Classification & Replication.pptx
- 1. Classification & Replication of Virus
- 2. DNA Viruses • Many viruses contain either single-stranded (ss) or double-stranded (ds) DNA genomes. • The genomes are replicated by direct DNA-to-DNA copying using DNA polymerase, which requires most DNA viruses to replicate in the host cell’s nucleus. • One exception is the poxviruses that replicate in the host cytoplasm, which means these viruses must carry the gene for their own DNA polymerase.
- 3. RNA Viruses • A large number of viruses contain either ssRNA or dsRNA genomes, which are replicated by direct RNA-to-RNA copying. • Some of the single-stranded viruses, such as the picornaviruses and coronaviruses, have their RNA genome in the form of messenger RNA (mRNA). These RNA viruses are referred to as positive- strand viruses.
- 4. • Other ssRNA viruses, such as the orthomyxoviruses and paramyxoviruses, have RNA consists of complimentary RNA strands and mRNA • These genomes are referred to as negative-strand viruses.
- 5. Retrovirus • These are usually grouped with the RNA viruses, the retroviruses are replicated indirectly through a DNA intermediate (RNA-to- DNA-to-RNA). • Each virion contains two copies of RNA. • During the infection process, a DNA intermediate will be formed using a reverse transcriptase enzyme carried within the virion.
- 7. • The process of viral replication is one of the most remarkable events in nature. • A virus invades a living host cell a thousand or more times its size, hijacks the metabolism of the cell to produce copies of itself, and often destroys the host cell when new virions are released. • Replication has been studied in a wide range of viruses and their host cells.
- 8. The Replication of Bacteriophages Is a Five-Step Process • One of the best studied processes of replication is carried out by bacteriophages of the T-even group (T for “type”). Bacteriophages T2, T4, and T6 are in this group. • They are large, complex, naked DNA virions with the characteristic head and tail of bacteriophages.
- 9. • It is important to note that the nucleic acid in a phage contains only a few of the many genes needed for viral synthesis and replication. • It contains, for example, genes for synthesizing viral structural components, such as capsid proteins, and for a few enzymes used in the synthesis; but it lacks the genes for many other key enzymes, such as those used during nucleic acid synthesis. • Therefore, its dependence on the host cell is substantial.
- 10. Phage replication in E. coli as a model for the Lytic cycle. 1. Attachment • The first step in the replication cycle of a virulent phage occurs when phage and bacterial cells collide randomly. • If sites on the phage’s tail fibers match with a complementary receptor site on the cell wall of the bacterium, attachment will occur that consists of a weak chemical union between phage and receptor site. • In some cases, the bacterial flagellum contains the receptor site.
- 11. 2. Penetration • Following attachment, the tail of the phage releases lysozyme, an enzyme that dissolves a portion of the bacterial cell wall. • The tail sheath then contracts and the tail core drives through the cell wall. • As the tip of the core reaches the cell membrane below, the DNA is ejected through the hollow tail core and on through the cell membrane into the bacterial cytoplasm. • The ejection process takes less than two seconds and the capsid remains outside.
- 12. 3. Biosynthesis • Having entered the cytoplasm, production of new phage genomes and capsid parts begins. • As phage genes code for the disruption of the host chromosome, the phage DNA uses bacterial nucleotides and enzymes to synthesize multiple copies of its genome. • Messenger RNA molecules transcribed from phage DNA appear in the cytoplasm, and the biosynthesis of phage enzymes and capsid proteins begins. • Bacterial ribosomes, amino acids, and enzymes are all enlisted for biosynthesis. • Because viral capsids are repeating units of capsomeres, a relatively simple genetic code can be used over and over.
- 13. 4. Maturation. • Once the phage parts are made, they are assembled into a complete virus. • The enzymes encoded by viral genes guide the assembly in a step-by-step fashion. • In one area of the host cytoplasm, phage heads and tails are assembled from protein subunits; in another area, the heads are packaged with DNA; and in a third area, the tails are attached to the heads.
- 14. 5. Release. • Mature phage particles now burst out from the ruptured bacterial shell. • For some phages, lysozyme, encoded by the bacteriophage genes late in the replicative cycle, degrades the bacterial cell wall. • The mature bacteriophages are set free to infect more bacterial cells.
- 15. • The time of attachment till release is known as Burst time. • For Bacteriophages, the burst time aeverages between 20-40 min. • At the end of cycle, 50-200 new phages emerge from host cells. This is commonly known as Burst size.
- 16. • Other phages interact with bacterial cells in a slightly different way, called a lysogenic cycle. For example, lambda phage also infects E. coli but may not immediately cause cell lysis. Instead, the phage DNA integrates into the bacterial chromosome as a prophage. • Bacteriophages participating in this cycle are known as temperate phages. • The bacterial cell survives the infection and continues to grow and divide normally. • As the bacterial cell undergoes DNA replication and binary fission, the prophage is copied and vertically transferred to daughter cells. • Thus, as cells divide, each daughter cell is “infected”; that is, it contains the viral genome as a prophage.
- 17. • Such binary fissions can continue for an undefined period of time. Usually at some point, the bacterial cells become stressed (e.g., lack of nutrients, presence of noxious chemicals). • This triggers the prophage to excise itself from the bacterial chromosomes and switch to a lytic cycle, lysing the bacterial cells as new phages are released.