Complete Notes on Viruses (With Easy Explanations of Key Terms
(From Chapter:
Bacteria and Viruses)
Introduction
- Living organisms are divided into five
kingdoms.
- Some creatures do not have cellular
organization but still show some characteristics of living things.
- Viruses are representatives of such organisms.
Structure of Virus
- Viruses are extremely small infectious agents.
- Visible only under electron microscope.
- Size range:
- 20 nm (nanometer = 1 billionth of a meter) → parvovirus
- 250 nm → pox viruses
- They are 10–1000 times smaller than
most bacteria.
- They can pass through pores of filter paper
(because of very small size).
Main Components
- Genome (Central Core)
- Made of nucleic acid (DNA or RNA,
not both).
- Capsid (Protein Coat)
- Surrounds genome.
- Gives definite shape to virus.
- Made of protein subunits =
capsomeres.
- Number and type of capsomeres
are specific for each virus.
- Nucleocapsid
- Genome + Capsid = Nucleocapsid
Examples
- Herpes virus → causes cold sores, chickenpox → 162
capsomeres.
- Adenovirus → causes common cold → 252 capsomeres.
Envelope in Viruses
- Some animal viruses have an additional
covering = Envelope.
- Envelope:
- Lipid-rich membrane.
- Derived from host cell.
- Viruses without envelope = Naked Viruses.
Shapes of Viruses
- Animal & Plant Viruses:
- Polyhedron (many-sided)
- Helical (spiral-shaped)
- Bacterial Viruses (Bacteriophages):
- Cubical
- Icosahedral (20 faces)
- Helical
- Complex (polyhedral head + rod-shaped
tail)
Structure of Bacteriophage
- Bacteriophages = viruses that attack bacteria.
- They are among the most complex viruses.
- Widely used in genetic engineering.
Use in Genetic Engineering
- Desired gene of interest is inserted
into bacteriophage DNA.
- Phage injects this DNA into bacterial
chromosome.
- Bacteria become transgenic (organism
whose genome has DNA from another organism).
- These bacteria can be cultured to produce copies
of gene or required protein.
T-Phages (of E. coli)
- T4 phage is well studied.
- Parts:
- Head:
- Elongated pyramidal/hexagonal
(six-sided prism).
- Made of proteins.
- Contains double-stranded DNA.
- Tail:
- Straight structure, attached to head
by a neck.
- Has inner core + outer
sheath (both protein).
- End Plate:
- At base of tail.
- Six tail fibers attached to end
plate.
- Function: helps bacteriophage attach
to bacterial wall.
Key Terms Simplified
- Nucleocapsid → Genome + Capsid together.
- Capsomeres → Protein subunits of capsid.
- Envelope → Outer lipid membrane (only in animal
viruses).
- Naked Virus → Virus without envelope.
- Bacteriophage → Virus that infects bacteria.
- Transgenic → Organism with DNA from another species.
- Icosahedral → Shape with 20 faces.
Structure of HIV (Human Immunodeficiency Virus)
Introduction
- HIV = Human Immunodeficiency Virus.
- Belongs to group Retroviruses (special group of animal viruses).
- Causes AIDS (Acquired Immunodeficiency Syndrome) → weakens immune system, patient becomes unable to fight infections.
- Fatal disease because no one can survive without immunity against bacterial and viral infections.
General Features of Retroviruses
-
Retroviruses contain:
-
RNA genome (not DNA).
Capsid (protein coat).
-
Envelope (lipid-rich membrane around capsid).
-
- Envelope contains glycoprotein spikes → help virus recognize and bind to host cell.
- Retroviruses are spherical in shape (~100 nm diameter).
-
Key distinguishing enzyme:
- Catalyzes reverse transcription → converts single-stranded RNA → DNA.
- DNA is then made double-stranded (double helix).
Structure of HIV
-
Genome (RNA) → two single-stranded RNA molecules.
-
Capsid → protein coat enclosing RNA.
-
Envelope → lipid-rich, derived from host cell.
-
Glycoprotein Spikes (gp120, gp41) → help HIV bind to target host cell (mainly CD4+ T-lymphocytes).
-
Enzyme Reverse Transcriptase → carries out reverse transcription (RNA → DNA).
Origin of HIV
- Evidence shows HIV originated in jungles of Africa among wild chimpanzees.
Transmission to humans:
- Monkey bites
- Eating monkey meat/brains
- In monkeys, HIV can multiply but does not cause AIDS (host-specific virus).
History of HIV and AIDS
- 1981 → AIDS disease first reported (patients were homosexuals).
- Later → found in non-homosexuals also (through blood transfusions from AIDS patients).
- 1984 → Identified that AIDS is caused by a virus.
- 1986 → Virus named Human Immunodeficiency Virus (HIV).
Key Terms Simplified
- Retrovirus → Virus with RNA genome that uses reverse transcriptase to make DNA.
- Capsid → Protein shell covering viral RNA.
- Envelope → Lipid-rich outer membrane (from host cell).
- Glycoprotein spikes → Protein-sugar structures for attachment to host cells.
- Reverse transcriptase → Enzyme that converts RNA into DNA.
- Host-specific → Can infect certain species but disease appears only in specific hosts.
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