The outermost layer of a virion fulfills which of the following functions of the virus?
a. replication
b. protection
c. recognition
d. both protection and recognition
e. both recognition and replication

The correct answer is D. The outermost layer of a virion fullfills both protection and recognition functions of the virus.

Explanation:

A virion is a single viral particle. In this form the virus is physically isolable but without metabolic activity. Many virions can even be crystallized.

A virion, infecting a single host cell, is capable of producing thousands of descendants using the mechanisms of DNA replication, transcription and translation of the infected cell. This viral multiplication is often sufficient to kill the host cell by breaking it (lysandola).

Nucleic acid gives infectious power to the virion. It is enclosed by a protein wrap called capsid which is always formed by a number of individual proteins, called structural subunits. These subunits, associating in a specific way, form larger complexes called capsomeres. The set of nucleic acid and proteins is called the nucleocapsid of the virus, which can be enclosed by a membrane called pericapside envelope. Viruses without this membrane are called naked. Virus membranes are made up of a lipid bilayer, often associated with virus-specific proteins.  The main function of the envelope is to help the virus enter the host cell. The surface glycoprotein serves to identify and bind to the receptor points in the host membrane. The viral envelope is then fused with the cell membrane, allowing the capsid and the viral genome to enter.

A step further down the evolutionary scale is the virino and below it the viroid and the prion; higher up is the virus.

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