Systematic Veterinary Virology

MORPHOLOGY

• The virions have a complex construction and consist of an envelope, a nucleocapsid, a nucleoid, and a matrix protein. Virions are enveloped, spherical to pleomorphic in shape and measure 80-100 nm in diameter. The surface has numerous glycoprotein spikes, which are 8nm in length. The core is rod-shaped and the nucleoid is concentric. The genome is dimeric (diploid) comprises of two monomers. The genome consists of non-segmented two molecules of linear positive-sense, single-stranded RNA joined by hydrogen bonds. Minor species of non-genomic nucleic acid are also found in virions. The genome is composed of 9200 nucleotides. The 5'-end of the genome has a methylated cap and the 3'-terminus has a poly (A) tract. The genome of retroviruses is unique in the following aspects.
• 1. They are the only viruses, which are truly diploid.
• 2. They are the only RNA viruses whose genome is produced by cellular transcriptional machinery
• 3. They are the only viruses whose genome requires a specific cellular RNA (tRNA) for replication.
• 4. They are the only (+) sense RNA viruses whose genome does not serve directly as mRNA immediately after infection. Hence, the nucleic acid is not infectious
• The genome is comprised of four genes gag, pro, pol and env. Each gene codes for a single or more than one protein. The important proteins are listed below.

Name	Protein	Function
MA	Matrix matrix protein (gag gene)	lines envelope
CA	Capsid capsid protein (gag gene)	protects the core most abundant protein in virus particle
NC	Nucleocapsid capsid protein (gag gene)	protects the genome; forms the core
PR	Protease	Essential for gag protein cleavage during maturation
RT	Reverse transcriptase (RNA dependant DNA polymerase)	Reverse transcribes the RNA genome
IN	Integrase Encoded by the pol gene	needed for integration of the provirus
SU	Surface glycoprotein	The outer envelope glycoprotein; major virus antigen
TM	Transmembrane protein	The inner component of the mature envelope glycoprotein