needed to support viral replication.
3. Special Conditions to Consider When Cultivating Viruses
Since viruses do not have the machinery to reproduce on their own, several factors must be taken into account when cultivating them: Temperature and pH : o Viruses have specific temperature and pH requirements for optimal growth. Many viruses are cultured at body temperature (around 37°C for human viruses) to mimic the natural conditions of infection. o The pH of the growth medium is also important, as it must be suitable for both the virus and the host cells. Atmosphere (Oxygen Levels) :
Bacteriophage replication can occur in two distinct phases: the lytic cycle and the lysogenic cycle. Below is an explanation and description of each cycle, as well as the steps involved, including the entry and exit of the phage from the bacterial cell.
1. Lytic Cycle (Virulent Phages)
The lytic cycle is the phase in which the virus hijacks the host cell’s machinery to replicate itself and ultimately causes the cell to lyse (break open), releasing new virus particles. The main steps in the lytic cycle are: Steps of the Lytic Cycle:
- Attachment (Adsorption): o The bacteriophage attaches to the surface of the host bacterium using its tail fibers, which bind to specific receptors on the bacterial cell wall. This step is crucial for the virus to initiate the infection.
- Penetration (Entry): o The phage injects its genetic material (DNA or RNA) into the host cell. The viral capsid remains outside, while the genetic material is transferred into the bacterial cytoplasm through the cell membrane. This can occur via direct fusion (for enveloped phages) or via the tail of the phage (for non-enveloped phages).
- Biosynthesis (Replication and Transcription): o The viral genome takes over the host cell’s machinery. The host’s ribosomes , enzymes, and other components are redirected to synthesize viral components. This includes: Replication of the viral genome. Transcription and translation of viral genes to produce viral proteins (capsid proteins, enzymes, etc.).
- Maturation (Assembly): o The newly synthesized viral genomes and proteins are assembled into complete virus particles (virions). The bacteriophage’s head is filled with the genome, and the tail is assembled.
- Release (Lysis): o The bacterium is lysed (broken open), often by the action of viral enzymes that degrade the bacterial cell wall. This causes the release of new virions into the environment, which can go on to infect other cells. This process results in the destruction of the host cell. Diagram of Lytic Cycle:
- Attachment (Adsorption) -> 2. Penetration (Entry) ->
- Biosynthesis (Replication & Transcription) -> 4. Maturation (Assembly) ->
- Release (Lysis) -> New virions released
2. Lysogenic Cycle (Temperate Phages)
The lysogenic cycle is a more subtle process in which the bacteriophage's genetic material integrates into the host's genome and can remain dormant for an extended period before being triggered to enter the lytic cycle. Steps of the Lysogenic Cycle:
- Attachment (Adsorption): o Similar to the lytic cycle, the bacteriophage attaches to the bacterial cell surface using its tail fibers and binds to specific receptors on the host cell.
- Penetration (Entry): o The phage injects its DNA into the host cell, where it enters the bacterial cytoplasm.
- Integration (Prophage Formation): o Instead of immediately taking over the host machinery, the viral DNA integrates into the host's chromosome. This integrated viral DNA is called a prophage. The prophage is dormant and replicates along with the host's genome during cell division. The bacterium carrying the prophage is called a lysogen.
- Replication of Lysogen (Cell Division): o The prophage is passed on to daughter cells when the bacterial cell divides. The viral genome remains inactive, replicating passively within the host’s genome.
- Induction (Triggering the Lytic Cycle): o Under certain stress conditions (e.g., UV light, chemical exposure), the prophage may be triggered to exit the host genome and enter the lytic cycle. This process is called induction. The viral genome is excised from the bacterial chromosome and begins the biosynthesis, maturation, and release stages of the lytic cycle.
- Lytic Cycle (If Triggered): o Once induction occurs, the virus enters the lytic cycle , replicating and assembling new phages, ultimately causing the host cell to lyse and release the new virions. Diagram of Lysogenic Cycle: 1. Attachment (Adsorption) -> 2. Penetration (Entry) -> ↓
- Integration (Prophage formation) -> 4. Replication of Lysogen (Cell Division) ->
- Induction (Trigger to lytic cycle) -> Enter lytic cycle (Replication, Maturation, Release)
Key Differences Between the Lytic and Lysogenic Cycles:
Lytic Cycle: o Virus immediately replicates and destroys the host cell. o Results in cell death and release of many new virions. Lysogenic Cycle: o Virus integrates into the host genome as a prophage and does not immediately destroy the host. o The prophage replicates along with the host cell and can remain dormant until induced.
Summary of Entry and Exit:
In the lytic cycle , the virus enters the host by injecting its genome , and exits by causing lysis of the host cell, releasing new viruses.
o Some viruses also depend on the immune environment or the physical conditions of a cell to successfully infect it. For example, some viruses can only enter certain cells when the cell is in a particular state (e.g., during a specific phase of cell division or in response to certain immune signals). o For example, the measles virus specifically targets cells in the respiratory system, as these cells provide the right receptors and the appropriate conditions for the virus to replicate.
Summary of Factors:
Receptor specificity on the host cell surface (e.g., CD4 for HIV, sialic acid for influenza). Co-receptors or co-factors may be required for the virus to enter the cell (e.g., CCR5 for HIV). The virus must be able to use the host cell’s machinery for replication and uncoating of the viral genome. Some viruses may need specific environmental conditions or cellular states to enter. Thus, the interaction between the viral proteins (or spikes) and the host cell receptors largely determines which cells a virus can infect, leading to the virus's specific tropism for particular tissues or organs.
- Explain three cytopathic effects viruses can cause in a cell. Cytopathic effects (CPE) refer to the observable structural and functional changes in a host cell as a result of viral infection. These effects are often used to identify the presence of a virus and assess its impact on the host cell. Here are three common cytopathic effects viruses can cause:
1. Cell Lysis (Cell Death)
Description : Many viruses cause lysis of the host cell, which is the breaking open or rupturing of the cell. This often occurs at the end of the viral replication cycle, when new virus particles are released. How it Happens : The virus replicates inside the host cell, hijacking its machinery to make viral components. Once sufficient numbers of viral particles are assembled, the cell's structural integrity is compromised, often due to the action of viral enzymes (such as lytic enzymes ) or a general overload of cellular processes. This leads to the rupture of the cell membrane, causing the release of viral progeny and the death of the host cell. Example : Influenza , poliovirus , and many bacteriophages cause cell lysis. This effect is commonly used to detect virus activity in cultures by observing the plaque formation (clear areas where the infected cells have been destroyed).
2. Syncytium Formation (Multinucleated Giant Cells)
Description : Some viruses cause the infected cells to fuse together, forming multinucleated giant cells or syncytia. This occurs when infected cells' membranes merge, allowing multiple nuclei to share the same cytoplasm. How it Happens : This effect typically results from viral proteins expressed on the surface of infected cells. These proteins cause the infected cell's membrane to fuse with nearby uninfected cells. The formation of syncytia facilitates the spread of the virus but can also disrupt the normal function of the host tissue. Example : The respiratory syncytial virus (RSV) , measles virus , and human immunodeficiency virus (HIV) can cause syncytium formation, which is commonly seen in respiratory and immune cells, respectively.
3. Inclusion Bodies Formation
Description : Some viruses cause the accumulation of inclusion bodies in the host cell. These are abnormal aggregates of viral particles, viral proteins, or host cell components that can be seen under a microscope. How it Happens : Inclusion bodies form when viral components (like nucleic acids or proteins) accumulate inside the host cell, often due to the virus hijacking the host's machinery to produce these components in large quantities. Inclusion bodies can form in the nucleus (intranuclear inclusions) or in the cytoplasm (cytoplasmic inclusions), depending on the virus. Example : o Herpesviruses (e.g., herpes simplex virus ) often cause the formation of intranuclear inclusion bodies , visible under a microscope. o Rabies virus causes the formation of Negri bodies (cytoplasmic inclusion bodies) in the neurons.
Summary of Cytopathic Effects:
- Cell Lysis : The infected cell bursts open, releasing new viral particles and killing the cell.
- Syncytium Formation : Infected cells fuse together to form multinucleated giant cells.
- Inclusion Bodies : The accumulation of viral particles or proteins in the host cell, often forming visible structures. These cytopathic effects can help in diagnosing viral infections and provide insight into the mechanisms of viral pathogenesis.
- Be able to draw and explain the steps involved in dsDNA viral replication including the cellular locations.
Double-Stranded DNA (dsDNA) Viral Replication
The replication of dsDNA viruses in a host cell follows a series of well-defined steps. These steps include the entry of the virus into the host cell, transcription of viral genes, replication of the viral genome, and assembly of new virions. Below is a description of each step, including the key cellular locations involved.
1. Attachment and Entry
Step Description : The virus attaches to the host cell surface via specific interactions between viral proteins (on the capsid) and cellular receptors. After attachment, the virus enters the host cell via endocytosis or fusion. Cellular Location : Cell membrane. Key Point : The viral envelope (if present) fuses with the host cell membrane (for enveloped viruses), or the virus is engulfed by the cell in a vesicle (for non-enveloped viruses).
2. Uncoating
Step Description : Once inside the host cell, the virus undergoes uncoating , where the capsid is removed, and the viral genome (dsDNA) is released into the nucleus. Cellular Location : Cytoplasm (initially), then transported to the nucleus. Key Point : The uncoating process releases the viral DNA into the host cell's nucleus , where it can be used for replication and transcription.
3. Transcription of Viral Genes
- Attachment & Entry:
- Virus binds to host cell receptors → Entry by endocytosis or fusion.
- Uncoating:
- Capsid is removed, and viral DNA is released into the host cell's nucleus.
- Transcription:
- Host RNA polymerase transcribes viral DNA into mRNA in the nucleus.
- Translation:
- Viral mRNA is translated by host ribosomes in the cytoplasm into viral proteins.
- Genome Replication:
- Viral DNA is replicated in the nucleus using the host's DNA replication machinery.
- Assembly:
- Newly made viral genomes and proteins are assembled into new virions.
- Maturation:
- Final processing of viral components to make the virions infectious.
- Release:
- New virions are released via lysis or budding from the host cell membrane.
Summary of Key Cellular Locations in dsDNA Replication:
Attachment & Entry : Cell membrane. Uncoating : Cytoplasm (initially), then nucleus. Transcription : Nucleus. Translation : Cytoplasm. Genome Replication : Nucleus. Assembly : Nucleus or cytoplasm. Release : Cell membrane (budding) or via lysis (for non-enveloped viruses). The nucleus is central to the replication of dsDNA viruses, where key processes like transcription, replication, and assembly occur, ensuring the production of new viral particles capable of infecting other cells.
- Be able to draw and explain the steps involved in +ssRNA viral replication in an animal cell including the cellular locations.
+ssRNA Viral Replication in Animal Cells
Positive-strand RNA (+ssRNA) viruses are a class of viruses whose genome can be directly used as mRNA for translation by the host cell’s ribosomes. These viruses follow a different replication strategy compared to dsDNA viruses. Below is an explanation of the steps involved in the replication cycle of +ssRNA viruses, along with the cellular locations where each step occurs.
1. Attachment and Entry
Step Description : The virus binds to specific receptors on the surface of the host cell. After attachment, the virus is taken into the host cell via endocytosis or, in some cases, membrane fusion (for enveloped viruses). Cellular Location : Cell membrane. Key Point : The viral glycoproteins (in enveloped viruses) or capsid proteins (in non-enveloped viruses) interact with host cell receptors, allowing the virus to enter the cell.
2. Uncoating
Step Description : After the virus enters the cell, the capsid is uncoated, releasing the viral genome into the cytoplasm. For enveloped viruses, the viral envelope fuses with the host cell membrane, releasing the viral RNA directly into the cytoplasm. Cellular Location : Cytoplasm. Key Point : The viral RNA is now free in the cytoplasm, ready to be translated and replicated by the host cell machinery.
3. Translation of Viral Genome
Step Description : The +ssRNA genome acts directly as mRNA. Host cell ribosomes translate the viral RNA into viral proteins, including structural proteins (capsid proteins) and enzymes (such as RNA- dependent RNA polymerase) required for replication. Cellular Location : Cytoplasm (on ribosomes ). Key Point : Since the +ssRNA genome is similar to the host's mRNA, it is translated by the host ribosomes in the cytoplasm to produce viral proteins.
4. Replication of the Viral Genome
Step Description : The viral genome needs to be replicated to produce more viral RNA. The host cell’s RNA-dependent RNA polymerase (which is either provided by the virus or encoded in the viral genome) synthesizes complementary negative-sense RNA (-ssRNA) from the viral genome. This negative-sense RNA serves as a template to produce more positive-sense RNA (+ssRNA). Cellular Location : Cytoplasm. Key Point : The replication process occurs entirely in the cytoplasm. The newly synthesized negative- strand RNA is used to generate more positive-strand genomes.
5. Assembly of New Virions
Step Description : Once sufficient viral genomes and proteins are produced, they are assembled into new virions (complete virus particles). The viral genome is packaged into capsid proteins to form new virions. Cellular Location : Cytoplasm (assembly occurs here, but for some viruses, this process may involve some interaction with the cell membrane ). Key Point : The viral genome is encapsulated with newly synthesized structural proteins to form complete virus particles.
6. Budding and Release
Step Description : New virions are transported to the host cell surface, where they are either budded off (in the case of enveloped viruses) or lysed (for non-enveloped viruses). Enveloped viruses acquire their envelope from the host cell membrane as they exit the cell. Cellular Location : Cell membrane (for budding). Key Point : For enveloped viruses, the viral particles acquire their envelope as they bud through the host cell membrane. Non-enveloped viruses are typically released through cell lysis.
Diagram of +ssRNA Viral Replication in an Animal Cell:
- Attachment & Entry:
- Virus binds to host cell receptors → Virus enters the cell by endocytosis or fusion.
Step Description : The retrovirus binds to specific receptors on the surface of the host cell. For example, HIV attaches to CD4 receptors on T-helper cells and also requires co-receptors such as CCR5 or CXCR4. After attachment, the virus enters the host cell via endocytosis or membrane fusion (for enveloped retroviruses). Cellular Location : Cell membrane. Key Point : The viral envelope proteins (like gp120 in HIV) mediate fusion with the host cell membrane, allowing the viral capsid and genome to enter the cell.
2. Uncoating
Step Description : Once inside the host cell, the viral capsid is uncoated, and the RNA genome is released into the cytoplasm. Cellular Location : Cytoplasm. Key Point : The viral RNA is now free in the cytoplasm, ready for reverse transcription into DNA.
3. Reverse Transcription (RNA → DNA)
Step Description : The reverse transcriptase enzyme converts the single-stranded RNA genome into double-stranded DNA (dsDNA). This process occurs in the cytoplasm. Cellular Location : Cytoplasm. Key Enzyme : Reverse Transcriptase. o Function : Reverse transcriptase synthesizes complementary DNA (cDNA) from the viral RNA template and then synthesizes a second DNA strand to create the double-stranded cDNA. The reverse transcription also involves the degradation of the RNA genome. Key Point : This is a crucial step in retrovirus replication because retroviruses must convert their RNA genome into DNA to integrate it into the host genome.
4. Integration of Viral DNA into Host Genome
Step Description : The newly synthesized double-stranded viral DNA is transported into the nucleus. The viral integrase enzyme facilitates the insertion of the viral DNA into the host cell's chromosome. Cellular Location : Nucleus. Key Enzyme : Integrase. o Function : Integrase catalyzes the integration of the viral cDNA into the host's genome by cutting the host DNA and inserting the viral DNA. Key Point : Once integrated, the viral DNA (now called a provirus ) becomes a permanent part of the host cell’s genome, where it can be transcribed and replicated during the host cell's division.
5. Transcription and Translation of Viral Genes
Step Description : Once integrated into the host genome, the proviral DNA is transcribed by the host's RNA polymerase II into viral mRNA. This mRNA can be used for both translation into viral proteins and for replication of the viral genome. Cellular Location : Nucleus (for transcription) and cytoplasm (for translation). Key Point : The host cell’s RNA polymerase transcribes the provirus into viral mRNA, which is then transported to the cytoplasm for translation.
6. Translation of Viral Proteins
Step Description : The viral mRNA is translated by the host cell’s ribosomes into viral proteins, including structural proteins (capsid proteins) and enzymes (like reverse transcriptase, integrase, and protease). Cellular Location : Cytoplasm (on ribosomes ). Key Point : These viral proteins are synthesized in the cytoplasm and will later be assembled into new viral particles.
7. Assembly of New Virions
Step Description : The newly synthesized viral proteins and viral RNA genomes are assembled into new virions (complete viral particles) in the cytoplasm. Cellular Location : Cytoplasm. Key Point : The viral RNA genomes are packaged with the viral proteins (including reverse transcriptase and integrase) to form new virions.
8. Maturation
Step Description : The newly formed viral proteins undergo proteolytic cleavage by the viral protease enzyme. This cleavage produces the mature, functional viral proteins needed for the new virion's infectivity. Cellular Location : Cytoplasm. Key Enzyme : Protease. o Function : The protease cleaves viral polyproteins into smaller, functional proteins (such as capsid proteins, reverse transcriptase, and integrase) that are necessary for viral maturation.
9. Budding and Release
Step Description : The newly assembled virions are transported to the cell membrane and budded off from the host cell, acquiring an envelope derived from the host cell membrane. For non-enveloped retroviruses, cell lysis may occur. Cellular Location : Cell membrane (for budding). Key Point : The virus exits the cell, ready to infect new cells. The cell is typically not immediately destroyed, and may continue producing virus for some time (chronic infection).
Diagram of Retrovirus Replication Cycle:
- Attachment & Entry:
- Virus binds to host cell receptors → Virus enters by endocytosis or fusion.
- Uncoating:
- Viral capsid is uncoated, releasing RNA genome into the cytoplasm.
- Reverse Transcription (RNA → DNA):
- Reverse transcriptase converts viral RNA into complementary DNA (cDNA) in the cytoplasm.
- Integration:
- Integrase integrates cDNA into the host cell genome (provirus) in the nucleus.
- Transcription & Translation:
- Host cell RNA polymerase transcribes proviral DNA into mRNA → mRNA is translated into viral proteins in the cytoplasm.
- Assembly:
