viruses are a unique group of pathogens with a simple a cellular organization and a distinct pattern of multiplication despite their simple structure they are a major cause of disease they have no cytoplasmic membrane cytosol or functional organelles but they can infect all types of cells and numerous viruses can also infect bacteria which are called bacteria phages viruses and bacteria phages are not capable of metabolic activity on their own so instead they invade other cells and use their metabolic machinery to produce more viral molecules nucleic acid and proteins which then assemble into new viruses viruses can exist either extracellular Lee or intracellularly in the extracellular state the virus is called a very on and isn't capable of reproducing a very on consists of a protein coat called a capsid surrounding a nucleic acid core which contains the genetic material or the viral genome the nucleic acid and the capsid are collectively called a nucleic acid some variants have a phospholipid membrane derived from the host cell called an envelope which surrounds the nucleo capsid the viruses that have an envelope are called enveloped viruses and these include the herpes viruses and HIV while the ones that lack the envelope such as poliovirus are called non enveloped or naked viruses once inside the cell the virus enters the intracellular state where the capsid is removed and the virus becomes active in this state the virus exists solely as nucleic acids that induce the host to synthesize viral components from which baryons are assembled and eventually released now the viruses are surrounded by an outer protein coating called the capsid which protects the viral genome and aids in its transfer between most cells also according to their capsid symmetry the viruses can come in many shapes and sizes there are three types of shapes helical icosahedral and complex first the helical viruses have a capsid with the central cavity or a hollow tube which is made by proteins arranged in a circular fashion creating a disc like shape the disc shapes are attached helical e creating a tube with room for the nucleic acid in the middle an example of a virus with helical symmetry is the tobacco mosaic virus which is the most studied example moving on to the icosahedral viruses which are made up of equilateral triangles fuse together in a spherical shape that fully encloses the genetic material these viruses are released into the environment when the cell dies breaks down and lysis thus releasing the variants some examples of icosahedral viruses include poliovirus Rhino virus and adenovirus finally the complex viruses have a combination of icosahedral and helical shape and may have a complex outer wall or head tail morphology the head tail morphology is unique to viruses that infect bacteria called bacteria phages and they have an icosahedral shaped head and a helical shaped tail these viruses use their tail to attach to the bacterium and then they create a hole in the cell wall and insert their DNA into the cell using the tail as a channel now the nucleic acid core of the virus contains the genetic material which can either be DNA or RNA and the total genetic content of the virus is called the viral genome the viral genomes are usually small and contain only a few genes that encode for proteins needed by the virus that's not present in the host cell the genetic material can be single-stranded or double stranded linear or circular in DNA viruses the viral DNA enters the nucleus and uses the hosts DNA polymerase to create more copies of itself as well as viral mRNA that gets translated into viral protein with RNA viruses many of them stay in the cytoplasm where the viral RNA is used by the hosts ribosomes to create viral RNA polymerase proteins these will help replicate the viral genome so more viruses can be created RNA polymerases are more likely to make copying errors than DNA polymerases and therefore often make mistakes during transcription this is why mutations and RNA viruses occur more frequently than in DNA viruses as a quick recap viruses are a unique group of infectious agents with a simple a cellular organization and a distinct pattern of replication they can exist either extracellular Lea where the virus is inactive and isn't capable of reproducing or intracellularly where the capsid is removed and the virus becomes active a virus in the extracellular state is called a Varian and consists of a protein coat called a capsid which protects the viral genome and aids in its transfer between host cells the capsid is surrounding a nucleic acid core which contains the genetic material or the viral genome which can be either DNA or RNA some variants have a phospholipid membrane derived from the host cell called an envelope which surrounds the nucleo capsid according to their capsid symmetry the viruses can come in many shapes and sizes and they can have three types of shapes helical icosahedral and complex