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11. Electron
microscopy has enabled the fusion process to be visualised. The diagram superimposed upon
the electron micrograph shows the viral structure: the nucleocapsid, viral proteins and
reverse transcriptase.
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12.Reverse
transcriptase is a viral enzyme which plays a key role in viral replication: in a stepwise
process, it synthesises a double stranded proviral DNA copy of the viral RNA.
This process involves different functions of reverse |
transcriptase:
synthesis of a first DNA strand
(blue); twisting into a double helix (red); synthesis of the second DNA strand and RNA
degradation (yellow). Many antiretroviral drugs are reverse transcriptase inhibitors, e.g.
zidovudine, 3TC and nevirapine.
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13.Reverse
transcriptase is involved in the production of a RNA-DNA double helix. A DNA copy of HIV
RNA is synthesised using the viral RNA as a template.
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14.Reverse
transcriptase twists the RNA and DNA strands into a RNA-DNA double
helix.
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15.Reverse
transcriptase degrades the viral RNA part of the double helix. A second DNA strand is
synthesised and combined with the original DNA strand to form a double helix. This double
helix is called proviral DNA.
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16.Enzymes, such as
reverse transcriptase, cut the proviral DNA into the correct size. Other enzymes, such as
integrase, transport the proviral DNA into the host cell nucleus. There it is integrated
into the target cells' DNA.
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17.The proviral DNA is
integrated into the host cell genome by integrase. Viral messenger RNA (mRNA) is then
synthesised, using the DNA as a template. Using this mRNA, the cell produces viral
proteins.
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18.New viral particles
are synthesised within the cell. The nucleocapsid, including HIV RNA and reverse
transcriptase, is assembled from different proteins. Digestion of a large polyprotein by
enzymes known as proteases
generates
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the different viral proteins. Proteases are the
target of a class of antiretroviral drugs known as protease inhibitors.
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19.After the
nucleocapsid has been assembled the newly replicated virion leaves the cell by budding
through the cell membrane. The virus acquires an envelope of host and viral proteins.
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20.Electron microscopy
has documented the process of the virion starting to bud from the cell. The outer envelope
begins to form around the virion.
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