Biological Classification/Cell & Its Division

Category : UPSC








Biological classification is the scientific procedure of arranging organisms into groups and subgroups on the basis of their similarities and dissimilarities and placing the groups in a hierarchy of categories. The purpose of biological classification is to organize the vast number of known plants and animals into groups that could be named, remembered and studied.




Classification is needed to

  • help in establishing relationship between different organisms and to know about their evolution.
  • help in the identification of organisms.
  • study one or two organisms of one particular group and give the sufficient information of that group. It gives an idea of whole range of diversity found in organisms.
  • it gives an idea of the evolution of various groups of organisms.



Systematics is the study of the units of biodiversity. It is the study of the diversification of organisms and their relationship among living things through time. It includes the following parts:

  • Identification: It is a process of finding the correct name and place of an organism in a system of classification.
  • Classification: It is the arrangement of an organism in a particular group on the basis of their similarities and dissimilarities.
  • Nomenclature: It is a system of providing proper and distinct name to one particular organism which helps in recognizing that organism.
  • Taxonomy: It is a science dealing with the description, identification, naming and classification of organism.


Classification of Organisms

It is the arrangement of organisms into taxonomic group according to their similarities and dissimilarties. System of classification is an attempt to organize different organisms into different categories that we can use to study.


Hierarchy in Classification

Hierarchy in classification involves many steps. Each step represents a rank or category. All categories or steps together constitute the taxonomic hierarchy.



The smallest tax on is species. At the species level organisms look alike and are able to breed with one another.



The next largest taxon is genus. At the genus level, there is a group of similar species that are closely related.


A group of two or more genera with common characteristics make a family. For example, lion (Panthera leo), tiger (Panthera tigris) and the domestic cat (Felis domesticus) make the family Felidae.



A group of related families make an order. For example the family of cat (Felidae) and the family of dogs, foxes, etc. (Canidae) is grouped under the Carnivora.



Related orders make a class. For example several orders like those of the tigers, cats, dogs, monkey, bats and human belong to class Mammalia.



A phylum is the largest category with related classes grouped together. For example the classes of mammals, birds, reptiles, amphibians and fishes together constitute the phylum Chordata. In plants, the corresponding category is named division.



Kingdom is the largest group of organisms differentiated on very general similarities. For example, plant and animal kingdom. The plant kingdom comprises all kinds of plants while animal kingdom comprises all kinds of animals.


Handy Facts

  • Ashoka was the first ruler in recorded history to order the establishment of wildlife sanctuaries.
  • Carolus Linnaeus was a Swedish naturalist also known as the father or Taxonomy as he developed a way to name and organize species that we still use today. He wrote a book named ‘Systema Naturae’ in which he describes the system of classification of nature. Two most important contribution of Linnaeus are:

          (i) Hierarchical Classification System

          (ii) System of Binomial Classification

  • Aristotle, also known as father of zoology classified animals on the basis or habitat into aquatic, terrestrial and aerial animal.
  • Theopharastus, also known as father of botany divided plants on the basis of form texture and habitat into four types as trees, shrubs, undershrubs and herbs. He wrote a book named ‘Historia Plantarum’.



Biologists have devised a technique for identification naming and grouping of various organisms.

There is a need to standardize the naming of living organisms such as particular organism. Carl Von Linnaeus devised a binomial system of nomenclature in which an organism is given two names.

  • A Generic name which it shares with other closely related organisms which has features similar enough to place them in the same group.
  • A specific name which distinguishes the organism from all other species. No other organism can have the same combination of genus and species.



This type of classification was proposed by R.H. Whittaker. The five kingdom proposed by Whittaker are Monera, Protista, Fungi, Plantae, Animalia.


Kingdom Monera

  • Monerans are single cell which may or may not move. It consists of primitive type of organism which includes bacteria, cyanobacteria, archae bacteria and mycoplasma. They can live in both living and non-living environment.
  • They can also survive in harsh and extreme climatic conditions like in hot springs, acidic soils etc.
  • These are classified in 4 groups:- cyanobacteria, Archae bacteria, Bacteria and Mycoplasma.
  • These are detected on the basis of Gram staining. Gram (+) thick Peptidoglycan cell wall. Gram (-) no cell wall/thin cell wall.


Economic importance of bacteria

  • Saprophytic bacteria causes decay and decomposition of organic matter (dead plants and animals) clean the environment and release minerals in the soil.
  • Antibiotics are the chemicals released by the bacteria which render the growth of other microorganisms.
  • Some bacteria are also involved in gobar gas plant and manure formation. In gobar gas plant bacteria are used to convert animal dung and other organic wastes into manure along with the fuel gas. Biogas consists of methane (50-75%), \[C{{O}_{2}}\] (25-50%),\[{{H}_{2}}S\](0-3%), \[{{N}_{2}}\](0-10%) and \[{{O}_{2}}\](0-0.5%).



Handy Facts


Streptomyces griseus


S. venezuelae


S. aureofaciens


S. ramosus


S. erythreus


Bacillus Licheniformis Vitamins


Clostridium butylicum

Cobalamin (Vit. \[{{B}_{12}}\])

Bacillus megatherium

Vitamin C

Escherichia coli


  • Nitrogen Fixation: Some bacteria are free living which are able to pick nitrogen from the air and soil and convert it into organic nitrogen in the form of amino acid. E.g., Azotobactor, Beijerinckia, Clostridium, etc. On the other part some are symbiotic which forms nodules in the root of the plant, e.g., Rhizobium.


Kingdom Protista

Protista are considered as a diverse group of eukaryotic organism. Protists can be unicellular or multicellular and also exists in colonial form. They do not have specialized tissue organization. Protists live in water, in moist terrestrial habitats, and as parasites and other symbionts in the bodies of multicellular eukaroytes.

These are classified into 3 groups :- protistan algal, slime molds, protozoan protists.


Handy Facts

Protozoan diseases

Causative agent


·           Trypanosome gambiense


central african sleeping sickness


·           Giardia Lambia


·           Leishmania donovani


·           Dermal leishmaniasis

Leishmania tropica

·           Plasmodium vivex


·           Entamoeba histolytica

Amoebic dysentery

·           Entamoeba gingivalis




Economic Importance of Diatoms

  • Diatoms are important sources of food to aquatic animals.
  • The oils extracted from some fishes and whales are actually produced by diatoms.
  • Diatoms are employed as a cleaning agent in tooth pastes and metal polishes, added to paints for enhancing night visibility, are also employed as insulation material in refrigerators, boilers and furnaces.
  • Diatomaceous earth is used to make sound proof rooms.


Kingdom Fungi

Fungi are basically multi-cellular. Yeast is an exception in being unicellular. The cell wall is generally composed of chitin (a nitrogen containing carbohydrate). They do not contain chlorophyll and hence are heterotrophic. Most of them are decomposers, hence fungi are also known as kingdom of multicellular decomposers. They may be saprophytic (depend on dead or decaying organic matter for their food) or may be parasitic depend on living organisms for their food). Kingdom is classified on the basis of Morphology of reproductive structures, which exhibits more variation.

These are classified into 4 groups.  Deuteromycetes, Oomycetes, Zygomycetes, Ascomycetes and Basidiomycetes.


Handy Facts

Lichens are dual organisms that are formed by permanent symbiotic association between an algae and a fungus. They co-exist for mutual benefit. This type of relationship is known as symbiosis. The alga manufactures food for itself and for the fungus. Fungus provides protection to alga and helps in fixation and absorption of water and minerals.


Economic Importance of Lichen

  • Lichens grows in a dry naked rocks, mountain, barren earth. It makes the way for growth of grasses and mosses.
  • These are used as a food in tundra for reindeer, caribou, musk, etc., and also used as food article in Iceland, Sweden and Norway.
  • These are used as dyes or biological stain obtained from Rocella tinctoria.
  • These are used as perfumes obtained from species Ramalina and Evernia.


Algal                Bryophyta

Red, Green,         Liverwort

Brown           Hornwort


Handy Facts

Mycorrhizae are the type of symbiotic association between fungus with the root of higher plants, in which both the organism are mutually benefited, these fungus secretes antimicrobial substances, that protects the plant root from harmful pathogens, fungus helps plants to absorb water and important nutrients from the soil. Fungus also derives nutrient from the roots.

Eg. Pinus, birch, etc.


Disease caused by fungi in Humans



Alternaria, phoma, Trichoderma,

Aspergillus, etc.

Ear infection

Aspergillus flavus

Valley fever



Mucor pusillus


Candida albicans



Kingdom Plantae

They are multicellular eukaryotes. All plants contain plastids. Plastids are double membrane organelle that possesses photosynthetic pigments. They are called chloroplast. They are usually autotrophic. Chloroplast contains a green colour pigment called chlorophyll and prepares own food by the process of photosynthesis. Cells have cell wall made up of cellulose.

Kingdom plantae shows a lot of diversity, because of which, it has been divided into four divisions. Algae, Bryophyta, Pteridophyta, and Spermatophyta (Gymnosperms and Angiosperms).

These are classified into 4 groups -:


Kingdom Animalia

There are diverse group of animals in the whole world they lives in different habitats. All animals are multicellular except protozoa, these are eukaryotic, lacks cell wall, heterotrophic, have power of locomotion and shows increased sensitivity through the nervous system. On the basis of presence and absence of vertebrate column. Animal Kingdom are broadly divided into vertebrates and invertebrates. Inverbrates consists 8-phylums named Porifera, Cnidaria, Platyhelmintus, Aschelminthus, Anellida, Arthropoda, Molluscs and Enchinodermata. Vertebrates or phylum chordata consists 3-subphylums, Urochordata, Cephalochordata and Vertebrata.





Cell is a structural and functional unit of life. In 1665, Robert Hooke, an English scientist, saw cells for the first time in a thin slice of cork with its microscope. He observed and described the cells as “Honey comb” like structures. He named the box-like compartments as cellulae or cells. The term “cell” is derived from a Latin word cella which means little room or hollow space.

In 1674, Van Leeuwenhoek, a Dutch Scientist, studied living cells for the first time with the help of an improved microscope.



  • In 1838, two biologists, J.M Schleiden and T. Schwann proposed the “Cell Theory”.
  • In 1855, Rudolf Virchow, a German pathologist proposed that all cells arise from pre-existing cells. He stated this in Latin as “Omnis cellula-e- cellula”.
  • Smaller Organisms, like bacteria are unicellular and larger organisms are made up of many cells, and are celled multicellular organism.

Cells theory, therefore, states that

  1. All living organisms are composed of one or more cells
  2. The cell is the basic unit of life.

iii. Cells develop from pre-existing cells. Virus is an exception to cell theory.



Prokaryotic cell

Eukaryotic cell


The size cell is small (0.1-5. \[0\mu m\]).

Cell Size is larger (5-100\[\mu m\]).


It consists one envelope organization.

It consists two envelope organization.


Unorganized nucleus is present. Hereditary material lies freely in cytoplasm.

Well-developed organized nucleus is present. Hereditary are embedded in nucleus covered by nuclear membrane. 


Membrane bound organelles like ribosomes, nucleus, endoplasmic-reticulum, golgi body, mitochondria, lysosomes, vacuoles etc. are absent.

Membrane bound organells are present.


DNA is naked.

DNA is associated by histone proteins.


Site of translation and transcription is cytoplasm ex- Bacteria, Cynabacteria, etc.

Site of Translation is cytoplasm and Transcription is nucleus ex-plants, animals, fungi.


Endocytosis and exocytosis does not takes place.

Endocytosis and exocytosis takes place in protists and animal cell.




Cell Wall

Bacterial cell wall is made up of peptidoglycans. The archean cell wall is made up glycoproteins and polysaccharides. The plant cell wall is mainly composed of cellulose hemi-cellulose glycoproteins, pectins and lignin. Animal cell lacks cell wall.


Plant Cell Wall

Plant cell walls are primarily made up of cellulose which is the most abundant micro molecule on the earth. Plant cell wall consists of three layers, the primary cell wall, secondary cell wall and middle lamella.


The middle lamella:

  • It is a first, thin, amorphous sticky layer, which get deposited during cytokinesis.
  • This layer is present in two adjacent cells, which are rich in Pectin.
  • It consists calcium and magnesium pectales, these pectic compounds are partially solubilised to make the ripen fruits soft.


Primary cell wall (0.1-3\[\mu m\])

  • It is formed inner to the middle lamella and are thin, flexible and extensive, ex-cells of cortex, pith, etc. It contains cellulose microfibril.
  • Microfibril embedded in primary cell wall provides high tensile strength to the wall.


Secondary cell wall (3-19\[\mu m\])

  • It is formed between the plasma - membrane and primary wall. Lignin, suberin, cutin, etc. are deposited in the wall.
  • These layers are thicker and provides mechanical strength to the cell. Xylan is present is this layer.


Handy Facts

  • The smallest cell found is a mycoplasma cell, which is about 0.1 micron in diameter.
  • The longest cells are the nerve cells, measuring about a metre in length.
  • The largest cells are represented by eggs of ostrich, which is about 170-135 mm.


Cell Membrane/Plasma Membrane

  • Plasma membrane is a living, selectively permeable membrane. It allows some selected materials to move in and out of the cell, and prevents the entry and exit of the other substances.

The plasma membrane is made up of a bilayer of lipids and proteins. Small carbohydrates are attached at places to outer surface of lipids and proteins.



  • The Fluid Mosaic Model proposed by Singer and Nicholson is widely accepted.

According to this model:

  • The cell membrane is composed of lipid bilayer of phospholipid molecules into which variety of globular proteins are embedded.
  • Each phospholipid molecule has two ends- an outer head hydrophilic (water attracting) and the inner tail hydrophobic (water repelling).






cell wall present

Cell wall absent

Plastids present

Plastids absent

Large one vacuole

Small vacuoles

Golgi body is present in the form of dictyosomes.

Golgi body is well developed

Nucleus lies in the peripheral cytoplasm.

Nucleus lies in center  

Centrosome and centrioles are absent

Centrosome with centrioles are present.

Cannot change shape

Can change shape

Lysosome absent

Lysosomes Present

Chloroplast present

Chloroplast absent

Ribosomes present

Ribosomes present

ER present

ER present

Cell wall and plasma membrane both are present

Only plasma membrane present

Microtubules or microfilaments are Present


Cytoplasm present




Function of cell membrane

  • It provides definite shape to the cell and acts as a mechanical barrier between external and internal environment of the cells.
  • It regulates the movement of molecules in and out of cell.
  • The flexibility of membrane helps the cell to engulf food and other substances from its external environment by endocytosis.



It is living portion or protoplasm of cell that comprises gelly like Substance called cytosol and organells with nucleus. It is present in both plant and animal cell. It includes, ER, Golgi bodies, Plastids, lysosomes, peroxisomes, ribosomes, Mitochondria, and Centrosomes.


Endoplasmic Reticulum

Endoplasmic reticulum is a complex network of membrane bound structure which runs through the cytoplasm. Cisternae are spaces within the folds of the ER membranes. It is connected to both the outer nuclear membrane as well as cell membrane. The membrane has the same structure as the plasma membrane but ribosomes do not have membranes.




  • Depending on presence or absence of ribosome on the surface of ER, it is divided into two types:

(i) Rough Endoplasmic reticulum (RER): It is lined with ribosomes and is rough in appearance, hence, named as rough endoplasmic reticulum. It is the site of protein synthesis.

(ii) Smooth Endoplasmic reticulum (SER): It contains no ribosomes and hence is smooth in appearance. It helps in lipid and steroid synthesis.


Functions of endoplasmic reticulum:

  • Endoplasmic reticulum helps in intracellular and intercellular transport of materials. It is the “transport system” of the cell. It transports chemicals between cells and within cells.
  • RER is the site of protein synthesis, and enzyme synthesis. It conjugates with golgi body and helps in the formation of lysosomes.
  • SER helps in lipid synthesis and detoxifying many drugs and poisons.
  • Proteins and lipids synthesized on ER are used for making cell membrane. The process is known as membrane biogenesis.


Golgi Bodies

Golgi body consists of smooth, flattened, membrane bound, sac-like structures called cisternae. The cisternae are stacked together; placed one above another in parallel rows. It is Golgi body is a single complex in animal cells while in plant cells, it is formed of separate units called dictyosomes. Membranes of Golgi body may develop connections with membranes of ER to form complex called extra membrane system.


Functions of golgi body

  • It is involved in the synthesis, repair of cell membrane, formation of lysosomes and peroxisomes.
  • Secretion is the major function of Golgi apparatus. All types of substances that are secreted and excreted are packed in vesicles by Golgi bodies for passage to the outside. It is the secretory organdie of the cell.
  • It also takes part in storage, modification and packaging of various biochemical products produced by different components of the cell.



Lysosomes (Lysis = Breaking down; Soma = Body):

Lysosomes are small, spherical vesicle covered by a single membrane. It is scattered all over the cytoplasm. It contains powerful digestive enzymes (about 40 in number) that are capable of breaking down the organic material. Thus, lysosome serves as an intracellular digestive system, and is called digestive bags. These are also known as suicidal bags.


Functions of lysosomes:

  • Lysosome helps in intracellular digestion of food particles as they are rich in various digestive enzymes.
  • They helps in destruction of foreign particles, as in white blood cell, and also help in cleaning up the cell by digesting damaged materials of the cell. They are therefore called cellular scavengers.
  • They digest the cell’s own damaged and dead cells. Hence, they provide energy during cell starvation by digesting cell’s own parts.


Vacuoles are membrane bound fluid-filled cavities or sacs present in the cytoplasm. They are surrounded by a membrane called tonoplast. The vacuole is filled with a liquid called “cell sap” that contains dissolved salts and sugars.


Functions of vacuoles

  • In plant cells Vacuoles help to provide turgidity and rigidity to the cell.
  • It acts as a store house of pigments and waste products. It also stores useful minerals and salts.



Mitochondria are rod shaped cell organelles surrounded by a double membrane. The outer membrane is smooth and porous while the inner membrane is folded into large number of finger like structures called cristae. Cristae increase the surface area of the inner membrane, which provides more surface area for the metabolic reactions to take place. The fluid inside the mitochondria is called the matrix.



Mitochondria are commonly known as “Powerhouse of the cell”. They contain enzymes necessary for the total oxidation of food and for the release of large amount of energy in the form of ATP molecules. The energy stored in this ATP is used for synthesis of new products and other metabolic process.


Function of Mitochondria

  • It also stores calcium when required during cell signalling, generation of heat, mediation of cell growth and death.



  • Plastids are semi-autonomous organelles having DNA, RNA, ribosomes and double membrane envelope. These are largest cell organelles in plant cell.
  • Leucoplasts: They are colourless plastids which generally occur near the nucleus in non-green cells and possess internal lamellae. They mainly store food materials and occur in the cells not exposed to sunlight, e.g., seeds, underground stems, roots, tubers, rhizomes etc.
    • These are of three types

(i) Amyloplast: Synthesize and store starch grains.

(ii) Elaioplast (Lipidoplast, Oleoplast): They store lipids and oils.

(iii) Aleuroplast (Proteinoplast): Store proteins.

  • Chromoplasts: Coloured plastids other than green are known as chromoplasts. These plastids are red, orange, yellow etc. coloured due to the presence at carotenoid. These are present in petals and fruits.
  • Chloroplast: Chloroplasts are green coloured plastids due to the presence of chlorophyll. They occur abundantly in green leaves and green parts of the shoot. They trap the solar energy which is used for manufacturing food. So, they are the sites of photosynthesis.
    • It is double membrane structure. Both membranes are smooth. The inner membrane is less permeable than outer but rich in proteins especially carrier proteins.



  • The inter-membrane space is called the periplastidial space. Inner to membranes, matrix is present, which is divided into two parts—

Grana: Inner plastidial membrane of the chloroplast is invaginated to form a series of parallel membranous sheets, called lamellae, which form a number of oval – shaped closed sacs, called thylakoids.

Stroma: It is transparent, protein aceous and watery substance. Dark reaction of photosynthesis occurs in this portions.


Functions of Plastids

  • It is the site of photosynthesis, (light and dark reactions).
  • Photolysis of water, reduction of \[NADP\]to \[NADP{{H}_{2}}\]take place in granum.
  • Photophosphorylation through cytochrome \[{{b}_{6}}f\]plastocyanin and plastoquinone etc.
    • They store starch or factory of synthesis of sugars.



  • The ribosomes are smallest known electron microscopic without membrane, ribonucleic protein particles attached either on RER or floating freely in the cytoplasm and are the sites of protein synthesis.
  • It has two subunits, one is small and another is large. Small submit reads RNA and large subunits joins amino acids to form a long polypeptide chain through which protein synthesis takes place. 70S ribosomes are found in prokaryotes, mitochondria and plastids of eukaryotes while 80S ribosomes are found in cytoplasm of eukaryotes.


Functions of ribosomes

  • Ribosomes are also called protein factories of the cell.
  • Enzyme peptidyl transferase occurs in large subunit of ribosome which helps in protein synthesis.



In eukaryotic cell, a framework of fibrous protein elements became necessary to support the extensive system of membranes. These elements collectively form cytoskeleton of the cell. There are of three types- Microtubules, Microfilaments and intermediate filaments.



The microtubules are electron-microscopic structures found only in the eukaryotic cellular structures like cilia, flagella, centriole, basal-body, astral fibres, spindle fibres. These are mainly formed of tubulin protein.


Functions of microtubules

  • These form a part of cytoskeleton and help in cell-shape and mechanical support.
  • The microtubules of cilia and flagella help in locomotion and feeding.



These are microscopic, long, narrow, cylindrical, non-contractile and proteins structures found only in the eukaryotic cytoplasm. These are present in the microvilli, muscle fibres (called myofilaments) etc. But these are absent in prokaryotes. These are mainly formed of actin-protein (contractile).


Functions microfilaments

  • The microfilaments forms a part of cytoskeleton and change the cell shape during development, motility and division.
  • The microfilaments bring about directed movements of particles and organelles along them in the cell.


Intermediate Filaments

They are supportive elements in the cytoplasm of the eukaryotic cells. They are missing in mammalian RBCs. The IFs are somewhat larger than the microfilaments and are about 10nm thick. They are solid, unbranched and composed of nonmotile structural proteins, such as keratin, desmin, vimentin.


Functions of intermediate filaments

  • They form a part of cytoskeleton that supports the fluid cytosol.
    • It maintains the shape of the cell.
  • They provide support to myofibrils, which is essential for their contraction.
  • Cilia (sing.: cilium) and flagella (sing.: flagellum) - (9+2) Pattern are hair-like outgrowths of the cell membrane.



  • They help in locomotion, respiration, cleaning, circulation, feeding, etc.
  • They show sensitivity to changes in light, temperature and contact.



Centrosome and Centriole

Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin.


Functions of centrosome

  • The centrioles help in organising the spindle fibres and astral rays during cell division.
  • They provide basal bodies which give rise to cilia and flagella.



Nucleus is the prominent, spherical structure found at the center of the cell. It is the largest organelle present in cell. Basically, nucleus is the controlling centre of all cell activities and hence, it has been described as the brain of the cell.

In plant cell, nucleus lies towards the periphery due to the presence of large central vacuole while in animal cell, nucleus lies in the central position.






Nuclear envelope or membrane

Nuclear membrane or nuclear envelope, consists of two parallel membranes inner and outer with a space between 10 to 50 nm called the perinuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm. The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.



  • It allows the passage of inorganic ions, small organic molecules, ribosomal subunits, RNAs and proteins through nuclear pores.
  • It maintains the shape of the nucleus.


The nucleolus

Nucleolus is a conspicuous, darkly stained spherical body found in nucleoplasm. It is composed of large amount of ribosomal proteins and ribosomal RNA. It is generally associated with nucleolar organizer region (NOR) of the nucleolar chromosomes.


Functions of nucleolus

  • It is seat of biogenesis of rRNA and also stores rRNA.
  • It plays important role in spindle formation during cell division.



  • It is transparent, homogenous, semifluid, colloidal, ground substance present inside the nuclear membrane.
  • Function the nucleoplasm helps in maintaining the shape of nucleus formation of spindle protein of NAD, ATP, DNA, RNAs and ribosomal subunits.


Nuclear matrix:

  • It is a fine network of proteinaceous fibrils that traverses the whole nucleus.


Function of nuclear matrix

  • It helps in maintaining shape of nucleus.
  • It provides anchorage to chromatin.



  • These are thread like structures which uniformly distributed in the nucleoplasm. They are observed only in the “interphase stage”. Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA.


Functions of Chromatin

  • Chromatin stores genetic information and forms chromosomes for equitable distribution of genetic information during cell division and reproduction.
  • They are DNA-protein hereditary structures which are formed by condensation of chromatin fibres for equitable distribution during cell division and reproduction.


Handy Facts

Giant Chromosome

It was discovered by E.G. Balbiani in 1881. These are Commonly present in salivary glands of insect, hence known as salivary chromosomes. Its length is 2000\[\mu m\]

Lamp brush chromosome

It was discovered by Flemming in 1882. It is larger in compared to giant chromosome. These are visible in diplotene stage of most animal 006ytes, spermatocytes. And giant nucleus of unicellular algae i.e. Acetabularia.


Micro bodies

  • Many membrane bound minute vesicles called micro bodies that contain various enzymes, are present in both plant and animal cells.


Peroxisomes (Uricosomes)

  • These were called peroxisomes because these contain peroxide producing enzymes (oxidases) and peroxide destroying enzymes (catalases).
  • These are found in photosynthetic cells of plants. In animals peroxisomes are found in vertebrates (cells of liver, kidney), brain, small intestine, testis and adrenal cortex), invertebrates and protozoans, e.g., Paramecium.
  • Their membrane is permeable to amino acids, uric acids, etc. They contain four enzymes of \[{{H}_{2}}{{O}_{2}}\] metabolism. The enzymes urate oxidase, d-amino oxidase, \[\alpha \]-hydroxy acid oxidase produce \[{{H}_{2}}{{O}_{2}}\] whereas the catalases plays a significant protective role by degrading \[{{H}_{2}}{{O}_{2}}\] because \[{{H}_{2}}{{O}_{2}}\] is toxic for cells.



  • These are found in fungi, some protists and germinating fatty seeds where insoluble lipid food reserves must be turned into soluble sugars. These are absent in animal cell.
  • They contain enzymes of metabolism of glycolic acid via glyoxylate cycle and bounded by a unit membrane. These also contain enzymes for \[\beta \]oxidation of fatty acids, produce acetyl CoA. It is metabolised in glyoxylate cycle to produced carbohydrates.





  • Rudolf Virchow (1855) observed that new cells always develop from pre-existing cells. He also produced cell lineage theory or law of cell lineage and doctrine of genetic continuity.
  • In unicellular organisms, cell division is the means of reproduction by which the mother cell produces two or more new cells. In multicellular organisms also, new individual develop from a single cell.



  • Cell division is a biological process in all living organisms in which mother cell divides into two daughter nuclei.
  • Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle. The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control.
  • The sequence of events which occur during cell growth and cell division are collectively called cell cycle. It was introduced by Howard and pole in 1953.


Phases of Cell Cycle

The period required to complete one cell cycle (from beginning of one cell division to the beginning of next) is called generation nine. It is 24 hours in human cells and 90 minutes in yeast. Cell cycle is simpler in prokaryotes and more complex in eukaryotes.


The cell cycle is divided into two basic phases:

  • Interphase
  • M Phase (Mitosis phase)/Dividing phase



  • It is the period between the end of one cell division to the beginning of next cell division.
  • It is highly metabolically active phase, in which cell prepares itself for next cell division.


Interphase is Completed into Three Successive Stages

  • G1 phase/Post mitotic/Pre-DNA synthetic phase/gap-I :
  • G1 phase corresponds to the interval between mitosis and initiation of DNA replication.


Fallowing events take place during this phase

(i)         Intensive cellular synthesis.

(ii)      Synthesis of rRNA, mRNA ribosomes and proteins.

(iii)        Metabolic rate is high.

(iv)        Cell size increases.

(v)        Synthesis of enzymes, amino acids, nucleotides etc. but there is no change in DNA amount.




  • S-phase/Synthetic phase

S or synthesis phase marks the period during which DNA synthesis or replication takes place.


Following events take place during this phase

(i)         DNA replicates and its amount becomes double. If the initial amount of DNA is denoted as 2C then it increases to 4C.

(ii)         Synthesis of histone proteins and NHC (non-histone chromosomal proteins).

(iii)        Duplication of centriole in the cytoplasm.

G2-phase/Pre mitotic/Post synthetic phase/Gap-II


Following events take place during this phase

(i)         Mitotic spindle protein (tubulin) synthesis begins,

(ii)         Chromosome condensation factor appears.

(iii)        Synthesis of 3 types of RNA, NHC proteins, and ATP molecule.

(iv)        Repair of damaged DNA occurs.


  • The cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (\[{{G}_{0}}\]) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.




Cell division



                \[\downarrow \]






\[\downarrow \]

                        \[\downarrow \]

                             \[\downarrow \]





·              Amitosis is also called as direct cell division.

·              Mitosis is also called indirect cell division or somatic cell division or equation division.


·       It is a division that occurs in a mature diploid reproductive cell (2x) in which nucleus divides twice but chromosome (DNA) replicates only once to form four haploid cells, each having half the number of chromosomes present in the parent cell. As it causes reduction in the number of chromosomes, it is known as reduction division.


·              In this division there is no differentiation of chromosomes and spindle. The nuclear envelope does not degenerate. The nucleus elongates and constricts in the middle to form two daughter nuclei. This is followed by a centripetal constriction of the cytoplasm to form two daughter cells. Examples: Prokaryotes, protozoans, yeasts, foetal membrane of mammals, cartilage of mammals etc.

·              In this, mature somatic cell divides in such a way that chromosomes number is kept constant in daughter cells equal to those in parent cell.




·                The growing regions of plants have meristematic cells (e.g. these cells are found in apical portion of root and stem and in the expanding leaf) in which mitosis takes place.







It is the longest phase of karyokinesis.



  • Chromatin fibres thicken and shorten to form chromosomes which may overlap each other and appears like a ball of wool.
  • Each chromosome divides longitudinally into 2 chromatids which remain attached to centromere.
  • Nuclear membrane starts disintegrating except in dinoflagellates.
    • Nucleolus starts disintegrating.
  • Spindle formation begins.



  • The degeneration of nuclear envelop and membrane vesicles are formed.
  • Emergence of kinetochores starts and spindle reaches to the chromosomes and get attached to the kinetochore.



  • Chromosomes become maximally distinct i.e., size can be measured.


  • Chromosomes move towards equatorial plane of spindles called congression and become arranged with their arms directed towards pole and centromere towards equator.
    • Spindle fibres attach to kinetochores.
  • Metaphase is the best stage for studying chromosome morphology (structure, size, number) and cancer studies.



  • Centromere splits from the middle and two chromatids gets separated.



  • Both the chromatids move towards opposite poles due to repulsive force called anaphasic movement.
    • The centromere faces towards equator.
    • Shape of chromosome is best studied at anaphase.



  • Chromosomes reached on poles by the spindle fibres and form two groups.
    • Chromosomes begin to uncoil and form chromatin net.
    • The nuclear membrane and nucleolus reappear.


  • In animal cells, astral rays and spindle fibres completely disappear in telophase. The two centriole pairs organise themselves into centrosomes.
    • Golgi complex and ER etc., reform.


  • Cytokinesis is division of cell having undergone karyokinesis to produce two daughter cells each with a daughter nucleus. It begins in mid-anaphase and is generally completed along with the completion of telophase.



  • Endo mitosis: Chromosomes and their DNA duplicate but fails to separate which lead to polyploidy e.g., in liver of man, both diploid (2n) and polyploid cells have been reported. It is also called endoduplication and endopolyploidy.



S. No.

Animal cell cytokinesis

Plant cell cytokinesis


Centrioles present at spindle poles.

Centrioles lacking at spindle poles.


Asters are formed (amphiastral).

No asters are formed (anastral).


Cytokinesis by furrowing of cytoplasm.

Cytokinesis mostly by cell plate formation.


Furrow extends centripetally.

Cell plate grows centrifugally.


Occurs nearly in all tissues.

Occurs mainly at meristems.


Cell becomes rounded and its cytoplasm more viscous at the time of mitosis.

Cell does not change from or nature at the time of mitosis.



  • It is a process in which, a single cell twice to produce four daughter cells.
  • Occurs in reproductive cells, in which cell have a half number of chromosomes present in parent cell.
  • It occurs only in reproductive cells.


Meiosis I

Meiosis II

Prophase I

Prophase II

Metaphase I

Metaphase II

Anaphase I

Anaphase II

Telophase I

Telophase II


Meiosis I

It results in the formation of two haploid cells from one diploid cell. The daughter cells are, therefore, haploid but with 2n DNA content. It is divided into four phases i.e., prophase, metaphase, anaphase, telophase.

Prophase-I: It is of longest phase of karyokinesis of meiosis. It is again divisible into five subphases i.e., leptotene, zygotene, pachytene, diplotene and diakinesis.

(i)         Leptotene/Leptonema

Chromosomes are long thread like with chromomeres (i.e. linear series of darkly stained swollen areas) on it. homologous chromosomes derived from different parents either paternal or maternal.

(ii)         Zygotene/Zygonema

Pairing or “Synapsis” of homologous chromosomes takes place in this stage.


Paired chromosomes are called bivalents, which by further molecular packing and spiralization becomes shorter and thicker.





(iii)        Pachytene/Pachynema

Crossing over It takes place by breakage and reunion of chromatid segments. Breakage called nicking, is assisted by an enzyme endonuclease and reunion termed annealing is added by an enzyme ligase.

(iv)        Diplotene/Diplonema

This stage the paired chromosomes begin to separate (desynapsis) terminalisation starts is formed at the place of crossing over between non-sister chromatids.

Homologous chromosomes move apart they remain attached to one another at specific points called chiasmata.

(v)        Diakinesis

Terminalisation of chiasmata. Occurs Nuclear membrane and nucleolus degenerates. Chromosome recondense and tetrad moves to the metaphase plate. Formation of spindle. When the diakinesis of prophase-I is completed than cell enters into the metaphase-I.



Chromosomes allign on the equator. Bivalents arrange themselves in two parallel equatorial or metaphasic plates.



In involves separation of homologous chromosomes which start moving towards opposite poles so each tetrad is divided into two daughter dyads. So anaphase-I involves the reduction of chromosome number, this is called disjunction.



Two daughter nuclei are formed but the chromosome number is half of the chromosome number of mother cell. Nuclear membrane reappears and after telophase I cytokinesis may or may not occur.


Significance of meiosis-I

  • It separates the homologous chromosomes to reduce the chromosome number to the haploid state, a necessity for sexual reproduction.
  • It introduces variation by forming new gene combinations through crossing over and random assortment of paternal and maternal chromosomes.



It is also called equation or homo typical division because the number of chromosomes remains same as after meiosis-I. It involves the separation of two chromatids of each chromosome and their movement to separate cells. It is divided in four phases i.e., Prophase-II, Metaphase-II. Anaphase-II and Telophase-II.


Significance of Meiosis-II

  • Constancy of chromosomes number in successive generation is brought by this process.
    • It helps in introducing variations and mutations.
    • It maintains the amount of genetic material.
    • The four daughter cells will have different types of dramatics.


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