Table of Contents
- What do viruses need to reproduce?
- Virus reproduction cycle
- Other Frequently Asked Questions
How do you think viruses reproduce? Many wonder how viruses get to reproduce especially as they differ from other microbes.
Viruses, as compared to other microbes depend totally on living cells to be able to function. A virus happens to be the smallest of all microbes and is made up of core genetic material. This genetic material could be RNA or DNA which is surrounded by a protective coat. The protective coat is called a capsid and is made up of proteins. Also, the capsid is surrounded by an additional spikey coat sometimes which is called the envelope.
Reproduction, however, is a biological process at which new individuals are produced and formed and viruses aren’t left out in the reproduction cycle. Individuals reproduce sexually or asexually and organisms that reproduce sexually require two individuals from each sex in order to reproduce. Whereas organisms that reproduce asexually can reproduce without another individual.
The way humans and most animals reproduce is a common example of sexual reproduction and organisms like bacteria, yeast, hydras, and viruses reproduce asexually. In asexual reproduction, an individual organism creates an identical replica or genetically similar copy of itself without any contribution of genetic material from another individual. Bacteria, for instance, reproduce asexually through binary fission. Some organisms like yeast and hydra reproduce by budding. Such organisms as well as viruses do not have different sexes (male and female). Hence, they are capable of multiplying and dividing themselves into two or more individuals.
What do viruses need to reproduce?
Viruses need a living cell to reproduce. As a result, how viruses reproduce is quite different from other organisms or microbes. This is because viruses themselves are only alive when they inhabit the cells of living organisms. Thus, a virus cannot reproduce on its own without a living cell. This is different from other microbes such as bacteria and archaea that can reproduce by themselves by separating their body into two new bodies.
These microbes duplicate their genetic material and then divide into two parts in a process known as cytokinesis. Then, each new organism receives one copy of DNA, for viruses it’s quite different as they depend on other living cells. Hence, for viruses to be alive or reproduce they need a host cell. This host cell could be cells of humans, animals, plants, or even bacteria. Yea there are some viruses that infect microbes like bacteria and archaea, such viruses are called bacteriophages.
Viruses are seen as parasites as they depend solely on living cells. They are capable of fastening onto a host cell and hijacking the cell for their replication. The replication of themselves in a cell is what produces other new viruses. Hence, reproduction in viruses is asexual. How viruses do reproduce asexually is basically just them taking control of the cell of a living organism to generate more viruses. They have adopted a mechanism that enables them to produce new virions. These new viruses then leave in search of other living cells to infect and reproduce in. This is how they gradually hijack the whole cellular system of an organism. Thus, causing disease.
Virus reproduction cycle
Answering questions like how do viruses reproduce needs one understanding of the reproduction cycle of viruses. Let’s look at the viral reproduction cycle and how viruses go about it.
Recall that viruses are only capable of reproducing and replicating themselves by taking control of the reproductive apparatus of living cells. They rather make these cells reproduce the virus’s genetic structure and particles instead. However, how viruses do this is entirely based on the type of nucleic acid they contain; either DNA or RNA.
Outside a cell, viruses cannot reproduce or function and depend totally on a host cell for survival. Moreso, most viruses attack specific species and some only infect a narrow range of animals, bacteria, fungi, or plants. Furthermore, there are two basic ways in which viruses reproduce. Even though some viruses reproduce by employing the two ways. However, how viruses reproduce involve two life cycle:
- Lytic cycle
- Lysogenic cycle
The lytic cycle is one of the viral reproduction cycles. This cycle ends up destroying the host cell and its membrane. Following a lytic cycle, the virus attaches to the host cell and injects its DNA. It then uses the cellular metabolism of the host to begin to replicate its DNA and form proteins. The fully formed viruses assemble and burst the cell. Hence, spreading the new viruses to other cells in order to continue the cycle.
In the lytic cycle, however, the DNA of the virus exists as a separate free-floating molecule within the host cell. Also, it replicates separately from the host cell DNA. This is different from the lysogenic cycle, where the DNA of the virus is located within the host DNA. The lytic cycle is a reproductive cycle with six stages. These stages are attachment, penetration, transcription, biosynthesis, maturation, and lysis.
During attachment, the virus attaches itself to the surface of the host cell to inject its DNA into the cell. In the process of penetration, the virus injects its DNA into the host cell. This is done by penetrating through the cell membrane. During transcription, the DNA of the host cell is degraded. The cell’s metabolism is directed to initiate viral biosynthesis.
In biosynthesis, the DNA of the virus replicates inside the cell, synthesizing new viral DNA and proteins. Then, maturation occurs as the replicated genetic material assembles into fully formed viruses. This leads to lysis, where the newly formed viruses are released from the infected cell. They burst and lyse the cell in the process as they exit the cell in search of new host cells to infect. A bacteriophage, T4 phage, for instance, is known to use this reproduction cycle.
The lysogenic cycle is one of the viral reproduction cycles. In this cycle, the virus attaches to the host cell and injects its DNA. The DNA of the virus then gets incorporated into the DNA of the host. This allows the DNA of the virus to be copied and passed on together with the host cell DNA. Hence, as the host cell replicates, the virus DNA replicates as well spreading its genome throughout the host. However, in a lysogenic cycle, the virus reproduces without destroying or lysing the host cell.
The lysogenic cycle doesn’t ruin the host cell. The virus is reproduced and replicated eventually in all daughter cells because the lysogenic cycle allows the host cell to stay alive and reproduce. The lysogenic cycle is a reproductive cycle with four stages. These stages are attachment, entry, integration, and cell division.
During attachment, the virus attaches to the host cell and as it enters the cell it inserts its DNA into the host cell. In integration, the DNA of the virus then recombines with the chromosome of the host cell and becomes integrated into the chromosome as a prophage. Then during cell division, new viruses are reproduced. Each time a cell that contains the prophage divides, the daughter cells of the host cell inherit the prophage. Once the host cell splits, the prophage is copied together with the DNA of the host. Hence, bringing forth new viruses that exit in search of other host cells to infect.
Also, under the right conditions during the integration phase of the lysogenic cycle, the prophage can become active and come back out of the chromosome of the host cell. This will set off the remaining phases of the lytic cycle. The phases involve DNA copying and protein synthesis, phage assembly, and lysis. The prophage in such a condition will exit the chromosome and becomes its own DNA molecule. Then, the lytic cycle will start.
Viral replication is the production of viruses during the process of infection in the target host cells. Viruses have to get into the cell first before viral replication can happen. Through the production of abundant copies of the viral genome and packaging of these copies, the virus continues to infect new host cells.
Stages of Viral replication
Listed above are the six stages that give a detailed explanation to answer the question of how do viruses reproduce. One should know that a virus comes across multiple obstacles as it tries to enter the host cells. The cellular and nuclear membrane are such barriers. Cellular membranes or plasma membranes pose as barriers for invaders. This barrier is the first obstacle that all viruses have to penetrate. The second barrier is the nuclear membrane. This is a barrier to some viruses that reproduce and replicate their genome in the nucleus of the host cell. How these viruses obviate the barriers in order to reproduce is what makes up the stages of viral replication.
The virus life cycle is in stages of attachment, penetration, uncoating, replication, assembly, and exit from the host cell. Entry and attachment is the first stage. This is the process in which a virus particle finds the host cell and attaches to the cell surface. In penetration, the virus particle reaches the cytoplasm. During uncoating, the virus sheds its capsid. After uncoating, the naked viral genome is employed for gene expression and viral genome replication. Then, the viral proteins and viral genomes are accumulated and assemble to form a progeny virion particle. They are then released extracellularly. Now, let us look at each stage in detail.
For viruses to reproduce and establish infection, they must enter the cells of the host organism and use those cells’ materials. In order to establish an entrance into the cells, the proteins on the surface of viruses interact with the proteins of the host cell. Hence, attachment happens between the viral particle and the cell membrane of the host.
Attachment happens to be the first contact of the virus particles with host cells. This involves two kinds of host proteins on the plasma membrane:
- Attachment factors
- Viral receptors
The role of the attachment factor on the surface of the host cell is to recruit and hold the virus particles. Therefore, it facilitates the interaction of the viral particle with the entry receptor. Actually, glycosaminoglycans such as heparins act as the attachment factor for several viruses, revealing the broader specificity of the attachment factors. Viral receptors, unlike attachment factors, promote the penetration of virus particles into cells when binding to the virus particles.
Viruses attach to a specific receptor site on the cell membrane of the host cell. This attachment is via attachment proteins in the capsid of the virus or through glycoproteins embedded in the viral envelope. The specificity of this interaction, however, determines the cells within the host (and the host) that can actually be infected by a specific virus. In addition, the presence of the receptor in a particular cell determines its susceptibility to a certain virus. Thereby, cell tropism to a great extent is determined by the receptor.
How viruses reproduce involves the second stage which is penetration. As a result of the attachment of the viral particle to the host cell, a hole is formed in the cell membrane of the host cell. The genome of the virus or the virus particles are then released into the host cell.
Penetration into the cytoplasm occurs after the attachment of the virus particle to the target cells. The mechanism for the penetration varies with viruses, be it enveloped viruses or naked viruses. Direct fusion and receptor-mediated endocytosis are the two mechanisms.
Enveloped viruses use one of the two mechanisms- i.e either direct fusion or receptor-mediated endocytosis. Whereas, nonenveloped naked viruses use receptor-mediated endocytosis for penetration.
In direct fusion, the two membranes which are the cell membrane and the viral envelope fuse together. During this mechanism, the viral nucleocapsid is delivered directly to the cytoplasm, leaving the viral envelope behind on the cell membrane. Retrovirus is a typical example of a virus that penetrates by direct fusion.
In receptor-mediated endocytosis, viruses depend on endocytic uptakes. In this mechanism, after the attachment of viral particles on the receptor, the virus particle-receptor complex sets off the endocytosis by forming a coated pit on the cell membrane that leads to endosome formation. Due to this, the virus particle becomes located inside the endosome. Next is to break down the endosome in order to penetrate to the cytoplasm. However, the process of endosome breakdown varies with enveloped or nonenveloped viruses.
For enveloped viruses, the membrane fusion between the endosomal membrane and viral envelope is set off by acidic pH at the early endosome. This causes the endosome breakdown. Furthermore, the fusion peptide embedded on the envelope glycoprotein becomes activated (exposed) as a result of conformational change upon low pH. The fusion is then set off by the fusion peptide.
For nonenveloped naked viruses, one of the capsid proteins induces the endosome lysis. In summary, the mechanism of penetration for enveloped viruses is membrane fusion. Whereas, the mechanism used by nonenveloped viruses for penetration is membrane lysis. After penetration, replication of the viral genome may begin.
Understanding how viruses reproduce is not complete without taking note of this third stage that involves uncoating. The virus particles move from the cell periphery to the perinuclear space, approaching the site of replication. As a result, the genetic material of the virus becomes exposed to cellular machinery for viral gene expression. This process is called uncoating. Uncoating is usually linked with the cytoplasmic trafficking or endocytic route.
However, for viruses that replicate and reproduce in the nucleus, the viral genome needs to enter the nucleus through a nuclear pore. Hence, they utilize multiple distinct strategies depending on their genome size. The viral capsid itself enters the nucleus for viruses with a smaller genome, like polyomavirus. Then, for viruses with a larger genome, the nucleocapsids are anchored to a nuclear pore complex which results in partial disruption of the capsid like in adenovirus. Also, it can induce a minimal change in the viral capsid like in the herpes virus. Thereby, allowing the transit of the DNA genome into the nucleus.
The viral genome replication strategies differ from each virus among the virus families. Actually, how these viruses go about genome replication is what defines the identity of each virus family. Moreso, the extent of dependency of each virus family on host machinery is diverse. It ranges from those that depend solely on the host machinery to those that are quite independent. However, all of them depend solely on host translation machinery and ribosome for their protein synthesis.
The mechanism of replication depends on the viral genome. DNA viruses for instance, normally use the proteins and enzymes of the host cell to produce additional DNA. This additional DNA is transcribed to messenger RNA (mRNA). It is then used to direct protein synthesis.
For RNA viruses, they use the RNA core as a template for the synthesis of viral genomic RNA and messenger RNA. The viral messenger RNA directs the host cell to synthesize viral enzymes and capsid proteins as well as to assemble new virions. Though, there are exceptions.
For example, if a host cell does not provide the necessary enzymes for viral replication, the viral genes supply the information needed to direct the synthesis of the missing proteins. Retroviruses, like HIV, have an RNA genome that has to be reverse transcribed into DNA, which is then incorporated into the genome of the host cell.
Retroviruses must contain genes that encode the virus-specific enzyme reverse transcriptase to be able to convert RNA into DNA. This transcribes an RNA template to DNA. However, reverse transcription doesn’t occur in uninfected host cells. The required enzyme reverse transcriptase is only obtained from the expression of viral genes within the infected host cells.
HIV produces some of its own enzymes. These enzymes are not found in the host. This has enabled researchers to develop drugs that inhibit these enzymes. Such drugs as well as the reverse transcriptase inhibitor AZT inhibit the replication of HIV by reducing the activity of the enzyme without affecting the metabolism of the host. This approach has successfully led to the production of a variety of drugs that are used to treat HIV. It has also been effective at reducing the number of infectious virions in the blood to non-detectable levels in many individuals affected with HIV.
For viruses to reproduce at this stage, they must take control of the replication mechanisms of the host cell. After regulation is established, the environment is made ready for the virus to begin making copies of itself. Then, replication occurs quickly by the millions.
Assembly occurs after replication. The capsid assembly follows as the viral genome and viral proteins abundantly accumulate. Capsid assembly involves two processes:
- Capsid assembly
- Genome packaging
These 2 processes can happen in a sequential manner or simultaneously, depending on the viruses. Picornavirus is an example of a virus that has these 2 processes occurring sequentially. Whereas, in a virus such as an adenovirus, it happens simultaneously.
In a picornavirus, the capsids which are the immature capsid or procapsid assembles first without the RNA genome. Then, subsequently, the RNA genome is packaged or introduced through a pore that is formed in the procapsid structure. Whereas, in an adenovirus, the capsid assembly occurs with the DNA genome packaging. Furthermore, how these viruses selectively package the viral genome is another thing. There is a packaging signal which is a cis-acting element in the viral genome. This signal is specifically recognized by the viral capsid proteins. These viral capsid proteins, however, selectively package either DNA or RNA.
During the stage of release or exit, the newly produced viruses are released from the host cell. This exit is either by causing the cell to lyse, waiting for the cell to die, or by budding off through the plasma membrane.
In the case of naked viruses, the newly produced virus particles are released through cell lysis of the host cell. This means the host cell burst open and no specific exit mechanism is needed. The plasma membrane that traps the assembled virus particles is dismantled and the viruses exit the host cell. Polyomavirus and adenovirus are examples of naked viruses.
Whereas, for enveloped viruses, envelopment takes place before the release. Envelopment is the process whereby the capsids are surrounded by a lipid bilayer. However, there are two mechanisms involved in relation to capsid assembly and envelopment.
Firstly, envelopment can occur after capsid assembly is completed. Then, the fully assembled capsids by the interaction of the viral capsids with viral envelope glycoprotein are recruited to the membrane. Examples of such viruses are the herpes virus and hepatitis B virus. Alternatively, in some viruses such as the retrovirus, the envelopment can happen simultaneously with the capsid assembly.
Furthermore, there are two cellular membranes used in regard to the membrane for envelopment. Some viruses use the plasma membrane as the site of envelopment. Examples are the retrovirus and influenza virus. Whereas others use endosomes, such as endoplasmic reticulum (ER) and Golgi bodies as the site of envelopment. Examples of such viruses are herpesvirus and hepatitis B virus.
Also, how these viruses exit from infected cells after reproduction is another thing. The majority of enveloped viruses are released extracellularly. This is through exocytosis and this process is also termed budding. Through budding, the envelopment occurs in a linked manner with the extracellular release.
In conclusion, the release is the last stage of the reproduction cycle as the new virions produced are released in the host organism. These virions are then able to infect nearby cells and repeat the reproduction cycle. As discussed earlier, some of them are released when the host cell dies, while others can leave infected cells by budding through the membrane without directly killing the cell. Also, after release, maturation occurs extracellularly. For viruses like picornavirus and retrovirus, maturation is crucial to enable infectivity.
Furthermore, some viruses can hide within a cell. This means they evade the immune system of the host cell and may enhance the long-term survival of the virus. This act of hiding is called latency. During the stage of latency, the virus does not produce any progeny. The virus stays inactive until external stimuli like light or stress trigger it to activate.
Other Frequently Asked Questions
How do viruses reproduce and cause disease?
How these viruses reproduce and cause disease is another frequently asked question. Already you should know that a virus has to use the living cell processes to reproduce. However, the reproduction cycle of the virus can cause severe biochemical and structural changes in the host cell. This may result in cell damage. These changes that cause cell damage are called cytopathic effects. It can change the functions of the cell or even destroy it.
Some infected cells end up dying through lysis or apoptosis as all the progeny virions produced are released at once. Lysis is the cell bursting whereas apoptosis is programmed cell death or cell suicide. The common cold virus, rhinovirus is an example of such a virus that causes lysis.
Therefore, symptoms of viral diseases are a result of the immune system response to the virus. The immune system in defense makes an attempt to stop and eliminate the virus from harming the body and cells. However several viruses like HIV exit the infected host cells via budding, with the virions leaving the cell individually.
During budding, the cell does not lyse and is not immediately killed. Though, the damage the virus causes to the cells makes them not function normally. Thereby, the cells may be alive but will be dysfunctional. The majority of viral infections and diseases follow similar stages. This involves attachment, penetration, uncoating, replication, assembly, and release.
Viruses can cause many diseases in humans. For instance, the influenza virus causes the flu. It is typical for viruses to stir up an immune response in the host and likely kill the virus. However, some viruses are not successfully attacked by the immune system and an example of such a virus is the human immunodeficiency virus (HIV). This results in a more chronic infection that is difficult to cure and only the symptoms are usually treated.
Viral infections are different from bacterial infections as antibiotics are ineffective at treating them. Usually, vaccines are used to best prevent viral infections. Also, some antiviral drugs can treat some viral infections. Most of these antiviral drugs work by interfering with the replication of the virus in the host. They prevent viral replication by stopping DNA synthesis.
How do viruses multiply?
How viruses multiply is the same thing as them replicating and reproducing. Viruses as obligate intracellular parasites must gain entry into target host cells and hijack the host cellular machinery in order to produce a progeny virus. They multiply after entering the cell and shedding its capsid.
The mechanism of multiplication depends on the viral genome. DNA viruses for instance, normally use the proteins and enzymes of the host cell to produce additional DNA. This additional DNA is transcribed to messenger RNA (mRNA). It is then used to direct protein synthesis. For RNA viruses, they use the RNA core as a template for the synthesis of viral genomic RNA and messenger RNA. The viral messenger RNA directs the host cell to synthesize viral enzymes and capsid proteins as well as to assemble new virions. Once, the environment is made ready for the virus to begin making copies of itself. Then, replication and multiplication of the virus occur quickly by the millions.
How do viruses reproduce in humans?
Questions such as how viruses reproduce in humans arise frequently. It is important to know that viruses use human cell machinery to reproduce in humans. Viruses are only capable of reproducing by taking control of the reproductive apparatus of the living cells of humans. They rather make these cells reproduce their own genetic structure and particles instead. Here is a video clip to help you understand better.
Do viruses reproduce asexually?
Questions like how viruses reproduce and whether it is done sexually or asexually come up frequently. As we all know reproduction is a biological process in which new individuals are produced and formed. In as much as viruses may be distinct entities, they aren’t left out in the reproduction cycle. Every living organism reproduces either sexually or asexually. Organisms that reproduce sexually require two individuals from each sex in order to reproduce. Whereas organisms that reproduce asexually can reproduce without another individual.
Viruses reproduce asexually. In asexual reproduction, an organism creates an identical replica or genetically similar copy of itself without genetic material from another individual. Such organisms as well as viruses do not have different sexes (male and female). Hence, they are capable of multiplying and dividing themselves into two or more individuals. The newly produced viruses are released from the host cell. This exit is either by causing the host cell to lyse, waiting for the cell to die, or by budding off through the plasma membrane.