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A) Characteristics of Viruses

• Chemical composition

All viruses contain nucleic acid, either DNA or RNA (but not both), and a protein coat, which encases the nucleic acid. And also someviruses are enclosed by a huge envelope of protein and fat.

• Replication

Steps of Viral Replication
1. Adsorption
The virus becomes attached to the cells, and at this stage, it can be recovered inthe infectious form without cell lysis by procedures that either destroy the receptors or weaken their bonds to the virions.
2. Penetration
It is the step that follows adsorption, and the viruscan no longer be recovered from the intact cell. The most common mechanism is receptor mediated endocytosis, which is the process by which many toxins and hormones enter cells.
3. Uncoating
It is animportant step, because is the acidification of the content of the endosome to a pH of about 5, owing to the activity of a proton pump present in the membrane. The low pH causes rearrangement of coatcomponents. They bind to the lipid bilayer of the membrane, causing the extrusion of the viral core into the cytosol.
4. Viral Nucleic Acid Replication
Viruses, either DNA or RNA, shut offcellular protein synthesis and disaggregate cellular polyribosomes, leading to a shift to viral synthesis

5. Maturation
6. Release

• Classification

Generally, viruses are classified by theorganism they infect, like animals, bacteria or plants. But viruses also are further classified into families and other groups based on three structural considerations:
• The type and size of theirnucleic acid
• The size and shape of the capsid
• Whether they have a lipid envelope

There are six types of families groups:

1) Double strand DNA viruses
2) Single strand DNA viruses
3) (+/-)Double-stranded RNA
4) Plus strand RNA viruses
5) Minus strand RNA viruses
6) Retroviruses

There are two kinds of shapes found amongst viruses: rods or filaments, and spheres. The rod shape is...
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