Important Point of The Chapter bacteria And Viruses


Taxonomy is the branch of biology which deals with the naming and classification of individuals.

The arrangement of organisms into systematic groups is called as classification.

More than 1.5 million species of animals and more than 0.5 million species of plants are known.

All organisms are related to one another at some point in their evolutionary histories.

Classification is based on homologies, comparative biochemistry, cytology and genetics. However the major base of classification is homologies.

A species is a group of natural population which can interbreed freely among themselves and produce fertile offspring’s, but are reproductively isolated from all other such groups in nature.

Each species has its own structure, ecology and behavior.

Species is the basic unit of classification. The taxonomic groups from species to kingdom form a classification ladder.

During’ 18th century, Carlous Linnaeus (1707 1778), a Swedish botanist, provided a system for naming and classifying the organisms.

Linnaeus published the list of names of plants in 1753. His system became popular and in 1758 he published the list of names of animals. Many of his names are in use today.

Linnaeus’s of giving each species a scientific name comprising words is known as binomial nomenclature.

The scientific names are mostly taken from Latin word. The scientific name is Latinizedor Italicized.

Examples of scientific names:

(i) Onion………………….Allium cepa

(ii) Amaltas………………….Cassia fistula

(iii) Man………………….Homo sapiens

(iv) Potato………………….Solanum tuberosum

(v) Tomato………………….Solanum esculentum

E-Chatton (1937) described two terms: PROCARIOTIQUE to describe bacteria and blue-green algae. EU-CARITIQCE to describe animal and plant cells.

Robert Whittekar (1969) proposed five kingdom system of classification. It is based on three levels of cellular organization (prokaryotes, Single call Eukaryotes and Multicellular Eukaryotes) and three types of nutrition (photosynthesis, absorption and ingestion).

KINGDOM MONERA includes prokaryotic unicellular organisms (bacteria and cyanobacteria).

KINGDOM PROTISTA includes eukaryotic unicellular organisms such as Euglena and Amoeba. This kingdom also includes simple multicellular organisms that were directly evolved from unicellular protists. However most protists are unicellular.

KINGDOM PLAN TAE includes eukaryotic multicellular autotrophs. They prepare their food by photosynthesis. Examples are mosses, ferns, gymnosperms and angiosperms.

KINGDOM FUNGI are heterotrophic having absorptive method of nutrition. Most fungi are decomposers -that live on organic material, secrete digestive enzymes and absorb small organic molecules which are produced by digestion. These are also called eukaryotic multicellular reducers (e.g. mushrooms).

KINGDOM ANIMALIA consists of eukaryotic, multicellular consumers. Animals live by ingesting and digesting food within special cavities. They can move and lack cellulose. Examples are birds and reptiles.

Lynn Margulis and Karlene Schwartz (1988) modified five kingdom classification of Whittakar by considering: cellular organization, mode of nutrition, cytology, genetics and organelles of symbiotic origin (mitochondria, chloroplast).

Binomial Nomenclature: The assignment of names to organisms using two Latin words, the first denoting the genus and the second descriptive name, the two, together constitute the name of species e.g., Homosapiens.

Carcinoma: A tumor arising in one of the epithelial sheets that cover the outer and inner surfaces of the body.

Kingdom: The most inclusive taxonomic grouping, such as the classification of all plants into the Kingdom Plantae.

Lytic Pathway: The serial events in which viral genes within a host cell begin to replicate independently, mature virus particles assemble and the host cell bursts, releasing the particles, which may then infect other host cells.

Viroid: A minute particle of RNA that lacks a protein coat and is capable of causing disease in both plants and animals.


Property / Character Value / No.
Adults immune to measles 60%
During lytic cycle after 25 minutes number of viruses produced 200
Largest viruses (Poxviruses) size 250 nm
Number of capsomeres in Adenovirus 252
Number of capsomeres in Herpes virus 162
Range of size of viruses 20 nm – 250 nm
Smallest viruses (Parvoviruses) 20 nm
Viruses smaller than bacteria 10 – 1000 times


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