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Viral Pathways | DAT Lytic and Lysogenic Pathways • Once inside its host cell, a virus can pursue a lytic or lysogenic pathway o Let’s look first at the lytic pathway as it pertains to a bacteriophage The Lytic Pathway • As we saw previously, the bacteriophage latches onto the host cell and injects its viral code o This marks the beginning of the latent period and the eclipse period • Inside the host, the viral DNA/RNA replicates itself • Using resources from the cell, new full-fledged viruses form o The formation of the first full virus ends the eclipse period • Ultimately, the bacterium will become over-packed with bacteriophages o At this point, the cell will either: • Break open in a process known as lysis, or • Release viruses one-at-a-time o This marks the end of the latent period • The viruses released during lysis can go infect other cells o Thereby repeating the cycle • A virus that infects using the lytic cycle is said to be virulent The Lytic Cycle (a) A bacteriophage (red) latches onto a bacterium (yellow). (b) The bacteriophage injects its viral DNA (red ring) into the bacterium. (c) The viral DNA replicates using the bacterium’s reproductive machinery. (d) Using resources from the cell, full fledged viruses develop within the cell. (e) When the cell has become overly-populated, it lyses, thereby releasing the newly-formed viruses. (f) Those viruses can go infect new cells. The Lysogenic Pathway • A virus that follows the lysogenic pathway is said to be temperate • The virus infects the cell, like, before o But this time, the viral code is incorporated into the host cell’s DNA (a) (b) Incorporation of Viral DNA into the Host Genome In the lysogenic pathway, the viral code injected in (a) is injected along with restriction enzymes. These enzymes help cut open the bacterial DNA, allowing the viral DNA to become inserted, as shown in (b). 1 © 2017 J Co Review, Inc., Accessed by Guest on 06-18-2017 Viral Pathways | DAT • • If the virus contains DNA, that DNA is introduced to the host genome via restriction enzymes If the virus contains RNA, it may also be injected with the enzyme reverse transcriptase o This allows the RNA to be transcribed to DNA Thus allowing it to be inserted into the host genome Incorporation of Viral RNA into the Host Genome Viruses that contain RNA (right) tend to also inject the enzyme reverse transcriptase. This allows the RNA to be converted to DNA (arrow). The DNA can then be inserted into the host genome. • • • When the host cell undergoes natural replication, the viral DNA will be replicated, too Once incorporated, the viral DNA might remain in a dormant or latent stage, such that it has no apparent effect on the cell o Such a virus is said to be a provirus, or a prophage Eventually, some environmental stress will activate the viral DNA o This converts the virus into a virulent virus In other words, the virus moves into the lytic cycle o In this respect, the lysogenic cycle is an offshoot of the lytic cycle The Lysogenic Cycle After the bacteriophage has injected into viral code into the cell, (a) that code is incorporated into the host cell’s genome. (b) When that host cell replicates, the viral DNA will replicate along with it. The viral DNA will remain dormant until a particular environmental stress moves it back into the lytic cycle. Types of Viral Code • Viruses can be somewhat diverse in the genetic code they carry • + strand RNA – Like mRNA, + strand RNA can readily be: o Transcribed to proteins o Reverse-transcribed to DNA via reverse transcriptase Viruses that reverse-transcribe RNA to DNA are called retroviruses • - strand RNA – The complement of mRNA o It must be converted to + strand RNA before it can be utilized • Viruses may also have double stranded RNA (or, of course, DNA) 2 © 2017 J Co Review, Inc., Accessed by Guest on 06-18-2017