hey everyone Dr D here and in this video we are covering chapter 7 from our Cohen 7th edition microbiology a systems approach textbook this chapter covers viruses and preons so let's go ahead and get started Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D explain stuff all right welcome back to chapter seven again covering viruses and prons so let's go ahead with this chapter what you should know is that viruses are not alive viruses are known as small infectious agents and these infectious agents what they do is they take over cells they infect cells and then they make the cell a virus Factory and that cell is reprogrammed to make copies of the virus and you're not spared if you are a living organism there is a virus for you there is a virus that can infect you so there are bacterial viruses known as bacteriophage there are viruses that infect algae there are viruses that infect fungi there are viruses for protozoa plants animals archa again you are not spared living organisms have some kind of virus which will infect them every type of organism is infected by some virus but one thing you should know is that not all organisms can be infected by the same virus right viruses have what's known as tropism tropism means that uh it's the range of different hosts a particular virus can have so for instance certain viruses can only infect one species of organism but other viruses might be able to infect multiple different species tropism refers to the variety of different tissues or cells or species that a virus can infect some viruses have narot tropism for instance HIV human immuno deficiency virus in specifically the helper te- cells or CD positive te- cells of humans while other viruses are known to infect multiple different species for example rabies virus would have a more broad tropism it would be able to infect a more broad variety of different organisms so it all depends on the virus as to which particular cells it infects and I'll explain why in a little bit now because viruses are not alive remember the reason viruses are not alive is because they are not composed of cells the basic unit of life is a cell if you don't belong to the domains bacteria ARA ukaria then you're not considered a living entity viruses are not cells they don't belong to any of the three domains of life therefore they are not alive so instead of saying a virus is alive or dead scientists use the terms active or inactive instead of alive or dead and that makes sense that's because they are small infectious agents they are not living and the main reason they are not living is because again a they they are not made up of cells B they are unable to multiply independently from a host cell amongst many other reasons as well remember the characteristics of life there are certain characteristics of life and living things tend to possess most of those characteristics of life well viruses lack most of those characteristics of life therefore they are not alive instead we call them obligate in cellular parasites obligate meaning strict or they must intracellular meaning inside of the cell and parasite meaning something that lives and becomes dependent on a host viruses are known as small intracellular parasites they are also small infectious agents so they are not alive but they require a host cell in order to reproduce with regard to the size of viruses they are small viruses are smaller than the average bacterium they are so small that you cannot study most viruses with a light microscope so in order to visualize a virus you're going to need an electron microscope electron microscopes are required to detect vir viruses in combination with special stains again viruses are much simpler than cells therefore they are smaller than cells you can't see viruses if you're studying them with a light microscope such as the ones we have in the lab remember that a light microscope has a resolution limit of one micrometer anything smaller than a micrometer is really lost l in to a light microscope so here you can see a size comparison of a UK carotic yeast cell see in in uh kind of teal color here you have a eukariotic yeast cell which appears to be budding and that's quite large just about seven micrometers across right so seven micrometers you're going to easily see that with a light microscope which has a minimum resolution of one micrometer and then eoli remember eoli and staf lakus or in this case sorry streptococus these are bacterium and they are procaryotic eoli is about two micrometers long so you could visualize it with a light microscope and spec streptococus is about one micrometer uh in diameter and so you could visualize that with a light microsc scope as well however notice that the viruses besides this ginormous Pandora virus which is about one micrometer but that's the exception notice that viruses tend to be smaller than one micrometer with the smallest viruses being about 22 nanometers in diameter so again because viruses are typically smaller than bacterial cells and shorter or smaller than the cut off limit or the size limit of a light microscope which is one micrometer you cannot visualize most viruses with a light microscope and instead what you're going to need as an electron microscope to visualize viruses and as I said before viruses bear no resemblance to cells they are not living they are not cells they lack protein synthesizing Machinery they don't have ribosomes but they're basically an external coating which is called a capsid protein coat this protein coat surrounds a core containing nucleic acids either DNA or RNA So at their core every virus is a nucleic acid either DNA or RNA surrounded by a protein coat called the capsid coat and that's essentially at the core of every virus is this protein capsid coat surrounding either a DNA or an RNA genome and as I just mentioned the most simple structure of a virus needs at least a piece of genetic material in the central core made by one or more nucleic acid strands of either DNA or RNA isn't that neat The genome the Genome of a virus can either be a DNA genome like you and me or an RNA genome viruses can have RNA genomes and remember surrounding that genome which is either a DNA genome or an RNA genome is an external protein coat called the capsid covering or the capsid coat and this protein coat covers the genetic material additionally some viruses will also have one or more enzymes as well these enzymes help the virus to replicate a virus is much simpler than a cell viral components include again a genome The genome could be either a DNA genome or an RNA genome this is known as the nucleic acid genome and surrounding that genome is remember a protein coat called the capsid the capsid is a protein shell that surrounds the nucleic acid genome by the way the genome plus the capsid has a name it's called the nucleo capsid nucleocapsid refers to the capsid and nucleic acid together and every single virus has at its core at least a nucleo capsid that nucleo capsid could be a Genome of RNA surrounded by a capsid coat or a genome of DNA surrounded by a capsid coat additionally some viruses have an envelope this means a membrane like a plasma membrane surrounding the nucleo capsid surrounding the protein coat usually this envelope is a modified piece of a host's cell membrane I'll show you this in a little bit though not all viruses have an envelope not all viruses have a membrane the viruses that lack an envelope are called naked viruses and lastly viruses have Spike proteins what's a spike protein a spike protein is found on the surface of both naked and enveloped viruses these Spike proteins are essential to the virus because they're what allows the virus to dock onto the host cell without Spike proteins the virus would not be able to attach to host cells and it would not be able to infect any cells and it doesn't matter if it's a naked virus with just a nucleocapsid or it's an enveloped virus with a plasma membrane or some kind of membrane either way these Spike prot proteins are on the surface of the virus and they allow the virus to attach to host cells and this is required for infection of the host cell now the term for a virus a specific viral particle is known as a virion a virion is a fully formed virus which is able to establish infection in a host so when I point to a virus that's known as a Varon and at the core of every Varon is the nucleocapsid the nucleic acid genome plus the protein capsid and remember whether that nucleocapsid is naked like the one on the left or enveloped like the one on the right these viruses have Spike proteins on the surface see these these Spike proteins on the surface which are required for binding to host cells what these Spike proteins do is that they bind specifically to receptors on host cells remember I mentioned tropism and I told you that tropism has to do with a virus's ability to infect a particular cell well that has everything to do with Spike proteins and which receptor they recognize if they recognize a receptor that's only present on a certain cell type well only that cell type is going to get infected by this virus but if these Spike proteins recognize a pro a protein or a receptor that's found on multiple different animals well then that means that multiple different animals are going to be infected by this virus so the spike proteins are so important because it's the spike protein binding to specific receptors on the host cells that dictates a virus's tropism how many different organisms and different tissues that the virus can infect isn't that interesting and again regardless of naked viruses on the left or envelope viruses on the right you have Spike proteins on the surface of the cell these Spike proteins are necessary for infection these Spike proteins attach to host cell receptors in a specific way without these Spike proteins the virus wouldn't be able to attach to a host cell and therefore it would not be able to infect a host cell now I told you what the capsid is the capsid is the protein coat or the protective outer shell of virus es but it's not made up of one protein the capsid is made up of what are known as proteins called capsom each of the proteins that make up the capsid are called capsom and these capsom are identical protein subunits identical protein subunits that spontaneously self assemble to form the capsid so again caps omir are the individual proteins which come together to form the capsid and these are identical protein subunits there are two main shapes for a capsid these are the two most common capsid shapes a helical capsid which is a rod-shaped capsomere that forms a continuous Helix around the nucleic acid which I'll show you on the next slide and a cassed capsid which forms a three-dimensional 20-sided figure kind of like a 20-sided dice if you played any uh Dungeons and Dragons game with 12 evenly spaced Corners so let me show you what I'm talking about when I'm talking about a helical capsid versus an aoso hedral capsid here you can see a helical capsid helical nucleo capsids notice that the capsids are made up of IND individual proteins these individual proteins remember are called capsomers these capsomeres for the helical capsid are Rod shape so these proteins come together to form a cylinder and these cylinders accommodate the nucleic acid the RNA or the DNA that RNA or DNA fits inside of the cylinder and it gets coated by these capsid proteins which begin to form a helix and the length of the virus depends on the length of the nucleic acid these viruses form helices what what you see with these viruses is that this capsid forms a long cylindrical tube kind of like a straw here you can see a virus with an electron microscope that virus is helical inst structure now let me tell you a funny story again this is a helical virus which has the nucleic acid inside of a protein coat that's helical it forms a long cylinder which kind of resembles a cigarette am I right this cigarette looking virus is actually known as tobacco mosaic virus which attacks the tobacco leaves of the tobacco plant isn't that interesting that a helical virus that looks like tiny cigarettes is the very virus that attacks the tobacco plants right tobacco leaves of the of the to tobacco plant isn't that interesting like an irony of life because tobacco leads to cigarettes you know and cigarettes are deadly to people however the tobacco mosaic virus attacks the tobacco leaves so it's one of those life's greatest ironies in my opinion it's kind of an interesting factoid uh so I hope you understand what a um helical virus looks like and again the length of this helical structure depends on the length of the genome inside and usually these have RNA genomes inside but that doesn't mean that all helical viruses are naked like this one here some helical nucleo capsids are enveloped such as this one you see inside are the nucleo capsids and outside is an envelope a membrane and this is a flu virus a flu virus is an enveloped virus which has helico you can see here helical nucleo capsids inside a flu virus is a what's known as a segmented genome virus the flu virus has several pieces of RNA there are several nucleocapsids inside of an envelope and the flu virus is made up of actually eight different pieces of uh RNA of nucleocapsid so a flu virus has eight different pieces of RNA all as a nucleo cap capid inside of a membrane inside of an envelope isn't that neat nucleo capsids can have a helical shape those shapes can be naked such as the one for tobacco mosaic virus or envelop such as the ones for the flu flu virus or influenza virus now the other major shape of a capsid is known as a aoso hedral capsid this looks like a 20-sided dice if you've ever played uh Dungeons and Dragons or even Boulders Gate 3 you know about the 20-sided dice aedin are essentially a 20 sided shape resembling that of a 20 sided dice and several viruses adopt an aedr C capsid a a capsid with 20 equilateral sides and these viruses can be naked where the 20 sided structure nucleocapsid is naked with the spike proteins pointing out or these eohed capsids can also be enveloped such as this right here but why the aedr shape well it turns out that EOS soedin is Nature's most effective way of packaging something so the aedin is the most effective way of packaging the DNA or the RNA of these viruses now this leads me to the last type of viral structure this is known as the complex capsid complex capsids take shapes that are not symmetrical they're not perfectly symmetrical like the helico capsid or they a cedrin these complex capsids have more to them here's an example this is the famous T4 bacterio Fage this is the virus that infects eoli this is the virus that infects bacteria and it's known as a type of bacterio Fage bacterio Fage are viruses that specifically infect bacteria and this is an example of a bacterio phage for example the T4 Fage this virus obviously is not just a simple shape is it it kind of has an acoso hedral head you see this is the acoso hedral head which houses The genome usually these guys have a DNA genome in side and but then look at this you have a sheath which is like an like an like a helical a helical tail in addition you have these tail fibers which serve as Spike proteins and tail pins as well so this virus here is a transmission electron micrograph of this virus and you can notice that this virus it's not just one simple shape is it it's a comp Plex capsid it has an acosto hedro head a helical tail in addition it has tail fibers and tail pins in this case we would call this a complex capsid U because it doesn't have a particular single shape to it so again viruses tend to have three different configurations right the capsid of viruses can be helical the capsid can be aedr or the capsid can be complex and again remember some viruses are nonenveloped or naked while other viruses have an envelope that means a membrane surrounding the virus here's an example of a naked capsid on the left and an example of a enveloped virus on the right here you can see this is the genome the DNA in the center in green here is the capsid in red and in blue you have the envelope so you might be confused as to where these viruses get their envelope where do these viruses get a membrane they're not cells are they remember viruses are not cells they are small infectious agents so where do they get their envelope from where do they get that membrane from well it turns out the viral envelope is composed of membrane from the host viruses do not create their own membrane or their own envelope they steal part of the host's membrane think about that isn't that interesting so a virus an enveloped virus did not make its envelope it did not make that membrane it stole that membrane from the the host and I'm going to show you how that occurs later on in this chapter that membrane is usually part of the cell membrane or even the nuclear membrane of the host cell that membrane can also have Spike proteins inside of it remember Spike proteins which allow that virus to attach to new host cells now remember viruses have at their core always a nucleo cap capid now remember at the core of every virus is the nucleo capsid which comprises the capsid coat surrounding a nucleic acid either DNA or RNA but what I need you to know is that it's either a DNA genome or an RNA genome but not both and that genome contains viral genes which are normally much smaller number of genes than a typical cell and that makes sense viruses are much simpler than cells they don't need as many genes as a cell does in order to function viruses only need the genes necessary to invade host cells and redirect their activity those genes are there to redirect the activity of the host cell and there to you know code for new capsid proteins new Spike proteins new enzymes and the enzymes necessary to copy all the viral components now how do they categorize or classify viruses let me introduce you to the Baltimore system of viral classification Baltimore was is a noel Prize winner David Baltimore at Caltech and he derived this new method of classifying viruses based on their genomes remember I said that some viruses have a DNA genome while other Earth have a RNA genome but it's a little more complex than that let me explain check it out according to the Baltimore system class one virus are any virus with a double strand DNA genome remember that you and I share a DNA genome that's double stranded so this is the type of genome that you and I are familiar with all organisms that are alive bacteria archa ukaria we all share double strand DNA genomes so double strand DNA genomes are similar to The genomes of animals and plants and bacteria and ARA and so some viruses have a doubl strand DNA genome for example herpes simplex virus these are known as class one viruses what's interesting is class 2 viruses also have a DNA genome but these are single stranded see SS stands for single stranded DNA genomes these viruses have a DNA genome that's single stranded for example parva virus B while class three viruses have what's known as doubl stranded RNA genomes I know you're not familiar with doubl stranded RNA uh most of you are familiar with singl stranded RNA for example mRNA but double stranded RNA exists and these viruses the viruses that are part of class three virus according to the Baltimore system are the double strand RNA viruses these include Rota virus class 4 viruses are actually single strand RNA viruses that have a positive sense what does that mean what is a singl stranded RNA positive sense virus for that answer let's head over to the board now doubl stranded RNA viruses have two complementary strands of RNA as their genome however one strand is known as the positive sense Strand and the complimentary strand is known as the negative sense strand what's the big difference the big difference is this the positive sense strand can be recognized by ribosomes while the negative sense strand cannot be recognized by ribosomes why is this important well remember ribosomes synthesize proteins and so ribosomes can recognize the positive sense strand of RNA they can attach to that positive RNA and they can actually make proteins they can translate the genes on that positive sense RNA into proteins while on the negative sense strand those ribosomes cannot recognize that as information they can use to make proteins so when we talk about single strand RNA viruses if we're talking about single strand RNA viruses that have a positive sense then ribosomes can recognize these uh genomes ribosomes can always make proteins directly from positive sense RNA genomes however when it comes to negative sense RNA genomes if it's a single strand RNA that's negative sense then you know ribosomes do not recognize that as a you know something they can build proteins from so these viruses are at a you know disadvantage they need to convert their negative sense RNA genome to a positive sense before they can make their proteins and that's the big difference between the positive and the negative sense rnas of various types of viruses so now you understand the class 4 positive sense single stranded rnas like those found in SARS CO2 the virus that causes covid-19 as well as other viruses like polio virus and hopefully now you understand the class five negative sense single strand RNA viruses which cannot be used as R mRNA directly remember that class 4 positive sense single strand RNA viruses can be used as Mr RNA directly that means that ribosomes recognize this type of genome and ribosomes can make proteins by directly binding to Positive Single strand RNA genomes while negative single strand RNA genomes cannot be directly recognized by ribosomes so these types of genomes need to be converted to positive sense single strand RNA genomes in order to be recognized by ribosomes and these viruses include the viruses like influenza virus now class six these are also single strand RNA genomes however the the thing that makes them their own class the thing that makes them unique is that these are known as the retr viruses that's because because these are known as the viruses that must copy their RNA genome to DNA before anything else can occur so these are the viruses that use an enzyme known as reverse transcriptase this is the enzyme that copies the single strand RNA genome back to double strand DNA so that these viruses can proceed with their life cycle I can explain this retrovirus at the board so now let me explain retroviruses retroviruses are interesting because although their genomes are single strand RNA it's a positive sense single strand RNA which would make you think that it behaves like the other positive sense single strand RNA viruses however ever these retroviruses they're in their own class because the first thing they do is convert that RNA they convert their RNA which is a single strand positive sense RNA they convert that to double strand DNA so they actually convert their RNA genome to double strand DNA and then they carry on with the viral synthesis and assembly Etc again retroviruses are unique in that they convert their RNA genome to double strand DNA before carrying on with the rest of the viral replication cycle and the reason they're called Ro is an interesting one let me explain that do you remember the central dogma of molecular biology which states that information flows from DNA to RNA to protein well the reason retroviruses have the term retro in their name is because retro means going back and look what happens with retroviruses they convert RNA to DNA which is going back in the central dogma if you're going from RNA to DNA you're going counteractive or retro with regard to the normal flow of information according to the central dogma of molecular biology that's how retroviruses got their name isn't that neat all right welcome back and lastly we have the class seven viruses these I'm not going to go into too much detail with but these are the double strand DNA viruses that have a interesting shape one of the strands has a gap and the other strand has a Nick and these doubl strand DNA viruses they have a unique unque life cycle which includes reverse transcriptase which we're not going to get into in detail in this class now remember when I mentioned that viruses have a nucleocapsid as well as Spike proteins but they also can possess enzymes here are just a short list of different enzymes viruses may or may not possess polymerases these are enzymes that can synthesize DNA or RNA replicases these are enzymes which are a type of polymerase that copies RNA reverse transcriptase these are enzymes that synthesize DNA from RNA reverse transcripts can be found in classes 6 and seven integrace this is a scary enzyme that inserts the viral DNA into the host DNA and perias an enzyme that Cleaves viral proteins to finalize them again some viruses have these enzymes some don't it all depends on the virus but these are some common viral enzymes now I personally prefer the Baltimore system of viral classification however you should be aware that there was a previous way of categorizing viruses based on similarities and this was an informal classification system which tried to group viruses based on their order their family their genre these classifications included suffixes like verali for the order of the of the virus ver day for the family of the virus and virus for the genre for example the order herpes Valles includes the family herpes ver day and the genus simplex virus followed by the species human Alpha herpes virus 2 essentially the committee the committee on the taxonomy of viruses in their 2020 issue reported 59 orders and 189 families of viruses I much prefer the Baltimore system to be frank the Baltimore system groups viruses based on their genomes and how the Gen gome is replicated how the virus is replicated remember there are seven different classes of viruses according to the Baltimore system however the old system which is run by the committee on the tonomy of viruses tries to group viruses inside of orders families genus species and as you can imagine that leads to problems and this is because viruses are not alive so trying to categorize them with taxonomical hierarchies similar to that of animals is really not very useful and again there are DNA viruses and various RNA viruses and fun fact Corona virus or SARS cov2 which is the virus that causes covid-19 is an example of an RNA virus Corona ver day and it is in the genus of Corona virus so that's pretty interesting to understand with that we are about halfway through the chapter why not take a quick break time with Gizmo and Wicket and see what these little guys are up to and we'll be right back for the second half of the [Music] chapter [Music] welcome back from Breaktime with Gizmo and Wicket now let's talk about how viruses multiply what do viruses do once they get inside the cell this is known as the multiplication cycle of an animal virus this includes General phases at absorption penetration uncoding synthesis assembly and release what does absorption mean adsorption is the attachment of the virus to the host cell do not confuse the words absorb with absorb absorb means to soak in like a paper towel like water into a paper towel absorb means to attach viruses must absorb to host cells in order to infect them and this is done with the spike proteins Spike proteins on the surface of the virus stick to specific receptors on the surface of the host cell and this specific binding is what allows infection to occur Ur and remember some viruses have a narrow Host range some viruses have a broad Host range it all depends on the spike protein and the host receptor and again The Host range is determined by this Spike protein to host receptor binding there are some viruses with a restricted Host range for example Hepatitis B which only infects the liver cells of humans you see how that's very restrictive that's a very small and narrow tropism on the other end of the scale you have broad Host range or broad tropism viruses such as rabies rabies virus infects all kinds of cells from mammals so basically all mammals can be infected with rabies virus so this virus exhibits a more broad Host range it has a broad tropism so now we've discussed absorption this is the attachment of the virus to a specific receptor on the host cell next we're going to discuss penetration and uncoding of the virus penetration involves the entry of the virus into the host cell and this can often occur via endocytosis where the entire virus is engulfed by the cell and enclosed in a vacu or vesicle here you can see a naked virus which touches the host membrane there will be specific binding between Spike proteins on the surface of the virus and The receptors on the host membrane this will cause an invagination to occur on the host membrane this is an inward movement an inward growth of the plasma membrane and that pops off as a vesicle a little virus inside of a membrane vesicles and at this point penetration is complete but uncoding needs to occur the virus needs to uncoat it will release itself from the vesicle and the genome will uncoat viruses must uncoat which means that the nucleocapsid needs to free itself from any kind of vesicle and the nucleic acid either a DNA or an RNA molecule this need to find their way into the cell so that the viral multiplication cycle can continue here is an example of penetration of a enveloped virus this is an enveloped virus you have the nucleo capsid in the center and the envelope on the outside the enveloped virus attaches with its Spike proteins to specific receptors on the host membrane this Fusion causes the envelope to fuse with the plasma membrane of the host releasing the nucleo caps inside of the cell and subsequently the DNA or the RNA genome will free itself from the capsid coat and this is known as uncoating following uncoding the genome can direct the cell to synthesize or make parts of the virus this includes genome replic copying the Genome of the cell and protein production making the proteins of the virus the viral nucleic acid takes control over the hosts synthetic and metabolic machinery and the way this occurs varies depending on whether we're talking about a DNA virus or an RNA virus because RNA viruses replicate in the cytoplasm while DNA viruses replicate in the nucleus again let's look at the multiplication cycle step by step notice here the enveloped virus binds to specific receptors on the cell's membrane this is via the spike proteins remember look at this this is called absorption the virus attaches to its host cell by specific binding of its spikes to cell receptors this determines the tropism of the virus here you can see the spike protein and purple is binding to the specific receptor in Gray and this is called adsorption Next there's penetration the virus must make its way into the cell and in this case it looks like it's making its way into the cell via endocytosis where the plasma membrane of the host cell invaginates to form this cavity allowing the virus to enter the cell next part three is called uncoding uncoding occurs to free the viral genome whether it's an RNA genome or a DNA genome into the cytoplasm see part three uncoding the viral genome whether RNA or DNA is released into the cell remember that RNA genomes replicate in the cytoplasm while DNA genomes replicate in the nucleus that RNA genome then undergoes what's known as synthesis synthesis includes replication and protein production here during the synthesis stage more genome is synthesized so if the genome in this case look it's an it's a positive sense RNA genome the positive sense RNA genome is copied to form more positive sense RNA genome this is done with RNA polymerase and this RNA genome because its positive sense can be recognized by right ribosomes which form new Spike proteins new capsomer for the new capsid as well as other enzymes as well this is known as synthesis of new viral molecules so next assembly occurs new verion are formed from the proteins that are synthesized as well as the nucleic acid that is constructed and lastly release release where enveloped viruses Bud like this one out of the membrane carrying away an envelope with spikes so check it out viruses nucleocapsids can Bud they can cause an evagination in the plasma membrane which then pinches off to form the envelope of the new virus so when you see a varion when you see a virus with an with a envelope with an envelope that envelope is actually stolen from the previous host cell during exocytosis and this process as we're going to mention in a little bit in a couple slides this process is known as budding budding occurs where the nucleo capsid of the new varion pushes its way out of the cell with exocytosis Tois and that results in stealing a part of the host's membrane and that's where I want you to know that that's where enveloped viruses pick up their membranes they pick up their membranes from the membrane of the host cell viruses do not construct their own membranes viruses steal their membranes from host cells so anytime you see an envelop virus you should think that it stole that envelope it stole that membrane from a host cell so each cell that is infected with a virus becomes a virus Factory a factory that is now instructed to make multiple multiple copies of that virus and here you can see an image of viral particles each one of these dark spots is a viral particle a new varion and you can see how there are hundreds upon hundreds of these varion within the new cell within the host cell again now we've touched on all the parts of the multiplication cycle of animal viruses including adsorption penetration uncoding synthesis assembly and release but let me go into to a little more detail regarding release just to finish off this part of the chapter during release of mature viruses or varion naked viruses are liberated usually by licing the cell while enveloped viruses are liberated by either budding or exocytosis this is the same thing actually budding and exocytosis are uh interchangeable terms remember that enveloped viruses they do not construct their own uh envelope they do not make their own membrane they budded or exocytosed out of the host cell and that's where they picked up the membrane so again typically naked viruses are naked because they left the host cell by destroying the host cell and licing the host cell pouring out of the host cell while enveloped viruses are liberated by budding or exocytosis which results in stealing the host cell's membrane and in animal cells this viral infection can lead to damage to the host cell the damage that's caused to a host cell by viruses is known as cytopathic effects or cpes for short these cytopathic effects can damage the cell and alter its microscopic appearance the cells will look different under the microscope than healthy cells do the way they could look different is that the cells can become disoriented they could undergo major changes in shape or size or develop intracellular damage you could also see inclusion bodies or compacted masses of viruses inside of the cell or damaged cell organel in the nucleus or the cytoplasm you could even see CIA which are fused cells Fusion of multiple host cells into a single large cell viral infection can cause two cells to fuse into one big cell this this table highlights various viruses and their cytopathic effects the responses in the animal cell different viruses can cause different problems different cytopathic effects or cpes you don't need to memorize this but this is a table showing you how different viruses might culminate in different cpes and un fortunately many viral infections are persistent there is no cure for these viral infections they can persist over time this occurs when the cell Harbors the virus but is not immediately liced this can also occur when a provirus forms this means that the viral DNA actually incorporated or inserted itself into the host's DNA you can see this with HIV human immuno deficiency virus where the host's DNA is invaded by the viral DNA the viral DNA has inserted itself into the host's DNA and a chronic Laten State can form with certain viral infections for example herpes virus where periodic reactivation occurs after a period of viral inactivity now in UK carots it's important to understand understand that viruses can cause cancer and it is estimated that up to 133% of all human cancers are caused by viruses this process is known as transformation The process by which a cell becomes cancerous a virus might be able to infect a cell and cause that cell to become a cancer obviously not all viruses can cause cancer but some viruses can and certain strains of viruses can cause cancer how viruses carry genes that can directly cause cancer or viruses produce proteins that induce a loss of growth regulation in the cell viruses can cause cancer and certain viruses are known for causing cancer for example there are certain strains of human papiloma virus that are known for causing cancer cervical cancer this is done by either bringing cancerous genes to the cell these are known as enco genes or by causing problems in your own genes known as Proto anaes either way certain viruses can cause cancer there are also viruses that specifically infect bacteria these are known as bacter Fage bacterio Fage mostly contain doubl stranded DNA and every known bacterial species is parasitized by at least one specific bacterio Fage the most studied bacterio Fage are known as the te even bacterio Fage these are bacterio Fage or viruses that infect ecoli eeral coli they're known as the teeven phage because they have a even number so for example T2 Fage or T4 Fage these are the viruses that have this complex capsid structure with an aedr head a helical tail tail fibers which serve as Spike proteins as well as tail pins here again like most bacterio phage these T2 and T4 bacterio phage contain double stranded DNA genomes now let's talk about bacterio life cycles the bacterial phage life cycle can be liic or lysogenic so let me explain the difference between the liic phase of the bacterial phase life cycle versus is the lysogenic cycle of the bacterial Fage now here we can see the multiplication cycle of a bacterio phage recall that a bacterial Fage could enter the liic cycle which is demonstrated here as this cycle right here on the right and bacteriophage often times can also enter what's called lys ogyny or the lysogenic state which is demonstrated here on the left so let me explain both I'm going to start with the litic cycle with the litic cycle we start with adsorption which makes sense the virus or Varon must attach to the host cell the eoli host remember that the bacterial phage is this complex uh birion with an aoso hedra head and a helical table tail the tail fibers serve as Spike proteins to bind to the surface receptors on the ecoli host the bacterial phage then squats down and penetrates the cell and this is pretty neat um this pink uh Circle represents the bacterial DNA this is the circular Genome of the bacterium however the virus if you notice the virus is injecting its double strand DNA genome into the ecoli cell and it leaves its capsid coat it's complex capsid coat outside again this is known as penetration now we have the double strand DNA inside of the host eoli cell and duplication of Fage components ensues this means that we want to replicate the viral genetic material this means we want to copy that double strand DNA and we want to synthesize new viral components as well right we want to synthesize new capsid Heads new helical Tails New Tail fibers New Tail pin all of the proteins and the nucleic acid of the virus is synthesized followed by assembly where those components come together to form new varion lastly we have maturation these Varon are finalized and remember it packaged inside of the eoso hedral head is the double strand DNA genome finally Lis of the weakened cell the ecoli will lice open releasing the Varon naked Varon in this case because there was no uh membrane stolen from the host through exocytosis this is an example of a cell Lis resulting in naked varion being released so that they can hunt down and infect new host cells again this is known as the liic cycle of a bacterio phage but remember in addition to the liic cycle varion can enter Once Upon penetration upon penetration these bacterial Fage can enter What's called the lysogenic state which is more of a dormant or chronic state let me explain remember that the the bacterial DNA is represented by this pink circle cuz it's circularized single chromosome the viral DNA is here in blue is is drawn here in blue the viral DNA enters the host cell and during the lysogenic State the viral DNA can actually insert itself into the host's DNA see here here in pink you have the bacterial DNA and the viral DNA inserted itself into that genome and this is known as a proage when the viral DNA inserts itself into the host genome this is known as a proage and this is wonderful for the virus because now that it's inserted itself into the bacterial cell every time the bacteria divides it has to also copy the proage it has to also copy the viral genome this is a way for the viral genome to propagate without the virus having to do anything so imagine the virus just the virus just piggy backs inside of the host cell every time this eoli doubles and remember eoli can double every 20 25 minutes um the viral proage the viral G genome is also replicated so imagine this you could propagate your genome your genetic information without having to do anything and what happens is this is wonderful because bacteria can you know divide so quickly that you could end up with a million cells after just 5 hours and a million copies of the virus now what can happen though is if the bacterium is found finds itself in a bad environment let's say that the bacteria finds itself in a nutrition starved environment or in a UV environment this can actually stress the bacterial cell and viruses these bacterio phage can sense that the bacteria is stressed can you believe it and when the bacteria is stressed the virus wants to get out of there before the bacteria dies uh so what happens is the viral genome can excise itself back out uh and then the the virus can re-enter the liic cycle when times get tough that means that you can duplicate assemble mature and lice out of the cell isn't that neat so again the purpose of entering the lysogenic state is so that the vir can go into a chronic or dormant State into the bacterium and replicate uh you know by laying low however if the bacterium is stressed or might die the virus will reactivate and it will excise itself or remove itself from the host genome and re-enter the litic state so that it can exit the cell and find a new host isn't that neat and here you can see with an electron micrograph an actual image of bacterio Fage these little guys bacterio Fage becoming released from a host eoli cell this is the bacterial cell and notice how the cell membrane has liced releasing these naked bacterial phage varion bacterial Fage that are capable of both the liic cycle here and the lysogenic State here are known as temperate Fage again temperate Fage can enter the lysogenic state which was the dormant State and then they can switch to the liic phase and remember the reason for the liic phase is to exit the cell or escape the cell if the conditions become poor recall that the proage refers to the inactive state in which the Fage DNA is inserted into the host chromosome during logy and then recall that when the going gets tough when the bacterium is starved for nutrients or when it's stressed this can induce the liic cycle this is known as induction or activation of a propage in a lysogenic cell to progress directly into viral replication and then liic cycle to release from the cell and lastly to finish off this chapter let's discuss other non-cellular infectious agents besides viruses there are other infectious agents for instance prons prons are infectious agents comprised entirely of protein protein and these these pron proteins can cause chaos in the nervous system of animals prons are actually normal brain proteins but these proteins can become unfolded to cause disease remember that proteins need to fold correctly in order to function well preon protein if they become unfolded or misfolded they become deadly this is because once a Pon becomes misfolded it is thought to trigger misfolding of other pron proteins leading to disease and the disease is called spongy form disease or spongy form encel opathy this means that the brain becomes damag the brain becomes so damaged that it forms spongy form which means it has literally holes in it that's the this extent of brain damage culminates in a spongy like brain and this spongy form encel opathy is a very traumatic and deadly brain disease that forms when these pron proteins misfold in the brain in humans there are preon disorders called crutz field yakob disease there's also variant crutz field yakob disease again there's no cure for this disease it causes spongy form enop ofy gradual degeneration and death although it's transmissible by an unknown mechanism it's been heavily linked to consumption of brain matter there are also several an animal examples of similar diseases these pron diseases Scrapy in sheep mink and elk and mad cow or Bine spongiform enop ofy in cows lastly let's talk about another form of non-cellular infectious agent these are known as viroids these are comprised entirely of RNA composed only of strands of RNA they lack any protein capsid or any other type of coating and these are significant pathogens in Plants they cause several plant disorders and the way they work is by a process known as RNA interference or RNA I certain plant genes are shut off and this can cause devastating consequences for the plant land and with that this leads us to the end of chapter 7 I hope it was informative thank you for joining me let me know in the comment box below if you have any questions and I'll catch you for the next chapter Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D A Dr D Dr D Dr D Dr D Dr D Dr D