Bacteriophages have a distinctive tadpole-like shape with a polyhedral head containing nucleic acid, a cylindrical neck, and a tail with fibres used to attach to bacterial cells.

Dmitri Iwanowski (1892) first observed the Tobacco Mosaic Virus (TMV) in tobacco plants, and W.M. Stanley isolated and crystallised it in 1935, proving viruses are non-cellular.

AIDS stands for Acquired Immune Deficiency Syndrome. The 'D' stands for Deficiency, referring to the deficiency of the immune system caused by the HIV virus.

HIV (Human Immunodeficiency Virus) is a retrovirus — it contains RNA as its genetic material and uses the enzyme reverse transcriptase to convert RNA into DNA inside the host cell.

Viruses lie on the borderline between living and non-living. They show characteristics of life (replication, genetic material) only inside a host cell; outside they behave as non-living crystals.

Viruses are generally not beneficial to humans — they are obligate parasites that cause diseases. Bacteria, fungi, and algae all have beneficial roles in industry, medicine, and ecology.

The protein coat surrounding the nucleic acid core of a virus is called the capsid. It is made of protein subunits called capsomeres and protects the viral genetic material.

Biological classification (taxonomy) is based on structure, evolutionary relationships, and common ancestry — not size, as organisms of vastly different sizes can belong to the same taxonomic group.

A virus contains only one type of nucleic acid — either DNA or RNA, never both. This single nucleic acid molecule carries all the genetic information of the virus.

Tobacco Mosaic Virus (TMV) contains single-stranded RNA as its genetic material. It was the first virus to be discovered and its RNA acts directly as mRNA in host cells.

TMV enters host cells as an RNA molecule only — the capsid is shed during infection. The viral RNA is then translated by host ribosomes to produce viral proteins.

Viruses are classified based on their morphology (shape and structure) and the type of nucleic acid they contain (DNA or RNA, single or double stranded).

In Whittaker's five-kingdom system (1969), all prokaryotes including bacteria and cyanobacteria are placed in Kingdom Monera, characterised by the absence of a true nucleus.

Lynn Margulis and Karlene Schwartz renamed Whittaker's Protista to Protoctista to include more eukaryotic microorganisms and their relatives that didn't fit into other kingdoms.

Carolus Linnaeus, working in the 18th century with only a two-kingdom system, placed bacteria in Kingdom Plantae since he considered them primitive plants.

A species is the basic unit of biological classification — a group of organisms that can interbreed naturally and produce fertile offspring. Members of different species cannot do this.

Allium cepa is the scientific (binomial) name for the common onion. It belongs to the genus Allium which also includes garlic (Allium sativum) and leek.

In binomial nomenclature developed by Carolus Linnaeus, the first word of the scientific name represents the genus (e.g., Homo), while the second word represents the species (e.g., sapiens).

Virology is the branch of biology (and medicine) that specifically studies viruses — their structure, classification, and the diseases they cause in humans, animals, and plants.

Wendell Stanley crystallised Tobacco Mosaic Virus (TMV) in 1935, proving that viruses are chemical entities (proteins) that can be purified. This earned him the Nobel Prize in 1946.

The capsid is made of protein subunits called capsomeres arranged in specific geometric patterns. The capsid protects the viral nucleic acid and helps the virus attach to host cells.

Icosahedral viruses have a capsid shaped like an icosahedron — a regular solid with 20 triangular faces and 12 vertices. Adenoviruses and poliovirus have this shape.

A lysogenic (temperate) phage integrates its DNA into the host chromosome as a prophage without killing the host cell. It can later be activated to enter the lytic cycle.

Tetanus is caused by the bacterium Clostridium tetani, not a virus. Smallpox, influenza, and polio are all caused by viruses.

Viruses reproduce by replication — they hijack the host cell's machinery to make copies of their nucleic acid and proteins, which are assembled into new viral particles.

HIV specifically infects and destroys CD4+ T-helper cells, which are central to the immune response. Their destruction leads to failure of the immune system, causing AIDS.

Hepatitis E virus (HEV) is transmitted via the faecal-oral route, often through contaminated water. It causes acute hepatitis, particularly dangerous for pregnant women.

Hepatitis A is called 'infectious hepatitis' because it spreads easily through contaminated food and water (faecal-oral route). It is self-limiting and rarely becomes chronic.

Hepatitis B is transmitted via blood, sexual contact, and from infected mother to child during birth. It can become chronic and lead to liver cirrhosis or liver cancer.

Poliovirus is one of the smallest known RNA viruses with a spherical (icosahedral) capsid of about 30 nm. It infects nerve cells and can cause paralysis.

A kingdom is divided into phyla (in animals) or divisions (in plants). The hierarchy is: Kingdom → Phylum/Division → Class → Order → Family → Genus → Species.

The viral capsid (protein coat) surrounding the nucleic acid is made entirely of protein. Some viruses also have a lipid envelope outside the capsid, derived from the host membrane.

Amaltas (the golden shower tree) has the scientific name Cassia fistula. It is a flowering plant known for its long drooping clusters of bright yellow flowers.

Viruses can only replicate inside living host cells because they lack the cellular machinery (ribosomes, ATP, enzymes) needed for reproduction. They are obligate intracellular parasites.

Kingdom Monera contains all prokaryotic organisms — bacteria and cyanobacteria — which are unicellular organisms lacking a membrane-bound nucleus and other true organelles.

Rabies is caused by the Rabies virus (a Rhabdovirus) and is transmitted to humans most commonly through the bite of an infected (rabid) dog or other mammals.

Viruses are much smaller than bacteria. Bacteria range from 1–10 micrometres, while viruses range from 20–300 nanometres — making viruses roughly 10–100 times smaller.

The capsid is the protein shell that surrounds and protects the nucleic acid core (genome) of a virus. It is assembled from protein subunits called capsomeres.

Viruses are obligate intracellular parasites — they can only replicate inside living host cells and cannot carry out any metabolic processes on their own outside a cell.

Viruses infect and replicate in all types of living cells — plant cells, animal cells, and microorganisms (bacteria are infected by bacteriophages). Each virus has specific host range.

The first step in bacteriophage replication is adsorption — the phage attaches to specific receptor sites on the bacterial cell surface using its tail fibres.

Retroviruses (like HIV) carry the enzyme reverse transcriptase, which converts their RNA genome into DNA inside the host cell — the reverse of the normal transcription process.

In biological classification, an Order is divided into Families. The hierarchy goes: Kingdom → Phylum → Class → Order → Family → Genus → Species.

AIDS presents with multiple symptoms including unexplained weight loss, swollen lymph nodes, persistent fever, and progressive failure of the immune system leading to opportunistic infections.

Single-stranded RNA tumour viruses (oncornaviruses) belong to the group Retroviruses. They integrate into the host genome and can transform normal cells into cancerous cells.

Bacteriophages are viruses that specifically infect bacteria. They can only replicate inside bacterial cells using the host's metabolic machinery.

A virus without a lipid envelope surrounding its capsid is called a naked virion. Examples include adenoviruses and poliovirus. Enveloped viruses (like HIV) have an extra lipid membrane.

The scientific name of modern humans is Homo sapiens (meaning 'wise man' in Latin), assigned by Carolus Linnaeus in 1758. Genus: Homo, Species: sapiens.

A lytic (virulent) phage takes over the host bacterial cell, replicates rapidly, and causes the cell to burst (lyse), releasing hundreds of new phage particles.

HIV is highly host-specific — it can only infect humans and specifically targets CD4+ T-helper lymphocytes, macrophages, and dendritic cells that express the CD4 receptor.