Gene Expression in Virus: Lytic and Lysogenic Cycle

Gene Expressions in Virus: Who would have ever thought that a small virus would shut the entire world? This pandemic created so much fear worldwide. The first wave, second wave and now third wave! The scare is not over yet. So, the question arises? Why is the Corona wave recurring even after many have been vaccinated? What causes the new wave? How does this covid-19 or any other virus operate? 

The virus is an acellular structure made up of a protein coat wrapped around the genetic material.  The viral genome is made up of DNA or RNA. When the virus enters a host, its genome attempts to hijack the host cellular machinery for multiplication. Gene expression of virus in the host cell leads to the multiplication of the virus. Here we are discussing the method of gene expression and much more. Read along to learn about the virus, viral genome and its expression.

Structure of Virus

Viruses have an acellular structure. They are diverse based on shape, size and genome. The outer covering of the virus is made up of a protein called the capsid, which is specific to a host-cell surface. This is the reason why viruses have a narrow host range. A layer of membrane called the envelope may be present on some viruses. Viruses have both prokaryotic and eukaryotic hosts. A virus that infects only bacteria is called a bacteriophage or a phage. Viruses that infect animal or plant cells are referred to as animal viruses or plant viruses. 

Viral Genome

The viral genome is very small in size. Viruses are broadly divided into DNA viruses and RNA viruses. Both DNA and RNA viruses can either be single-stranded or double-stranded, with a circular, linear or segmented arrangement.  Viral transcription and gene expression depend upon host cell machinery and the type of polymerases present.

Fig: Structure of a Virus

Examples:

Reproductive Cycle of the Virus

Viruses do not have any mechanism or enzyme of their own, and they depend on the host completely for replication, gene expression and reproduction; hence they are also referred to as obligate parasites.  There are two kinds of the life cycle: The lytic cycle and the lysogenic cycle.

Lytic Cycle

The lytic cycle represents viral hijack over the cellular mechanism of the host cell. The virus takes over the host cell to multiply, leading to the death of the cell in the process. The cell burst open to release the new virus. The stages of a lytic cycle can be divided into five stages.

Fig: Steps of Lytic Cycle

1. Attachment: The virus attaches itself to the host cell with its protein coat.
2. Penetration: Genetic material of the virus enters the cytoplasm of the host by membrane fusion or endocytosis. The virus genome enters through contraction of the tail sheath, which acts like a hypodermic needle to inject the viral genome through the cell wall and membrane. It is important to note that the protein body of the virus remains outside of the cell. 
3. Gene expression and biosynthesis: This step is crucial in deciding the fate of the virus life cycle. The viral genome tries to block the host’s defence mechanism. Viruses block the host transcription and begin viral replication and gene expression. The interaction of these regulatory proteins controls gene expression in the virus. The nature of the transcription factors present in a cell can control whether the viral infection will result in the multiplication of viral elements or a long-lived asymptomatic latent infection. Once the virus takes over the host genome, the biosynthesis of viral components begins. 
4. Assembly and maturation: Capsid proteins start to assemble and form new viruses.
5. Lysis and release: Newly formed virus is released from the host cell by lysis of plasma membrane. The host cell disintegrates and dies.

Lysogenic Cycle

The lysogenic cycle begins exactly like the lytic cycle.  First attachment and then penetration, but the difference begins at the biosynthesis stage. The lysogenic cycle is commonly observed in bacteriophages. During the lysogenic cycle, the virus genome integrates into the host genome. The integrated phage genome is called a prophage. The bacterial host with a prophage is called a lysogen, and the process is called lysogeny. During lysogeny, the prophage remains integrated to the host and even passed on to the daughter cells. During unfavourable conditions, the viral genome takes over the host cell machinery through a lytic cycle. 

Fig: Lysogenic Cycle

Retroviral Mechanism

Retroviruses are enveloped animal viruses containing a single-stranded RNA genome. Once a retrovirus penetrates the host cell, the viral RNA genome is converted into double-stranded DNA with the help of enzyme reverse transcriptase. Now the retroviral  DNA is integrated into host DNA by an enzyme called integrase.  Host-cell ribosomes, tRNAs, transcription factors and translation factors are used in the synthesis of all viral proteins in the host. The virus multiplies by either lytic or non-lytic pathway, leading to the release of new viruses by budding through the host-cell plasma membrane or by lysis of the host plasma membrane.

Summary

Viruses are acellular structures made up of capsid protein and genetic material. The genetic material of a virus is DNA or RNA. The genetic material can be double-stranded or single-stranded. Since the virus lacks enzymes for multiplication or cellular components, it depends upon the host for multiplication and gene expression. Once a virus enters its host, it may enter either the lytic pathway or the lysogenic pathway. The pathway depends upon the defence system of the host cell. If the defence is weak, the viral genome enters the lytic cycle; if not, the virus enters the lysogenic pathway. 

The lytic pathway begins with the attachment of the virus to the host, followed by the entry of the genetic material of the virus into the host cell. Once the viral genome enters the host, it disables the defence mechanism of the host. It uses transcription factors, enzymes, ribosomes and other resources of the host to multiply and then burst open the host cell.  The lysogenic cycle begins and ends just like the lytic cycle. If the host genome has a strong defence system, the viral genome integrates itself into the host and even is transferred to the daughter cells. Once the host cell is in stressful condition, the viral genome takes over and enters the lytic cycle.

Frequently Asked Questions from Gene Expression in Virus (FAQs)

Q.1. How do viruses inhibit host gene expression?
Ans: A virus inhibits host gene expression by interfering with the host gene expressions. A virus codes for proteins that prevent host gene expression.

Q.2. What is the major difference between the lytic and lysogenic life cycles of a virus?
Ans: In the lytic cycle, the host cell is destroyed and killed, whereas in the lysogenic cycle, the host is not destroyed, but the viral genome is integrated into the host. The virus multiplies to produce multiple new viruses in the lytic cycle. In the lysogenic cycle, viruses do not multiply.

Q.3. What is the name for a virus that infects a bacterium?
Ans: Viruses that infect bacteria are called bacteriophages or simply phages.

Q.4. What are some examples of viral diseases?
Ans: The examples of viral diseases include polio, chickenpox, AIDS, common cold etc.

Q.5. What is the relative size of the viral genome?
Ans: Genome sizes of bacteriophages and viruses range from about 2 kb to over 1 Mb.

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