Microbiology

Microbiology Study Guide

circular and does not contribute to the nucleocapsid symmetry. Viruses also contain spikes, which are projections erupting out of the envelope. These spikes support the virus in attaching themselves to the host cells (e.g., the AIDS virus). The viruses that metabolize and multiply within bacteria are known as bacteriophages. The structure of these viruses consists of icosahedral heads and helical tails. Such viruses have DNA. Multiplication of Viruses Viruses make use of host cells to replicate. At the time of replication, the virus encourages these host cells to fuse the components necessary for the production of new viral particles. After that, the new particles gain some structure and then migrate to other cells and infect them. There exist a number of phases in the process of replication of viruses. The very first step is attachment, in which the virus attaches itself to the host cell, develops high specificity, and joins together with the receptors of the surface of the cell with the help of the spikes of the virus. These receptors may be present in any of the cell membrane, pilli, or flagella of the bacteria. After attaching itself, the virus penetrates into the cell, in the phase known as penetration. Some of the ways to achieve penetration are phagocytosis, blending the envelope of the virus with the membrane of the cell, or injecting the virus genome in the host cell. The next phase is replication, in which the genome is detached from the cytoplasm of the cell by stripping off the protein capsid from the genome. In the case of an RNA genome, the enzymes are synthesized with the help of the signals received by the RNA molecule that provides the necessary genetic codes needed for replication. Finally, synthesis takes place between the viral genomes and capsomeres, and new viruses are produced. In contrast, if the genome is DNA, the genome signals and provides the genetic codes for the synthesis of RNA molecules and then replication of new virus cells can take place. New virions are formed after the synthesis of genomes and capsomeres has been completed. They then reside in the cytoplasm or nuclei of the host cells. Once this assembly is done, the virions migrate to other cells and replicate again and infect them. Lytic Acid There are many circumstances that help release virions into the environment. Virions may be released once the host cells that helped the virus replicate have been biologically exhausted. Further, in the case of the viruses with envelope, the process of budding takes place. During this the nucleocapsids force themselves out of the host cell membrane and migrate to other cells. During the budding process, the nucleocapsid that escape from the host cell derive new envelope from the cell membrane of the host cell. It should be noted that if the host cells die during the process of replication, it is termed as lytic cycle of reproduction.

©2018

Achieve

Page 36

of 132