2. Taxonomy of viruses – 2475 sp.
Reported so far
Baltimore system of International Committee for
classification Taxonomy of Viruses
By International union of microbiological societies
Based on genetic material and its replication strategies
5. Retroviruses (Family Retroviridae)
family of single-stranded RNA
viruses having a helical envelope
and containing an enzyme (RT)
that allows for a reversal of
genetic transcription, from RNA
to DNA rather than the usual DNA
to RNA.
the newly transcribed viral DNA is
incorporated into the host cell's
DNA strand for the production of Schematic cross section through a
new RNA retroviruses (Integrase) retroviral particle. Transmembrane (TM)
and the surface (SU) components linked
together by disulfide bonds, matrix (MA)
protein, capsid (CA) protein, and
nucleocapsid (NC) protein. Major products
of the pol-coding region are reverse
transcriptase (RT) and integrase (IN). The
protease (PR) is derived from the pro gene
between gag and pol.
6. Retroviridae
Taxonomy:
Synonym(s): RNA tumor virus group (and related agents).
Retroviruses are currently classified into 7 genera.
Host:
Virus infects vertebrates.
Morphology:
Virions enveloped; slightly pleomorphic; spherical; 80-100 nm
in diameter. Surface projections of envelope small (surface
appears rough), or distinct (8 nm long glycoprotein); spikes;
dispersed evenly over all the surface. Capsids isometric (to
spherical), or rod-shaped (or a truncated cone eg HIV).
Nucleoid of spherical nucleocapsid symmetric, or
asymmetric.
7. Retrovirus genome
RNA. Single stranded. Linear; genomic nucleic acid
positive sense. Genome dimeric. Total genome of one
monomer 7000-11000 nucleotides long.
Genome of retroviruses is flanked at
each end by repeated sequences ("R")
the gag gene is translated into molecules of the capsid protein
the pol gene is transcribed into molecules of reverse
transcriptase
the env gene is translated into molecules of the envelope
protein
8. Family Retroviridae - Taxonomy
Genus Gammaretroviruses - Mouse mammary tumor virus
Genus Episilonretroviruses - Reticuloendotheliosis virus
Subgenus: Reptilian type C oncovirus virus group - Viper retrovirus
Reticuloendotheliosis virus group - Reticuloendotheliosis virus
Genus Alpharetroviruses - Avian leukosis virus
Genus Betaretrovirus group - Mason-Pfizer monkey virus
Genus Deltaretroviruses - human T-lymphotropic virus (HTLV)
Genus Lentivirus
Subgenus: bovine lentivirus group - bovine immunodeficiency virus
Subgenus: Equine lentivirus group - Equine infectious anemia virus
Subgenus: Feline lentivirus group - feline immunodeficiency virus
Subgenus: Ovine/caprine lentivirus group - caprine arthritis encephalitis virus
Subgenus: Primate lentivirus group - HIV
• Genus Spumavirus Chimpanzee foamy virus
9. Life cycle - Retroviruses
When a retrovirus
infects a cell
its molecules of reverse
transcriptase are carried into
the cell attached to the viral
RNA molecules.
The reverse transcriptase
synthesizes DNA copies of the
RNA.
These enter the nucleus and
are
inserted into the DNA of the
host.
These inserts are transcribed
by the host's enzymes into
fresh RNA molecules which
re-enter the cytosol where
some are translated by host
ribosomes
10. Human Immunodeficiency Virus (HIV)
ICTV Group: +ssRNA RT
Baltimore Group( Earlier Classification): Group VI
(ssRNA-RT viruses (+)sense RNA with DNA
intermediate in life-cycle)
Family: Retroviridae
Genus: Lentivirus
Species: Human Immunodeficiency Virus (HIV)
12. HIV
An HIV particle is around 100-150 billionths of a metre in diameter. That's
about the same as: 0.1 microns
one seventieth of the diameter of Host Cell- human CD4+ white blood cell.
Unlike most bacteria, HIV particles are much too small to be seen through an
ordinary microscope. However they can be seen clearly with an electron
microscope.
HIV particles surround themselves with a coat of fatty material known as
the viral envelope (or membrane). Projecting from this are around 72 little
spikes, which are formed from the proteins gp120 and gp41 that act as
receptors.. Just below the viral envelope is a layer called the matrix, which is
made from the protein p17.
The viral core (or capsid) is usually bullet-shaped and is made from the protein
p24. Inside the core are three enzymes required for HIV replication called
reverse transcriptase, integrase and protease. Also held within the core is HIV's
genetic material, which consists of two identical single strands of RNA.
13. Different types of HIV
There are two types of HIV: HIV-1 and HIV-2. Both types
are transmitted by sexual contact, through blood, and from
mother to child, and they appear to cause clinically
indistinguishable AIDS. However, it seems that HIV-2 is
less easily transmitted, and the period between initial
infection and illness is longer in the case of HIV-2.
Worldwide, the predominant virus is HIV-1, and generally
when people refer to HIV without specifying the type of
virus they will be referring to HIV-1. The relatively
uncommon HIV-2 type is concentrated in West Africa and
is rarely found elsewhere.
14. Life Cycle - HIV
Virus entry into lymphocytes is receptor mediated
After entry it releases the 2 RNA genome and 3 proteins viz.,
protease, reverse transcriptase and integrase into cytosol.
Viral RNA undergoes transcription to cDNA with help of RT
Viral RNA removed with viral enzymes and sscDNA undergoes
duplication to form dsDNA
dsDNA moves to the nucleus and integrates (with the help of
integrase ) with the host cell DNA which undergoes
transcription to viral RNA and translation to viral proteins – core
proteins (further processed by protease)
2 viral RNA mol, integrase, RT and protease come together forms
the core of the virus and acquires the envelope / lipid coat and
penetrates out of the cell as infective virions.
