how viruses reproduce

where do viruses reproduce

• Viruses can only reproduce by infecting living cells and using the protein-building machinery of their host cells to produce new viral particles

how viruses attack host cells

• bacteriphages inject their genome into the host bacterial cell, but the main part of the viral material remains outside the bacterium the viral DNA forms a plasmid within the bacterium

• viruses that infect animals enter the cells in several ways. Soome types enter cell by endocytosis and the host cell then digests the capsid, releasing the viral genetic material

• most commonly, the viral envelope combines with the host cell surface, releasing the rest of the virus inside the cell membrane

• plant viruses usually enter the plant cell using a vector to penetrate the cellulose cell wall

what are the two different routes of infection?

• the lysogenic pathway

• the lytic pathway

what happens in lysogenic pathway

• Many DNA viruses do not cause disease on first entering the cell. Viral DNA known as a provirus is inserted into the host DNA, but a viral gene coding for a repressor protein prevents the viral DNA from being transcribed and translated

• Every time the host DNA copies itself, the inserted viral DNA will also be copied

• This is called latency and the time during which it occurs is known as a period of lysogeny

• However under certain conditions (e.g. when the host cell is damaged) viruses in this state are activated. At this stage the amount of repressor protein drops, and viruses enter the lytic pathway, becoming virulent.

what is lytic pathway

• Occasionally, viral genetic material is replicated independently of the host DNA, as soon as it enters the host.

• Genetic material transcribed and translated to give viral proteins.

• Proteins assembled into capsid and assembles into mature viruses that accumulate inside the host cell

• eventually the host cell bursts, releasing large numbers of new virus particles to invade other cells.Cell bursting is known as cell lysis.

• the virus is now disease-causing and the process of replicatin and killing cells is known as the lytic pathway

what is retrovirus

a type of RNA virus that controls the production of DNA corresponding to the viral RNA and inserts it into the host cell DNA. For example HIV virus

life cycle of retrovirus

• retrovirus have RNA which cant be used as mRNA but is translaed into DNA by the specific enzyme reverse transcriptase in the cytoplasm of the host cell

• this viral DNA enters the nucleus of the host cell where it is inserted into the host DNA

• host transcriptase enzymes then make viral mRNA and new viral genome RNA and protein coat

• new viral material is syntehsised and the new viral particles leave the cell by exocytosis

• the host cell continues to function as a virus- making factory, and at the same time the new viruses continue to infect other cells

how viruses cause the symptoms of disease

• by causing the host cells to release their own lysosomes and digest themselves from the inside or by the production of toxins that inhibit cell metabolism

why virus infections specific to particular tissues

due to presence or absence of cell markers on the surface of host cells. Viral attachment proteins that are complimentary to receptors on the cell suface membrane of T helper cell

each type of cell has its own recognition markers and differnet types of virus can only bind to particular marjers

the prescence or absence of these markers can affect whether a group of livign organisms is vulnerale to attack by viruses