1. Structure of a virus.
2. Lyctic and lysogenic cycles.
3. Viroids and Prions.
1.Viruses are not plants, animals, or bacteria, but they are the quintessential parasites of the living kingdoms. Without a host cell, viruses cannot carry out their life-sustaining functions or reproduce. They cannot synthesize proteins, because they lack ribosomes and must use the ribosomes of their host cells to translate viral messenger RNA into viral proteins. Viruses cannot generate or store energy in the form of adenosine triphosphate (ATP), but have to derive their energy, and all other metabolic functions, from the host cell. They also parasitize the cell for basic building materials, such as amino acids, nucleotides, and lipids. All viruses contain nucleic acid, either DNA or RNA (but not both), and a protein coat, which encases the nucleic acid. Some viruses are also enclosed by an envelope of fat and protein molecules. Capsid - The capsid is the protein shell that encloses the nucleic acid; with its enclosed nucleic acid, it is called the nucleocapsid. This shell is composed of protein organized in subunits known as capsomers. Envelope - Many types of virus have a glycoprotein envelope surrounding the nucleocapsid. The envelope is composed of two lipid layers interspersed with protein molecules (lipoprotein bilayer) and may contain material from the membrane of a host cell as well as that of viral origin. Nucleic Acid - Just as in cells, the nucleic acid of each virus encodes the genetic information for the synthesis of all proteins. While the double-stranded DNA is responsible for this in prokaryotic and eukaryotic cells, only a few groups of viruses use DNA. Most viruses maintain all their genetic information with the single-stranded RNA.
2. The lyctic cycle ends in the death of the host cell by rupturing it. In thic cycle, a bacteriophage injects its DNA into a host cell and takes over the host cell's machinery to synthesize new copies of the viral DNA as well as protein coats. In the lysogenic cycle the bacteriophage's DNA becomes incorporated into the host cell's DNA and is replicated along the host cell's genome. The viral DNA is called prohage.
3. Viroids, smaller and simpler than even viruses, consist of tiny molecules of naked circular RNA that infect plants. Their several hundred nucleotides do not encode for proteins but can be replicated by the host’s cellular enzymes.These small RNA molecules can disrupt plant metabolism and stunt plant growth, perhaps by causing errors in the regulatory systems that control plant growth. Prions are infectious proteins that spread disease.They appear to cause several degenerative brain diseases including scrapie in sheep, “mad cow disease,” and Creutzfeldt-Jakob disease in humans.Prions are likely transmitted in food.
video on the cycles: http://www.youtube.com/watch?v=wLoslN6d3Ec
5 facts on this chapter:
. Retroviruses are RNA viruses that use the enzyme reverse transciptase to transcribe dna from an rna template. the new dna then permanently integrates into a chromosome in the nucleus of an animal cell.
. Viruses have a limited host range. This means they can infect only a very limited variety of hosts.
. A phage that reproduces only by a lyctic cycle is a virulent phage.
. Phages capable of using both modes of reproducing within a bacterium are called temperate phages.
. Viruses enter plant cells through damaged cell walls or are inherited from a parent.
In this chapter, we learned the components of a virus and the differences between lyctic and lysogenic cycles.
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