The virus is a tiny infectious agent composed of a protein coat known as a capsid that envelopes the genetic material. Some viruses also have an envelope that encases the capsid. Viruses do not have protoplasm.
Viruses are so little that a microscope is required to see them, and their structure is quite basic. When a virus particle is self-contained, it consists of a viral genome, or genetic material, encased in a protein shell known as a capsid. There are three types of viruses: DNA viruses, which include both human and animal viruses; RNA viruses, which include plant viruses; and retroviruses, which include HIV.
A virus must make copies of itself for this infection to be successful. This means the virus must find a new cell to infect with its DNA or RNA. A virus can do this by either attaching to a cell's surface or using its special proteins to penetrate the cell membrane and enter the cell. Once inside the cell, the viral DNA or RNA directs the production of more viruses. These new viruses then go on to find other cells to infect!
Since viruses are made of molecules of carbon, hydrogen, oxygen, nitrogen, and some other elements, they are actually part of a class of organism called organisms. Organisms are divided into two broad groups: prokaryotes and eukaryotes. Prokaryotes are organisms without true nuclei (their cells contain membranes instead). Eukaryotes have true nuclei and are therefore more complex than prokaryotes.
Viruses are made out of nucleic acids, either RNA or DNA, but never both. A protein coat (capside) or shell covers and protects the nucleic acid core in all viruses. A lipid envelope or membrane surrounds the nucleocapsid core of certain viruses. Viral proteins may also be found associated with the nucleocapsid core or enclosed within membranes. They can be classified into several groups based on their chemical composition and function.
RNA virus cores contain ribonucleoproteins that consist of an RNA molecule attached to a nucleus-specific protein called RNP complexing with other viral proteins. The ribonucleoprotein core is surrounded by a lipoprotein layer derived from the virus membrane. Virus-associated proteins are required for replication and transcription of the genome and assembly into virions. Some remain bound to the genomic material after infection is established while others are removed during maturation of the virus particle. Proteins are also released from infected cells in large quantities during lysis to provide nutrients for viral replication. These proteins act as enzymes, providing new surface receptors for infection of additional cells. Some become incorporated into new virions and some are degraded by proteases present in infected cells.
DNA viruses contain protein capsids that enclose their genetic material. The capsid is composed of several hundred copies of a single protein. Certain viruses also contain a second type of protein called scaffolding that provides structure for assembling the capsid.
A virus is a non-cellular, infectious organism composed of genetic material and protein that can only penetrate and replicate within the live cells of bacteria, plants, and mammals. For example, a virus cannot reproduce outside of the host cell. When it does, it requires the help of another virus (or virus-like particle) for delivery into another cell.
Viruses are divided into three groups on the basis of their shape: spherical, rod-shaped, and spiral. They can also be classified by their method of transmission: contact, vector-borne, and airborne.
Spherical viruses range in size from 20 to 250 nanometers. They have a shell made up of protein molecules wrapped around a core of DNA or RNA. Spherical viruses do not use vectors to transfer themselves from one host cell to another. Instead, they jump from cell to cell by attaching to specific receptors on the surface of human cells and then using its tail to enter an adjacent cell. Once inside the cell, the viral capsid is destroyed by cellular enzymes and the genetic material is used to create new copies of itself.
Rod-shaped viruses range in size from 250 to 1000 nanometers. They look like rods with tails at both ends. Like spherical viruses, they do not use vectors to transfer themselves between cells.