Lytic and Lysogenic cycle of virus
Virus reproduce by undergoing the lytic cycle or lysogenic cycle. In both cases the virus attaches to the cell wall of the host, and injects its genome into the cytoplasm of the host cell. In lytic cycle the virulent virus genome destroys the host genome and takes over the cell functioning to replicate its genome and structural elements. After assemble of new virions the host cell wall is lysed to release them.
In the lysogenic cycle, the viral genome integrates with the host genome and replicates harmlessly as a prophage. When the cell health deteriorates, the viral genome triggers the lytic cycle.
More than 4000 species have been identified, but it is believed there could be many more (1). Virus has a genome that be made of RNA or DNA, and no cytoplasm within a capsid (1). Some may have also a glycoprotein cover over the capsid with spikes (2).
Virus are considered non-living as they cannot perform any of the functions of a cell, like cellular respiration or metabolizing proteins. So they cannot grow or replicate on their own. They are completely parasitic, and need other organisms to be able to perform any functions, and to reproduce. (1, 2).
They do this by infecting other organisms, single celled prokaryotes or multi celled eukaryotes. The virus infect only specific hosts, and particular cell types (1).
Replication of Virus
Virus multiple by two types of infection cycles called lytic and lysogenic cycle (3). Viruses use these two methods to infect bacteria, plants and animals (4). Some of the steps like attachment, and injection are common to both the processes (5).
In both cases the virus attaches itself to the host cell membrane. This can happen physically, for example with the help of the spikes in the glycoprotein membrane. Many virus, typically bacteriophages, the virus which infect bacteria, have a tail with they use to attach to the host cell membrane. The tail releases proteins that bind to the membrane at specific receptor site (3, 4, 6).
The virus releases enzymes that weakens or breaks down the host cell membrane. In some cases particularly in bacteriophages, the nuclear strands (DNA or RNA) are injected through the weak spot. In other cases the membrane is broken down to let the whole virus in. The capsid and glycoprotein layers are then lost by dissolution to release the genome into the cytoplasm of the host (3, 4, 6). This process is called uncoating (4). At this point the cell is infected and can be attacked by the immune system of the host (4).
After this the two cycles take a different pathway. The lytic cycle ends in the destruction of the host cell, while the lysogenic cycle doesn't harm the host cell (4).
The lytic cycle is the way predominantly used by virus for replication, and these virus are called virulent (4).
Once inside, the virus ensures that no other virus infect its host cell, by producing repressors. Then the virus takes control of the cell mechanism to replicate (3). Different viruses use two different methods used to do this.
In some cases the DNA of the host cell is deactivated, so that the virus can replicate, as in T4 pages infecting E. coli (4).
Other DNA viruses produce proteins to destroy the host DNA. The virus use the host mechanism by transcribing themselves into the messenger RNA of the cell and directing the ribosome work. One of the first proteins formed by this RNA, destroys the host cell DNA (3, 4).
Next, the virus genome produce the RNAs, which then make the necessary proteins. In the second phase the viral genome is synthesized using the proteins. In the third phase the structural components of the virus are synthesized (4, 5).
After the different components are ready, they are assembled into complete viruses, with the help of proteins coded by the virus genome. The entire host cell machinery is now involved in this process (5).
When the cell is full of new infectious virions, 100-200 of them, an enyzme is produced to break the host cell-wall and release the new virus. These go on to infect the neighboring host cells. At this point the organism is infected with the virus and develops symptoms (2, 3, 4).
The lytic cycle can also occur without lysis, where the new virus bud of the host cell, by taking some cytoplasm with them. This is the method used by virus that cause HIV and common influenza, and many others that infect eukaryotes (4).
The stages from injection to the end of replication when all component parts are ready is called eclipse period. The time from injection to the lysis of the host is called latent period. In T2 phages the eclipse period takes just 12 minutes, and the latent period 18 minutes (7).
In this cycle the virus does not take over the cell mechanisms (7).
Integrating with host genome
The viral genome does not destroy the host DNA nor does it affect its normal functioning (5, 7). The DNA or RNA of the virus integrates with the host DNA and becomes inactive and is called a prophage (5, 8, 7, 9). The viral DNA duplicates along with the host DNA as part of it each time the cell undergoes mitosis or cell division. Some of the expression of the host DNA can be affected, but the changes to the cell functioning are minimal (3). Due to this reason, the organism that hosts the virus doesn't develop any symptoms (3).
This can continue until the cell health deteriorates due to decrease in nutrients or occurrence of stress like mutation, or radiation (3, 5, 7, 8 ,9). Then the viral genome initiate replication through the lytic cycle (3, 7, 8, 9). The viral genome remains as prophage by expressing genes that inhibit the lytic cycle, when this inhibition does not occur, the lytic cycle starts and produces new virus (8). At this point the host organism will develop infection symptoms (3).
When the viral genome remains in the lysogenic stage, some virus actually boost host cell health, and are called 'morons' (8). In some cases it can increase the infection capacity of bacteria or a non-toxic bacteria can start producing toxins. For example, Corynebacterium diphtheriae
does not produce any toxin. It can produce the toxin leading to diphtheria only when it is infected by phage β (8).
Many of the bacterial phages are called temperate phages as they can choose to go through the lysogenic cycle or the lytic cycle (8, 9).