These transcripts may be packaged into viral particles or further processed by splicing into mature mRNAs for translation. The pol portion of this precursor contains the reverse transcriptase protein. This mechanism supplies only a small amount of the enzyme, which is logical since it can act catalytically. Proteases - these are enzymes that cleave the polyprotein precursors. Behind every difficulty, there is opportunity.
Some retroviruses carry oncogenes.
These are genes derived from the host that when mutated or improperly expressed cause cancer. The host copies of these genes have normal functions, often involving the regulation of cell proliferation, e.g. src - a kinase. Viral particles are assembled, but this is not sufficient to produce virions. Budding An envelope consisting of a lipid bilayer from the host cytoplasmic membrane is acquired as viral particles are released from the host cell. This is reminiscent of M13.