Chapter ninteen: viruses

Q1: is viruse alive?

A1: no, because viruses have to inject to a host cell to live and work.
Q2: what's the basic structure of viruses?
A2: viruse contain geetic materials DNA and RNA, and enclosed by protein.
Q3: who was the first person come up with the idea of viruses?
A3: Wendell Stanley.


Five Facts:
1. a virus consists of a nucleic acid surrounded by a protein coat.
2. viruses reproduce only in host cells
3. viruses, viroids, and prions are formidable pathogens in animals and plants
4. lytic cycle and the lysogenic cycle are two alternative way of reproduction
5. researches discovered viruses in the late 1800s by studying a plant disease, tobacco mosaic disease


Figure:
bacteriophage T4, like other "T-even: phages, has a complex capsid consisting of an icosahedral head and a tail apparatus.


Summary:
Many viruses' genomes consist of double or single-stranded DNA or RNA, depending one the kind of virus. The protein shell enclosing the viral genome is called a capsid. Capsids are built from a large number of protein subunits called capsomeres. Some viruses have accessory structure that help them infect their hosts. Viral envelope, which are derived form the membane of the host cell, contain host cell phopholipids an membrane protein.Many of the most complex capsods are found among the viruses that infect bacteria, called bacteriophage, or phage. Each type of virus can infect cells of only a limited variety of hosts, called the host range of the virus. This host specificity results from the evolution of recognition systems by the virus. Virus identify host cells by a "lock and key" fit.

A phage reproductive cycle that culminate in death of the host cell id known as a lytic cycle. A phage that reproduces only by a lytic cycle is a virulent phage. When phage DNA successfully enters a bacterium, the DNA often identifitied as foreign and cut up by cellular enzymes called restriction enzymes. The lysogenic cycle allows replication of the phage genome without destroying the host. Phage capable of using both modes of reproduing within a bacterium are called temperate phage. The RNA animal viruses with the most complicated reproductive cycles are the retroviruses. These viruses are equipped with an enzyme called reverse transcriptase, which transcrbes an RNA template into DNA, providing an RNA-DNA information flow. The integrated viral DNA, called a provirus, never leaves the host;s genime, remaining a permanent resident of the cell.

A vaccine is a harmless variant or derinative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen. Virods are circular RNA molecules tha infect plants. Prions appear ro cause a number of degenerative brain diseases in various animal species.



Video:
http://www.youtube.com/watch?v=41aqxcxsX2w