Retroviruses Use both RNA and DNA

The retroviruses infect animals and include the notorious AIDS virus. They are unique in having both RNA and DNA versions of their genome. The virus particle contains single-stranded RNA (Fig. 17.22) inside two protein shells surrounded by an outer envelope layer. The retrovirus envelope is made from the cell membrane of its previous victim. This membrane layer has retrovirus proteins both inserted through it and covering its surface (Fig. 17.23).

What makes a retrovirus "retro" is that upon entering a host cell it reverses the normal flow of genetic information by making a DNA copy of its RNA genome. Upon infecting an animal cells the retrovirus converts the ssRNA into a double-stranded DNA copy by using the enzyme reverse transcriptase (Fig. 17.24). The retrovirus DNA is then inserted into the host cell DNA. Once integrated, the retrovirus DNA is never excised but remains permanently inserted in the host genome. Consequently, retro-viruses are impossible to get rid of completely after infection and integration, at least using current medications and procedures.

AIDS (aquired immunodeficiency syndrome) Disease caused by human immunodeficiency virus (HIV) that damages the immune system retroviruses A family of animal viruses with single-stranded RNA inside two protein shells surrounded by an outer envelope.They possess reverse transcriptase which is used to convert the ssRNA version of the genome to a dsDNA copy reverse transcriptase An enzyme that uses single-stranded RNA as a template for making double-stranded DNA

FIGURE 17.21 Tobacco Mosaic Virus Structure

Tobacco mosaic virus has a simple cylindrical structure. (A) Its single-stranded RNA genome is packaged inside a shell of proteins that are arranged as a helix. (B) Electron micrograph of Tobacco mosaic virus rod-shaped particles. Copyright 1994, Rothamsted Experimental Station.

FIGURE 17.22 Particle

Retrovirus

Retroviral particles such as HIV have two single-stranded RNA molecules surrounded by a double protein shell. Like many other animal viruses, the retrovirus has an outer membrane derived from the host cell. This envelope contains viral proteins that coat the entire surface of the virus particle. Some of the proteins are embedded in the outer envelope while others sit upon the surface.

Inner shell of protein

Envelope (stolen from host cell)

Virus proteins

Inner shell of protein

Virus proteins

Envelope (stolen from host cell)

FIGURE 17.23 Structure of Retrovirus Particle

Three dimensional cut-away view of a retrovirus particle. The two coils at the center (green and pale blue) are the copies of the virus genomic RNA. The purple and dark blue layers surrounding the RNA are protein shells. The outermost fawn layer is the viral envelope, with protruding proteins. Copyright 2002, The Universal Vi rus Database of the International Committee on Taxonomy of Viruses.

FIGURE 17.23 Structure of Retrovirus Particle

Three dimensional cut-away view of a retrovirus particle. The two coils at the center (green and pale blue) are the copies of the virus genomic RNA. The purple and dark blue layers surrounding the RNA are protein shells. The outermost fawn layer is the viral envelope, with protruding proteins. Copyright 2002, The Universal Vi rus Database of the International Committee on Taxonomy of Viruses.

Reverse transcriptase synthesizes DNA from an RNA template.

Retrovirus particles carry ssRNA of the plus conformation. Although this RNA has a sequence identical to a messenger RNA, it is not used as an mRNA. Instead it is used as a template for reverse transcriptase to make a DNA copy of the retrovirus genome (Fig. 17.24). Reverse transcriptase first uses ssRNA to make a complementary DNA strand. It then degrades the RNA and uses the first DNA strand as a template to make a second DNA strand. The process of making a double-stranded DNA copy from an RNA sequence is known as reverse transcription and its discovery forced the first major revision to the Central Dogma of Molecular Biology. Previously it was believed that information flowed only from DNA to RNA to protein, never in reverse.

reverse transcription The process in which single-stranded RNA is used as a template for making double-stranded DNA

normal Reverse

TRANSCRIPTION TRANSCRIPTION

dsDNA

ssRNA

Template strand

MAKE complementary

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