Are Viruses Living? LivingNonliving

Viral Reproduction Bacteriophages reproduce via 2 cycles: lytic cycle and the lysogenic cycles

Lytic Cycle - reproductive cycle that results in death of the cell - viruses that undergo the lytic cycle are referred to as virulent

Lytic Cycle (1) Bacteriophage attaches itself to a host cell. (2) This virus releases an enzyme that creates a hole in the host cell wall. (3) The tail of the virus then contracts and forces its DNA into the host cell. (4) The phage DNA causes the host cell then to produce viral DNA and proteins. After a while the host cell is full of phages, it then bursts and releases these new phages to infect other cells.

Draw the Lytic Cycle Here

Lytic Cycle - as many as 300 phages can be made in 1 host cell - this process can take minutes if conditions are right

Lysogenic Cycle - in this cycle the viral nucleic acid is reproduced without destroying the host cell (1) Phage DNA is injected into bacteria host cell. (2) Phage DNA becomes incorporated into the bacterial chromosome (plasmid) creating a PROPHAGE. (3) When the bacterial cell undergoes cell division the viral DNA is copied as well, passing it on to future bacterial generations. (4) Sometimes the viral DNA exits the bacterial chromosome – this starts the lytic cycle.

Lysogenic Cycle - usually external factors such has chemicals or radiation cause the lytic cycle to begin

Lytic Lysogenic Viral DNA destroys Cell DNA, takes over cell functions and destroys the cell. The Virus replicates and produces new viruses. There are symptoms of viral infection. Virulent viral infection takes place. Viral DNA merges with Cell DNA and does not destroy the cell. The Virus does not produce new viruses. There are no symptoms of viral infection. Viral DNA is replicated every time the cell divides. Viral DNA is permanently in the host DNA.

Immune Responses

- viruses are specific = “VIRAL SPECIFICITY” - some are pathogenic (pathogens can be viruses, bacteria, fungi, protozoans, parasitic worms...) - vertebrates have three lines of defense 1) Body Surfaces 2) Nonspecific Responses 3) Immune Responses (Specific Responses)

Body Surfaces ● Skin ● low moisture, low pH, thick layers of dead cells ● natural fauna of bacteria out compete most harmful bacteria ● Mucous Lining ● Digestive tract and vagina (helpful bacteria) ● Infection-fighting Chemicals ● tears, saliva, gastric fluid Flushing effect of tears, saliva, urination, and diarrhea

Nonspecific Responses ● Inflammation (redness, warmth, swelling, pain) ● Macrophages engulf and destroy pathogens ● Complement Proteins (chain reaction)

Specific Responses ● Self/Non-self Recognition ● Non-self invaders have proteins on their surface different from self cells called antigens ● B-cells (lymphocytes) – make antibodies specific to pathogens ● T-cells – directly kill virus-infected cells, tumor cells, transplanted cells ● Memory – a second exposure to the same pathogen will result in a faster response