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The steps of specialized transduction. Step 1 is viral attachment and penetration. This is when the phage infects a cell. This shows the virus sitting on the outside of a cell and injecting DNA into the cell. Step 2 is integration when the phage DNA becomes incorporated into the host genome. Step 3 is excisionwhen the phage is excised from the bacterial chromosomes along with a short piece of bacterial DNA. The DNA is then packaged into newly formed capsids. When the virus particles are assembled the DNA contains both viral and host segments. Step 4 is infection when the phage contains both viral and bacterial DNA infects a new host cell. Step 5 is recombination when the phage DNA along with the attached bacterial DNA are incorporated into a new cell. The image shows a new bacterial cell with virus DNA as well as other bacterial DNA in its genome.
This flowchart illustrates the mechanism of specialized transduction. An integrated phage excises, bringing with it a piece of the DNA adjacent to its insertion point. On reinfection of a new bacterium, the phage DNA integrates along with the genetic material acquired from the previous host.
  • Which phage life cycle is associated with which forms of transduction?

Life cycle of viruses with animal hosts

Lytic animal viruses follow similar infection stages to bacteriophages: attachment, penetration, biosynthesis, maturation, and release (see [link] ). However, the mechanisms of penetration, nucleic-acid biosynthesis, and release differ between bacterial and animal viruses. After binding to host receptors, animal viruses enter through endocytosis (engulfment by the host cell) or through membrane fusion (viral envelope with the host cell membrane). Many viruses are host specific, meaning they only infect a certain type of host; and most viruses only infect certain types of cells within tissues. This specificity is called a tissue tropism . Examples of this are demonstrated by the poliovirus , which exhibits tropism for the tissues of the brain and spinal cord, or the influenza virus , which has a primary tropism for the respiratory tract.

Steps of influenza infection. Step 1 is attachment when the influenza virus becomes attached to a target epithelial cell. This image shows a spherical virus binding to the surface of a host cell. Step 2 is penetration when the cell engulfs the virus by endocytosis; this shows the virus within a vacuole. Step 3 is uncoating when the viral contents are released; the image shows the virus being released from the vacuole. Step 4 is biosynthesis when the viral RNA enters the nucleus where it is replicated by RNA polymerase. Step 5 is assembly when the new phage particles are assembled. Step 6 is release when new viral particles are made and released into the extracellular fluid. The cell, which is not killed in the process continues to make new viruses.
In influenza virus infection, viral glycoproteins attach the virus to a host epithelial cell. As a result, the virus is engulfed. Viral RNA and viral proteins are made and assembled into new virions that are released by budding.

Animal viruses do not always express their genes using the normal flow of genetic information—from DNA to RNA to protein. Some viruses have a dsDNA genome like cellular organisms and can follow the normal flow. 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 a genome is ssDNA, host enzymes will be used to synthesize a second strand that is complementary to the genome strand, thus producing dsDNA. The dsDNA can now be replicated, transcribed, and translated similar to host DNA.

If the viral genome is RNA, a different mechanism must be used. There are three types of RNA genome: dsRNA, positive (+) single-strand (+ssRNA) or negative (−) single-strand RNA (−ssRNA) . If a virus has a +ssRNA genome, it can be translated directly to make viral proteins. Viral genomic +ssRNA acts like cellular mRNA. However, if a virus contains a −ssRNA genome, the host ribosomes cannot translate it until the −ssRNA is replicated into +ssRNA by viral RNA-dependent RNA polymerase (RdRP) (see [link] ). The RdRP is brought in by the virus and can be used to make +ssRNA from the original −ssRNA genome. The RdRP is also an important enzyme for the replication of dsRNA viruses, because it uses the negative strand of the double-stranded genome as a template to create +ssRNA. The newly synthesized +ssRNA copies can then be translated by cellular ribosomes.

Viruses with −ssRNA (negative single-stranded RNA) use RdRP (viral RNA-dependent RNA polymerase) to make +ssRNA (positive single stranded RNA). RdRP can also be used to covert +ssRNA to −ssRNA. +ssRNA uses host ribosomes to make viral proteins.
RNA viruses can contain +ssRNA that can be directly read by the ribosomes to synthesize viral proteins. Viruses containing −ssRNA must first use the −ssRNA as a template for the synthesis of +ssRNA before viral proteins can be synthesized.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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