the cell theory is a set of principles that defines what it means for an organism to be a living organism now the cell theory states that the basic unit of life is a cell and all living organisms consist of one or more cells it also states that cells arise from other pre-existing cells and this means that living cells can replicate and reproduce to form other cells now the focus of this lecture will be viruses and viruses are basically small agents that infect other cells and this includes animal cells plant cells as well as all different types of other cells now viruses do not actually satisfy the cell theory for example as we'll see in just a moment viruses cannot actually replicate and reproduce on their own and that means viruses are not considered living organisms they are non-living now all the viruses can come in many different forms and shapes and types all viruses contain nucleic acids either RNA or DNA but viruses NE contain both RNA and DNA they only contain one or the other viruses also contain a protein covering that is found outside the nucleic acid inside inside the viruses and this protein covering is known as a capsid now some viruses also contain another covering that is made of lipids and this is known as the lipid envelope and the lipid envelope actually comes from other living cells now the lipid envelope doesn't actually only act in protecting the cell it also serves as an attachment it basically allows the virus to attach onto living cells so the lipid code can have receptor proteins that can recognize and bind to other living cells and infect those living cells now as I mentioned earlier viruses cannot actually reproduce on their own and this is because they do not contain the proper Machinery to actually reproduce and create the molecules needed for reproduction and division for example virus es do not have ribosomes and that means they cannot themselves produce proteins however instead of actually replicating and dividing on their own our viruses can actually infect living cells and use the Machinery the organel of the living cells to actually reproduce and divide and form other viruses so one very common example of a virus one very common common type of a virus is a bacteria Fage and a bacteria Fage is a virus that only targets and infects bacterial cells so basically the structure of our bacteria Fage looks something like this we have our nucleic acids either DNA or RNA found inside the protein capsid this is the protein capsid also known as the head of our bacteria Fage we also have a protein midsection as well as the protein tail and in order to actually attach the virus onto the cell membrane of the bacteria this tail the bottom portion of the tail actually has to attach to the proper receptor on the cell membrane of that bacteria and once our attachment actually takes place so following attachment our bacteria phage in ejects the nucleic acids either DNA or RNA never both into the cytoplasm of the cell and within a short period of time the cell begins to basically translate and synthesize the proteins encoded by the viral nucleic acid now the bacteria Fage usually undergoes a pathway a cycle known as the litic cycle but other types of bacteria exist that can or other types of viruses exist that can also undergo the lysogenic cycle so when the virus into the cell there are two types of Pathways that can be taken by that virus we have the litic cycle the litic pathway as well as our lysogenic cycle lysogenic pathway so let's begin by discussing the litic cycle which is basically followed by our bacteria FAS viruses that infect bacterial cells so under this pathway as soon as the virus actually injects itself into the cell or injects the nucleic acid into that cell in the case of the bacteria Fage the bacteria Fage only actually injects the nucleic acid into the cell it leaves the protein tail the protein midsection and the protein captured outside of that that cell so on this pathway as soon as the virus is inside the cell it directs the cell to synthesize new viruses by using the host cells machinery and this includes the nucleus it includes the endoplasm reticulum it includes our ribosomes so for example ribosomes synthesize the protein coding arac capsid and the viral nucleic acids are essentially replicated ated and placed inside those protein capsids eventually our cell basically fills up with these new viruses and eventually the pressure as a result of the many different viruses inside the cell causes our cell to actually burst open and this process is known as licing and that's exactly why this is known as the liic cycle now once our cell lies it releases all all these new Pro all these new viruses and these viruses that are found outside the whole cell are known as virant now another method by which our cell can basically release the viruses onto the outside of the cell is by undergoing a type of process that looks like exocytosis basically using the cell membrane to take our viruses and bring them outside of that cell out now the period between when our virus actually infects the cell and right before the cell Lis is known as the latent period and this diagram basically describes the process by which the bacteria Fage undergo our litic cycle so in Step One our bacteria or our virus approaches the cell membrane of our bacterial cells so we have the cell membran braan we have the bacteria Fage so our bottom portion of our tail protein of this bacteria Fage attaches itself onto the proper receptor region on that cell membrane at that point this capsid essentially injects that nucleic acid into that cell through the M through the midsection and this capsid SE and this uh tail section so this entire protein tail and caps in the midsection is Left Behind on the outside of that bacterial cell while this nucleic acid either DNA or RNA found in the capsid is injected entirely into that bacterial cell now in step three we have the assembling process so now the cell has been infected and the cell is in the latent period so basically the cell is producing many new virus as shown in the following diagram and in the final step because we have so many different viruses that increases the pressure and causes the cell to actually burst open and lies and in this point the cell basically dies releasing all the new virin into the outside portion surrounding that cell and this process is known as the liic cycle now a different process that other types of viruses on under go is known as the lysogenic cycle under this pathway the viral DNA or RNA that enters that cell is basically incorporated into the host genome the host DNA now certain viruses such as HIV do not contain DNA they contain RNA and they also contain special types of enzymes known as reverse trans cryptate and these enzymes basically reverse transcribe the RNA into DNA and then that DNA is integrated is incorporated into the host cell genome the host cell DNA now once integrated into our DNA of the host the cell can basically live on and show no sign of actual infection and this stage this phase is known as as the dorment period now of course eventually we can have some type of environmental factor that can cause the cell to basically undergo the litic cycle for example UV radiation is one form of environmental factor that can basically take a cell under the lysogenic cycle and transform it into the litic cycle forced to go into the liic cycle and produced many different viruses and eventually lice so a cell whose genome contains a viral DNA section inside that cell is known as a pro virus so these are the two types of cycles of Pathways that can be followed by our cell once the virus actually injects itself or injects the nucleic acid into that whole cell a specific timee type of virus that only infects and targets bacterial cells is known as the bacteria phage and this usually undergoes the litic cycle there are also different types of viruses for example the HIV viruses a virus that doesn't only undergo the litic cycle it can also undergo the lysogenic cycle in the lysogenic cycle the DNA of that virus is actually Incorporated it's integrated with the DNA of that whole cell now what exactly is the main difference between the litic cycle and the lysogenic cycle well the litic cycle isn't very useful because it actually kills off that whole cell so once the cell dies the viruses have to find other cells to actually infect however the lysogenic cycle is more useful because the virus doesn't actually directly and immediately kill off that cell it incorporates into the cell and can undergo the ltic cycle any time it wants to basically when it is stressed by certain types of environmental factors such as for example heat or UV radiation