BI111 Lecture Notes - Lecture 92: Mitochondrion, Commensalism, Pediococcus
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Biology review – bacteria, viruses etc.
Viruses: not cells and do not grow. Have only one characteristics of life: reproduction. A
virus can only reproduce inside a host cells, the process of reproduction is the lytic cycle.
- 2 virus categories
1. DNA- are stable and do not mutate rapidly. Are single or double stranded.
2. RNA- unstable and mutate rapidly. Single or double stranded
Capsid: surrounds the virus, made of proteins. Proteins located on the capsid make the virus
specific for receptor sites on the cell.
Host Range: the limited number of host species, tissues or cells that a virus or other parasites
can infect
Viral replication→
They cannot multiply on their own; they require a host cell, using the cells machinery
to copy its nucleic acid and duplicate
Lytic cycle→
1. Attachment- proteins on surface of virus act as keys that fit into matching shapes on
surface of host cell
2. Entry-virus injects nucleic acid into the cell, leaving the empty capsid outside the cell
3. Replication- is copied and codes for making pieces of the virus by host cell
4. Assembly- the pieces are put together to make a virus
5. Lysis and Release- host cell breaks open, dies and releases new viruses
Lysogenic Cycle →
• Proteins on the surface of the virus bind with proteins on the surface of the host cell
• Virus attaches itself and injects DNA into cell
• Viral DNA attaches to host DNA, becoming a new set of genes called prophage
• When host cell divides the new gene is replicated and passed to new cells
2 possibilities…
1. Prophage is permanent part of host cell DNA
2. External stimuli causes prophage to become active (lytic cycle begins)
VIRUS UNDERGOING LYTIC CYCLE= A VIRULENT VIRUS
VIRUS UNDERGOING LYSOGENIC CYCLE= TEMPERATE VIRUS
Defense against viruses→
• Easy to destroy outside living organisms
• They are weak
• Viruses can mutate and develop resistance really quickly
Our natural defense→
• White blood cells- engulf viruses in the blood and digest them
• Interferon’s- proteins produced by cells when exposed to a virus. Bind to membranes
of neighboring cells and stop the ability of a virus to attach and enter the cell
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Document Summary
A virus can only reproduce inside a host cells, the process of reproduction is the lytic cycle. 2 virus categories: dna- are stable and do not mutate rapidly. Are single or double stranded: rna- unstable and mutate rapidly. Proteins located on the capsid make the virus specific for receptor sites on the cell. Host range: the limited number of host species, tissues or cells that a virus or other parasites can infect. They cannot multiply on their own; they require a host cell, using the cells machinery to copy its nucleic acid and duplicate. 2 possibilities : prophage is permanent part of host cell dna, external stimuli causes prophage to become active (lytic cycle begins) Defense against viruses : easy to destroy outside living organisms, they are weak, viruses can mutate and develop resistance really quickly. Our natural defense : white blood cells- engulf viruses in the blood and digest them.