Describe The Life Cycle Of A Cell – All viruses depend on cells for reproduction and metabolic processes. Viruses themselves do not encode all the enzymes necessary for viral replication. However, inside the host cell, the virus can use cellular machinery to produce viral particles. Bacteriophages replicate only in the cytoplasm because prokaryotic cells do not have a nucleus or organelles. In eukaryotic cells, most DNA viruses can replicate within the nucleus, except for large DNA viruses such as poxviruses, which can replicate in the cytoplasm. RNA viruses that infect animal cells often replicate in the cytoplasm.
The life cycle of bacteriophages has been a good model for understanding how viruses affect infected cells, because for eukaryotic viruses cells can die quickly or develop a latent or chronic infection. Virulent stages usually lead to cell death through cell lysis. On the other hand, latent stages can become part of the host chromosome and recombine with the cell’s genome to form viruses, or progeny viruses.
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Describe The Life Cycle Of A Cell
During the lytic cycle of the viral stage, the bacteriophage engulfs the cell, produces new stages, and destroys the cell. T-even phage is a good example of a well-characterized class of viral stages. There are five stages in the bacteriophage lytic cycle (see Figure 1). Attachment is the first step in the infection process, where the phage interacts with specific bacterial surface receptors (eg, lipopolysaccharides and the OmpC protein on the host surface). Most stages have a narrow range and can only infect a single species of bacteria or a single strain within a species. This unique recognition can be used for targeted treatment of bacterial infections, phage therapy, or phage typing to identify unique bacterial species or strains. The second stage of infection is entry or penetration. This is caused by the contraction of the tail sheath, which acts like a hypodermic needle to inject the viral genome through the cell wall and membrane. The phase head and other components remain outside the bacteria.
Cell Differentiation Definition, Process & Examples
Figure 1. The viral phase represents only the lytic cycle described here. In the lytic cycle, the phage replicates and lyses the host cell.
The third stage of infection is the biosynthesis of new viral components. After the virus enters the host cell, the bacterium synthesizes virus-encoded endonucleases to degrade the chromosome. It then hijacks the host cell to copy, transcribe, and translate the viral components (capsomeres, coat, basal plates, tail fibers, and viral enzymes) needed to assemble new viruses. Polymerase genes are usually expressed early in the cycle, while capsid and tail proteins are expressed later. During the mature stage, new virions are formed. To release the free stages, the bacterial cell wall is broken down by phage proteins such as golin or lysozyme. The last stage is the release. In a process called lysis, mature viruses are released from the host cell and progeny viruses are released into the environment to infect new cells.
In the lysogenic cycle, the phage genome enters the cell by entry and penetration. A prime example of a phase with this type of life cycle is the lambda phase. During the lysogenic cycle, instead of killing the host, the phage genome integrates into the bacterial chromosome and becomes part of the host. The integrated phage genome is called a propagule. The bacterial host is called a lysogen. The process of bacterial infection with the intermediate stage is called lysogeny. Characterized by latent or inactive temperature phases within the cell. As the bacterium replicates its chromosomes, it also replicates the host’s DNA and passes it on to new daughter cells during reproduction. The presence of phage can change the bacterial phenotype because it can introduce additional genes (eg, toxin genes that increase bacterial virulence). This change in host phenotype is called lysogenic conversion or phase conversion. Some bacteria, e.g
, is less viral in the absence of propaganda. Infecting stages of these bacteria carry toxin genes in their genomes and increase the virulence of the host when the toxin genes are expressed. In case
Solution: Cell Division Lab2
, the toxin can cause paralysis. During lysogeny, propagation will continue until induction on the host chromosome, which results in the removal of the viral genome from the host chromosome. After induction, the intermediate stage can undergo a lytic cycle and undergo lysogeny in the newly infected cell (see Figure 2).
Figure 2. An average bacteriophage has lytic and lysogenic cycles. In the lysogenic cycle, phage DNA is incorporated into the host genome, creating a promoter that is passed on to the next generation of cells. Environmental stressors such as starvation or toxic chemicals can trigger infection and enter the lytic cycle.
This video shows the stages of the lysogenic life cycle of a bacteriophage and the transition to the lytic phase.
Transduction occurs when a bacteriophage transfers bacterial DNA from one bacterium to another during successive infections. There are two types of transduction: generalized and specialized transduction. During the lytic cycle of viral replication, the virus hijacks the host cell, condenses the host chromosome, and produces viral genomes. While collecting the DNA and loading it into the phase head, the packaging occasionally fails. Instead of packaging the viral DNA, it takes a random piece of DNA and inserts it into the capsid. Once released, this virion will inject the previous host’s DNA into the newly infected host. Asexual transfer of genetic information allows DNA recombination, thus providing the new host with new genes (eg, an antibiotic resistance or sugar replacement gene).
The Birth And Death Of Cells
Generalized transduction occurs when a random fragment of bacterial chromosomal DNA is transferred during the lytic cycle. Specialized transduction occurs at the end of the lysogenic cycle, when the promoter and bacteriophage enter the lytic cycle. Because the phage is integrated into the host genome, the promoter can replicate as part of the host. However, certain conditions (such as UV light or chemical exposure) induce induction, which causes this stage to excise from the genome, enter the lytic cycle, and generate new stages to leave the stem cells. A single step in the excision process from the host chromosome may remove some bacterial DNA near the viral integration site. Phage and host DNA from one or both ends of the integration site are packaged into a capsid and transferred to a new, infected host. Since the phase-transferred DNA is not packaged randomly, but instead has a specific piece of DNA near the integration site, this mechanism of gene transfer is called specific transduction (see Figure 3). The DNA can then fuse with the host chromosome, giving the latter new characteristics. Transduction plays an important role in the evolutionary process of bacteria, providing them with a mechanism for the asexual exchange of genetic information.
Figure 3. This diagram shows a specific transduction mechanism. Integral phase excises, bringing with it a piece of DNA adjacent to the insertion site. When infecting a new bacterium, the DNA phase combines with the genetic material from the previous host.
Lytic animal viruses undergo the same stages of infection as bacteriophages: attachment, penetration, biosynthesis, maturation, and release (see Figure 4). However, the mechanisms of entry, nucleic acid biosynthesis and release differ between bacterial and animal viruses. After binding to host receptors, animal viruses enter by endocytosis (by the host cell) or by membrane fusion (viral envelope with the host cell membrane). Most viruses are host-specific, meaning they only infect a specific host. and most viruses infect certain cells in tissues. This property is called tissue tropism. Examples of this are poliovirus, which shows a tropism for brain and spinal cord tissue, or influenza virus, which has a primary tropism for the respiratory tract.
In influenza virus infection, viral glycoproteins bind the virus to host epithelial cells. As a result, the virus spreads. Viral RNA and viral proteins are synthesized and assembled into new virions, which are released by budding.
Pine Life Cycle
Animal viruses do not always express their genes using the normal flow of genetic information—from DNA to RNA to protein. Some viruses, like cellular organisms, have dsDNA genomes and can follow a normal course. However, others may have ssDNA, dsRNA or ssRNA genomes. The nature of the genome determines how the genome is replicated and expressed as viral proteins. If the genome is ssDNA, host enzymes will be used to synthesize a second strand that complements the genomic strand, thus producing dsDNA. The dsDNA can now be copied, transcribed and translated to mimic the host DNA.
If the viral genome is RNA, a different mechanism must be used. There are three types of genomic RNA: dsRNA, positive (+) single-stranded
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