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Replication of Viruses Viruses are specific to a particular host cell because part of the capsid bind to the receptor on the host cell’s plasma membrane in a lock and key fashion. The virus then will inject its viral nucleic acid into the host cell and it will take over the metabolic reaction of the cell so new viruses will be made. Replication of Bacteriophages Bacteriophages are viruses that are parasitic to bacteria. These phages must use bacteria to replicate. Phages have two cycles. The lytic cycle is when the host bacterial cell lysis and breaks open to release the newly formed viruses. The lysogenic cycle is when there is viral replication, but will be complete some time in then future. Lytic Cycle There are five key stages to the lytic cycle. 1) Attachment Stage – Capsid combines with the bacteria’s receptor in a lock and key fashion. 2) Penetration Stage - Enzymes from the virus start to eat away the bacterial cell wall and will inject its’ viral DNA in the bacterial cell. 3) Biosynthesis Stage – at this stage the virus inactivates the bacterial genes and takes over the metabolic functions to produce copies of DNA and produce the capsid. 4) Maturation Stage – During this stage, the viral DNA and capsids are put together. 5) Release Stage – Lysosomes are made and produce hydrolytic enzymes to eat a hole in the cell wall of the bacteria. At this time, the cell will lyse and the newly formed viruses will leave the host cell. Lysogenic Cycle After the attachment the penetration stage, the viral DNA attaches to the bacterial DNA, but does not replicate and destroy the bacterial DNA. During the lysogenic cycle, the phage becomes a prophage that is integrated into the host genome (DNA). At a later time, the phage may start the lytic cycle. When a virus is latent, not replicating, it is called a prophage. At any time, factors such as x-rays or ultraviolet light may initiate the prophage to start the lytic cycle. Replication of Animals Cells Animal cells are quite different when replicating. In fact, the entire animal virus enters the bacterial cell. Once inside, the animal virus loses its capsid and either the viral DNA or RNA and biosynthesis occurs. Another difference that occurs in animal’s cells is that when animal viruses leave the cell, it leaves by a process called budding. This means that a portion of the animal cells membrane is wrapped around the virus. Retroviruses Retroviruses are viruses that have the nucleic acid RNA. This virus contains the enzyme called reverse transcriptase. This enzyme allows the RNA to do DNA transcription. The resulting DNA strand is a copy of the viral genome. New viruses are produced when the DNA is transcribed and is released from the cell by budding. An example of a retrovirus is HIV and some cancers.