Viruses
 
Viral Anatomy:
 
Viron: transmissible state of a virus. These structures contain a nucleic acid core wrapped in a protein coat. This coat is called a capsid. Some virons contain an envelope, constructed from the host cell membrane. The nucleic acid core may be DNA ( double strand, single strand), or RNA (single strand, double strand), but never both. The shape of these virons vary from spherical, cylindrical, bullet-shaped , to amorphous shaped particles. They vary in diameter from 18 to 300 nm. Virons must infect a living cell. Viruses are obligate intracellular parasites. Some virons are hardier than others (hepatitis virus can withstand short periods of boiling; most virons are destroyed by this).
 
 
Viral Replication: most viruses carry the enzyme RNA replicase to make complementary RNA strands. Some viruses contain the enzyme reverse transcriptase, which produces DNA , then RNA. These are called retro viruses. HIV is an eample of a retro virus. The three basic patterns of genome replication are: DNA--->DNA, RNA---->RNA, and RNA------->DNA---->RNA. Viral components are held together by weak bond ( van derWaals forces and hydrogen bonds).
Host Specificity: Each type of virus can infect and parasitize a limited range of host cells. They are guided by external recognition areas of the host cell. This may keep a virus from infecting many different types of cells.
 
Lytic Cycle: Life cycle of virulent phages.
 
1. The phage lands on the host cell, and using 140 ATP's in its tail fibers, injects its nucleic acid core into the bacterium.
 
2. The host's DNA is cut up and destroyed by the enzymes produced by the host from the new DNA.
 
3. The now unassembled nucleotides are reassembles into phage DNA, which produces copies of the capsid.
 
4. The genes also produce the enzyme lysozyme, that digests the bacterial cell wall, causing the newly assembled viruses to escape.
 
 
 
Lysogenic Cycle: Viruses that reproduce without killing their hosts are called temperate viruses. These viruses have 2 alternate forms of reproduction. The lytic cycle or the lysogenic cycle. The process begins very similar to the lytic infection or inserting the genome into the host's genome and becoming dormant. During the lysogenic cycle one of the temperate phage's genes remains active. It codes for a repressor protein keeping the prophage genes inactive. From here on the cells reproduce regularly. Each carrying the hidden genes of the virus. Under normal circumstances harmless bacteria remain so until infected by these viruses; then they can become pathogenic. Example Botulism, diphtheria, and scarlet fever bacteria.
 
Diseases caused by viruses: small pox, chicken pox, cold sores, herpes, Rubella, yellow fever, AIDS, rabies, mumps, influenza, measles,and some forms of cancer.
 
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