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VIRUSES § Viral replication results in genetic variation, and viral infection can introduce genetic variation into the hosts. How does viral replication differs from other reproductive strategies? ? How does viral replication generate genetic variation?

§ The genome of viruses can be ss. DNA, ds. DNA, ss. RNA, or ds. RNA § Viruses are called DNA viruses or RNA viruses depending on their nucleic acid.

§ Made of a protein shell (capsid) enclosing the nucleic acid. Capsids are built from a large number of protein subunits and exist in a variety of shapes. § Some viruses, especially those which infect animals, have a membrane (called an envelope) surrounding the capsid. The envelope is derived from the host cell membrane and contains both viral and host proteins and glycoproteins.

§ Viruses lack organelles. § They are intracellular parasite - able to reproduce only inside a host cell. § Infection begins when the viral genome enters the host cell and begins to direct the host cell machinery to make new capsid proteins and copies of the viral genome. § To produce new virus particles, the virus gains all the necessary enzymes, nucleotides, amino acids, energy, etc. from the host. § After the capsid proteins and nucleic acid are synthesized, they usually spontaneously assemble into new virus particles. § Hundreds or thousands of new viruses exit the cell, often destroying the cell in the process. § The new virus particles are then free to infect more cells.

This life cycle ends with the death of the host cell. Viruses with this type of life cycle are called virulent viruses. 1. The virus attached to the cell and injects it’s nucleic acid. 2. The viral genes take over the cell, directing it to make viral proteins and nucleic acid which then selfassemble into new viruses. 3. One viral gene codes for an enzyme which digests the cell wall. 4. Without a cell wall, the bacterial cell lyses (hence, lytic cycle) as a result of the osmotic uptake of water. 5. The lysed cell then releases up to 200 phages to infect nearby cells.

§ In this cycle, the viral genome is copied without killing the cell. § Viruses with this type of life cycle are called temperate viruses. § The bacteriophage lambda provides a good example. This virus can follow either a lytic or lysogenic cycle.

1. 2. 3. 4. 5. 6. The phage attaches to the surface of the host cell (E. coli) and injects it’s DNA. During a lytic cycle the phage follows the pattern outlined earlier. During a lysogenic cycle, the viral genome is incorporated into the host cell genome and is called a prophage. One gene codes for a protein that suppresses most of the other prophage genes. Each time the bacterium divides, the phage DNA is replicated and passed on to the daughter cells. This way, the virus increases in numbers without destroying the host cells. (diabolical) A chemical or environmental trigger of some kind causes the prophage to exit the lysogenic phase and begin a lytic infection.

§ Viruses with envelopes 1. Once the virus binds with the host cell, the envelop fuses with the host cell plasma membrane, transporting the capsid and nucleic acid into the cell. 2. After cellular enzymes remove the capsid, the viral genome takes over the cell.

§ Viruses with envelopes 3. The ER makes proteins for the new envelopes and clusters them in patches on the plasma membrane. 4. New viruses bud (like exocytosis) from the areas of these clusters. Thus, the envelop is derived from the host cell membrane although some of the proteins contained in it are of viral origin. Note that this cycle does not necessarily kill the host cell.

§ RNA viruses known as retroviruses contain an enzyme cal led reverse transcriptase which can transcribe DNA from an RNA template. § The host RNA polymerase then transcribes the DNA into RNA which function as both m. RNA for the synthesis of viral proteins as well as for new viruses.

§EQ: How does viral replication generate genetic variation?

1. Virus replication allows for mutations to occur through usual host pathways. 2. RNA viruses lack replication error-checking mechanisms, and thus have higher rates of mutation. 3. Related viruses can combine/recombine information if they infect the same host cell. 4. Documented evidence: HIV is a well-studied system where the rapid evolution of a virus within the host contributes to the pathogenicity of viral infection.

§ 1. Viruses transmit DNA or RNA when they infect a host cell. § • Transduction in bacteria § • Transposons present in incoming DNA § 2. Some viruses are able to integrate into the host DNA and establish a latent (lysogenic) infection. These latent viral genomes can result in new properties for the host such as increased pathogenicity in bacteria.