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Since the beginning of civilization there have been numerous attempts to categorize living living organisms. This was done naturally and not by using rules that were It is a scientific concept, but it was born from the necessity to utilize organisms for our own purposes for clothing, shelter and food. Aristotle was the first to explore a way to provide more Scientific basis for classification. He used morphological characters that were simple to categorize the plants into shrubs, trees and even herbs. He also classified animals. divided into two groups: the ones with red blood, and those that were not. In Linnaeus” time, there was In the time of Linnaeus there was a Two Kingdom system of classification was in place that was accompanied by Plantae as well as Animalia kingdoms was created which comprised all animals and plants. Best NEET Coaching in Dimapur.
This system did not differentiate between the prokaryotes and the eukaryotes multicellular and unicellular organisms Photosynthetic (green algae) and non-photosynthetic (fungi) organisms. The classification of organisms into animals and plants was easy. It was done and simple to comprehend, however it was a huge number of organisms. Didn’t fall in any category. Therefore, that the classification of two kingdoms. For a long period of time, the method it was discovered to be inadequate. Apart from that the gross anatomy. There was also a desire to include other traits like cell structure, nature of wall, the mode for nutrition methods of reproductionEvolutionary relationships, etc.
These X-shaped structures, are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years. The final stage of meiotic prophase I is diakinesis. This is marked by terminalisation of chiasmata. During this phase the chromosomes are
fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. Diakinesis represents transition to metaphase. Best NEET Coaching in Dimapur. Metaphase I: The bivalent chromosomes align on the equatorial plate (Figure 10.3). The microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes. Anaphase I: The homologous chromosomes separate, while sister chromatids remain associated at their centromeres (Figure 10.3). Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as dyad of cells (Figure 10.3).
Systems of classification for living things Organisms have undergone numerous changes throughout the years.The animal and plant kingdoms have existed for a long time, they are a constant throughout allDifferent systems, the understanding of what kinds of groups/organisms can beThese kingdoms are changing. The numbers and the type of kingdomsThe nature of other kingdoms has also been interpreted differently by diverse scientists over time.R.H. Whittaker (1969) suggested the idea of a Five Kingdom Classification. The Kingdoms he defined were known as Monera, Protista, Fungi, Plantaeand Animalia. The principal criteria used for his classification are Cell Structure, body structure body structure, nutrition mode reproduction, and Phylogenetic relations. Best NEET Coaching in Dimapur.
The five kingdoms have their own characteristics. The system of three domains has been suggested to divide the Kingdom Monera splits into two domains leaving the other Eukaryotic kingdoms to the third domain , thereby creating the classification of six kingdoms. The information you will receive on the system in depth at higher levels.Let’s examine this classification of five kingdoms to help us understand the challenges and other factors that have affected and influenced the classification system. and other factors that influenced the classification system. The classification systems comprised blue green algae, bacteria, fungi, mosses gymnosperms, ferns and angiosperms listed under the heading ‘Plants’.. The naturethe thing that unites this entire kingdom was that all organisms in the kingdom shared the same cells’ cell wall.
This brought together groups that were vastly different in Other features. It brought together prokaryotic bacteria as well as the blue algae (cyanobacteria) together with other groups that were eukaryotic. It also includes the unicellula and multicellular organisms. For instance, Chlamydomonas and Spirogyra were put together under algae. Best NEET Coaching in Dimapur. The classification did not distinguish between heterotrophic and heterotrophic group – fungi, as well as the autotrophic green plants although they also demonstrated There is a distinct different in the composition of their walls The fungi contained Chitin in their walls whereas the plants with green leaves had cells that were cellulosic. If this is the case, features were analyzed, and the fungi were categorized, and then placed in a different kingdom – Kingdom Fungi. The prokaryotic bacteria were placed in one group. Together with Kingdom Monera and the unicellular Eukaryotic Organisms.
They were incorporated into Kingdom Protista. Kingdom Protista brings people together. Chlamydomonas, Chlorella (earlier placed in Algae within Plants and both with cell walls) that are surrounded by cell walls) Paramoecium as well as Amoeba (which were previously placed within the animal kingdom that have no cell wall). The cell wall has been put in different organisms. In earlier classifications, were put in distinct kingdoms. This occurred because the criteria were changed in the classification process. This sort of The changes are likely to occur in the future, too, based on the improvements in our understanding of the characteristics and their evolutionary connections. Over time, A bid was made to develop an organization system that is reflective of not just the morphological, physiological and reproductive affinities, However, it is also phylogenetic i.e. it is built on the evolution of relationships. Best NEET Coaching in Dimapur.
In this chapter, we will look at the specifics that characterize Kingdoms Monera, Protista along with Fungi are part of the Whittaker classification system. The Kingdoms Plantae and Animalia often known as animals and plants Kingdoms will be treated separately in chapters 3 and 4. Spore Flagellum Cocci Bacilli Spirilla Vibrio Figure 2.1 Bacteria in various shapes 2.1 KINGDOM MONERA Bacteria is the sole member in Monera’s Kingdom. Kingdom Monera. They are the largest. abundant micro-organisms. Bacteria are everywhere. Hundreds of bacteria can be found in the soil of a handful. They also inhabit extreme environments like deserts, hot springs deep oceans and snow, in which there are very few Life forms can live. A lot of them reside in or on other living things as parasites.
Bacteria are classified into four categories according to their shape. Spherical Coccus (pl. cocci) is the rod-shaped Bacillus (pl. : Bacillus) The rod-shaped Bacillus (also known as cocci) comma-shaped Vibrium (pl. the word vibrio)) as well as The circular Spirillum (pl. : Spirilla) While the bacterial structure is extremely simple, they are extremely complicated in their behavior. Best NEET Coaching in Dimapur. In comparison to other species of organisms, bacteria is an entire group They have the greatest metabolic diversity. Some bacteria Autotrophic, i.e., they make their own food by synthesising organic substrates. They can be photosynthetically autotrophic, or chemosynthetic. The majority of bacteria is heterotrophs. i.e. they depend on each other. Organisms, or dead organic matter that is used for food. 2.1.1 Archaebacteria They are distinct because they reside in some of the most difficult environments for example, extreme salty regions (halophiles) Hot springs (thermoacidophiles) and and (methanogens).
Archaebacteria are different from other bacteria. by having a distinct cell wall and this structure is what is responsible for their ability to survive in extreme conditions. Methanogens can be found in the gut. of various ruminant animals including buffaloes and cows and are that are responsible for the responsible for the production of methane (biogas) in the waste of the animals. Figure 2.2 A filamentous blue-green algae – Nostoc 2.1.2 Eubacteria There are a myriad of Eubacteria or “true” bacteria’. They are distinguished in the absence of Cell wall that is rigid, and if mobile or it is a flagellum. The Cyanobacteria (also called blue-green algae) contain chlorophyll, which is similar to green plants. photosynthetic autotrophs (Figure 2.2). Best NEET Coaching in Dimapur. The Cyanobacteria can be colonial, unicellular or filamentous freshwater/marine algae or terrestrial algae. The colonies They are usually surrounded by a gelatinous sheath. Theytypically, blooms form when polluted bodies of water.
These organisms are able to fix atmospheric nitrogen Specialized cells known as heterocysts e.g., Nostoc and Anabaena. Chemosynthetic autotrophic microbes Oxidize various inorganic compounds like ammonia, nitrites, nitrates and nit and then use the ammonia released energy to support the ATP production. They play an important function in recycling nutrients like nitrogen, phosphorous, sulfur and iron. Heterotrophic bacteria are the most prevalent in nature. A majority of them are important decomposers. A lot instances have major impact on the human affairs. They can be useful in the production of curd made from milk, production of antibiotics and fixing nitrogen in legume roots etc.
Certain pathogens cause harm to humans to farm animals, crops, and animals. Typhoid, Cholera and citrus canker are all well-known diseases that are caused by different bacteria. Bacteria reproduce mostly through fission (Figure 2.3). In some instances, in unfavorable circumstances, they make spores. Best NEET Coaching in Dimapur. They reproduce as well. Sort of sexual reproduction that is achieved by taking the A primitive form of DNA transfer between a single bacteria one to the to the. Mycoplasma are Mycoplasma are the Mycoplasma, which are organisms that They are completely devoid of a cell wall. They are the tiniest living cells are known to exist and are able to survive without oxygen. Mycoplasma are a variety of can be pathogenic to animals as well as plants.
2.2 KINGDOM PROTISTA Single-celled eukaryotes all fall under Protista however, there are boundaries Of this kingdom, the terms are not clearly defined. The definitions of ‘photosynthetic’ Protistan to one biologist could be a plant to one. In this book, we They include Chrysophytes, Dinoflagellates, Euglenoids such as Slime moulds, Chrysophytes, Dinoflagellates Protozoans in Protista. The members of Protista are mostly aquatic. This kingdom is linked with other kingdoms dealing with animals, plants, and plants. and as well as. As eukaryotes, the protistan cell body has a clearly delineated the nucleus as well as other organelles that are membrane bound. Some are flagella-like or cilia. Best NEET Coaching in Dimapur. Protists reproduce sexually and asexually by means of a process that involves Cell fusion and zygote formation.
2.2.1 Chrysophytes Diatoms belong to this group, as do gold algae (desmids). They can be commonly found in freshwater and marine in marine. They are tiny and floating passively in and float in the currents of water (plankton). They are the majority photosynthetic. Diatoms’ cell walls are formed by two thin shells that are overlapping, They can be joined to form which can be used as soap boxes. The walls are coated with silica Therefore, the walls and the walls are impervious to destruction. So, diatoms have left their mark huge amount from cell wall deposits found in their natural habitat. This accumulation has occurred over billions of years is often referred to as ‘diatomaceous Earth’. Being a bit gritty, this The soil is used for polishing as well as filtration of oils and syrups. Diatoms are the diatoms that are used in polishing, filtration and syrups. primary producers of the oceans. 2.2.2 Dinoflagellates These species are mostly photosynthetic and marine.
They appear as green, yellow or brown, blue, red based on the primary pigments that are present on the main pigments present in their cells. They are mainly pigmented by the main pigments present in their. The cell wall is composed of cellulose plates that are stiff on the outside. The majority of These flagella are two in number One is located longitudinally, the other on they have a second flagella that is located longitudinally. Other transversely, in a furrow across the walls. Best NEET Coaching in Dimapur. Most often Red dioflagellates (Example: Gonyaulax) They multiply at such a rapid rate that they form the Sea appears in red (red seas). Toxins released from such huge The numbers could even cause the death of other marine animals, like fishes.
2.2.3 Euglenoids The majority of them are freshwater organisms that are found in stagnant water. Instead of a cell wall they are proteins The pellicle is a thick layer that allows their body to be elastic. They have two flagellas, one of which is a long and short one. However, they have two flagellas. They are photosynthetic in presence of sunlight. When they’re not getting sunlight, they behave as heterotrophs Predating on smaller organisms. It is interesting to note that the the pigments in euglenoids match to the pigments found in Higher plants. Example: Euglena (Figure 2.4b). 2.2.4 Slime Moulds They are saprophytic prototists. The body is moved on decaying twigs, leaves and that are engulfing organic material. Best NEET Coaching in Dimapur. In the right conditions they will make up an The plasmodium aggregation process can develop and It is spread over a few feet.
In times of extreme weather, The plasmodium transforms and produces fruiting bodies with spores that are attached to their the tips. They have walls. They are very durable and can last for a long time, even under adverse conditions. The spores disperse through air through air. 2.2.5 Protozoans All protozoans are heterotrophs . They are predators, or parasites. These are thought to be the earliest cousins of animals. There are four main groups of protozoans. Amoeboid protozoans The organisms are found in the fresh air. sea water, or soil that is moist. They move and catch their prey using the pseudopodia (false feet) like Amoeba. Marine Forms contain silica shells on their surfaces. Certain forms, such as Entamoeba are parasites. Protozoans with flagellated flagellation: The members of this category are either free-living or parasitic.
They also have flagella. They can cause diaseases, such as sleeping sickness. Example: Trypanosoma. Ciliated protozoans are aquatic, active organisms due to due to the existence of hundreds of the presence of thousands of. They are able to open an opening (gullet) that can be opened towards the outside of the on the surface of the cell. Best NEET Coaching in Dimapur. The coordination of the movement of rows of cilia allows the water brimming with food to flow to the bottom of the gullet. Example: Paramoecium (Figure 2.4d). Sporozoans: This is a broad category of organisms with an infectious The stage that resembles spores in its life. The most well-known is Plasmodium (malarial parasite) which is the cause of malaria. an illness that is shocking influence on human populations.
2.3 KINGDOM FUNGI The fungi are a unique family that includes heterotrophic species. They exhibit a wide range of both habitat and morphology. You’ve probably observed Fungi that live on breads with moisture and rotten fruit. The most common mushroom you consume And toadstools too are fungi. The white spots on leaves of mustard may be due to to parasitic fungi. Certain unicellular fungi, e.g., yeast can be used to produce beer and bread. Other fungi cause disease in animals and plants; wheat The rust-causing Puccinia is a good instance. Certain are the cause of antibiotics, e.g., Penicillium. Best NEET Coaching in Dimapur. Fungi are multi-cultural and are found in water, air, soil, on plants and animals. They prefer growing in humid and warm environments. Have thought about why you store food items in our refrigerators ? Yes, It is designed to stop food from spoiling because of fungal or bacterial illnesses.
Except for yeasts, which are monocellular They are filamentous. Their bodies are made up of threads with a length that is slender and long. structures The term hyphae is used to describe the known as hyphae. The hyphae network is called mycelium. Certain hyperhae Continuous tubes are that are filled with multinucleated cytoplasm they are known as coenocytic hyphae. Other varieties have septae, or cross walls. hyphae. Fungi’s cell wall are made of polysaccharides and chitin. The majority of fungi are heterotrophic. consume organic matter that is soluble dead substrates, and are classified as saprophytes. The ones that depend On living animals and plants, they are referred to as parasites. They can also be found in Symbionts that are in contact with algae, such as lichens, and also with roots of higher Plants as mycorrhiza.
The reproduction process in fungi takes place via vegetative methods – Fragmentation, fission and budding. The process of sexual reproduction is through conidia, Zoospores and sporangiospores. sexual reproduction are caused by ascospores and oospores and basidiospores. The diverse spores are formed in distinct structures known as fruiting bodies. Best NEET Coaching in Dimapur. The sexual cycle is comprised of the following steps: (i) The fusion of protoplasms in two motile or non-motile gametes known as plasmogamy. (ii) (iii) Fusion of two nuclei, a process known as karyogamy. (iii) Meiosis in zygote results from haploided spores. If an fungus reproduces sexually two haploid Hyphae of mating types that are compatible are joined and fuse. In some fungi, the union of two cells that are haploid It is immediately adiploid cell (2n). But, in other cases, it is possible to have diploid cells (2n) that are immediately formed. Fungal species (ascomycetes as well as basidiomycetes) and an interfering dikaryotic stage (n + n, i.e., two nuclei per cell) occurs;Such a situation is referred to as a dikaryon.
The term dikaryophase is used to describe the fungus. The parental nuclei The cells fusion and they become diploloid. The fungi develop fruiting the bodies where reduction division takes place, which leads to The formation of haploid spores. The morphology of mycelium and the mode of spore production the formation of fruiting bodies and the foundation of the The division of kingdoms into several classes. 2.3.1 Phycomycetes The phycomycetes can be found in the aquatic ecosystems and decaying wood in humid and moist places, or on decaying wood in damp and moist places or Insane parasites that feed on plants. Mycelium is aseptate and coenocytic. The process of reproduction is asexual. Zoospores (motile) as well as by Aplanospores (non-motile). Best NEET Coaching in Dimapur. They are The spores are produced endogenously in the sporangium. A The zygospore forms by the it being formed by the fusion between two gametes.
Gametes are similar in morphology (isogamous) or Different (anisogamous or anisogamous or). A few of them are Examples include Mucor (Figure 2.5a), Rhizopus (the bread The mould we that was mentioned earlier) in addition to Albugo (the parasitic fungus mentioned earlier) and Albugo (the on mustard). 2.3.2 Ascomycetes Often referred to as sac-fungi the ascomycetes are mainly multicellular e.g., Penicillium or, more often, monocellular e.g. the yeast (Saccharomyces). They are saprophytes and decomposers. parasitic or coprophilous (growing on dung). Mycelium can be branched and septate. Conidia, which are asexual spores, are created Exogenously, on the specific mycelium known as conidiophores. Conidia on germination produce mycelium.
Sexual spores, also known as ascospores which are made endogenously by sacs similar to and asci (singular ascus). They are created endogenously by sac like asci. asci can be found in different kinds of fruiting bodies known as ascocarps. There are several examples: Aspergillus (Figure 2.5b), Claviceps and Neurospora. Neurospora is extensively used in both genetic and biochemical work. There are many members such as truffles and morels are edible and considered to be delicacies. 2.3.3 Basidiomycetes The most well-known forms of basidiomycetes are fungi, brackets and mushrooms. or puffballs. Best NEET Coaching in Dimapur. They are found in soil, on stumps of trees and logs as well as in living Plant bodies as parasites, e.g. Rusts, rusts and the smuts. Mycelium can be branchedand septate. The asexual spores are usually not present, but the vegetative The reproduction process is often fragmented.
The sex organs are not present, But plasmogamy is induced by the fusion of two plants or somatic cells that are of various genotypes or strains. This results in a structure that is called dikaryotic which eventually gives rise to that eventually leads to. Meiosis and Karyogamy take The basidium produces four basidiospores. The basidiospores Exogenously, they are produced on the basidium (pl. : basidia). The basidia are They are arranged in fruiting bodies known as basidiocarps. Common species They are Agaricus (mushroom) (Figure 2.5c), Ustilago (smut) and Puccinia (rust fungus). 2.3.4 Deuteromycetes Often referred to as imperfect fungi, because it only the sexual or The vegetative phases in these species of fungi is called vegetative phases. Best NEET Coaching in Dimapur. The sexual varieties are present, they are known as When these fungi were identified, they were then absorbed into classes where they were able to are part of.
There is also a possibility that both the vegetative and sexual stages have been given a single name (and put under the deuteromycetes) and sexual Stage one stage (and put under a different class). Then, when the linkages are established They were identified, and the was identified and removed from deuteromycetes. Once complete (sexual) stage of the members of dueteromycetes were identified and they were frequently transferred to ascomycetes and basidiomycetes. The deuteromycetes reproduce exclusively through Asexual the spores Conidia is also known as commonly referred to as conidia. Mycelium is septate and is branched. Some species are parasites, saprophytes or saprophytes while the majority of them Decomposers of litter aid in the process of mineral cycling. Examples includeAlternaria, Colletotrichum and Trichoderma2.4
The KINGDOM PLANTAE Kingdom Plantae includes all eukaryotic chlorophyll-containing Organisms that are commonly referred to as plant-like organisms. Some members are part heterotrophic like insectivorous species or parasites. Bladderwort and Venus fly traps are examples of plants that eat insects. Cuscuta is one. parasite. Plant cells possess an eukaryotic form with distinct the cell wall and chloroplasts are primarily comprised of cellulose. The study will focus on the The structure of eukaryotic cells is described at Chapter 8. Plantae includes algae, bryophytes and gymnosperms, pteridophy and angiosperms. Best NEET Coaching in Dimapur. The life cycle of plants comprises two distinct phases: the diploid and the sporophytic and the gametophytic haploid – which alternate with one another. The lengths of the diploid and haploid of the diploid and haploid phases, and whether these of the phases are freeof the diploid and haploid phases.
living in dependence or dependent on others, differ between different plant groups. This phenomenon is known as the alternation of generations. You can study more details about more details about this kingdom in Chapter 3. 2.5 KINGDOM ANIMALIA This kingdom is characterized by heterotrophic eukaryotic species Multicellular, the cells do not have cellwalls. They either directly or Indirectly, they depend on plants for food. They process their food via an internal way. cavity, and stores food reserves in the form of fat or glycogen. The way they eat is through their nutrition is holozoic through eating food. They exhibit a certain pattern of growth and then grow into adults who have a distinct size and shape. Higher forms exhibit a complex sensory and neuromotor mechanisms for sensory and neuromotor. They are the majority capable of moving around. Best NEET Coaching in Dimapur. The reproduction of sexuality occurs through the copulation of female and male which is then followed by through embryological development.
The most important characteristics of different species are as described in chapter 4. 2.6 VIROIDS, VIRUSES, LICHENS, PRIONS In the classification of five kingdoms of Whittaker there is no mention of lichens. as well as some cell-based organisms, such as prions, viruses, and viroids. They are Briefly introduced briefly here. Anyone who has felt the effects of the common colds or the flu have experienced the ill effects of common cold or ‘flu. how viruses impact in us, even when don’t think of it as a part of the way we live. condition. The virus could not be found an appropriate classification because they aren’t is considered to be truly living, in the sense of living being living organisms. They have a cell structure. They are non-cellular organisms which It is distinguished by its inert crystalline structure inside cells that are living.
When they infect a cell, they take over the machineries that is in the cell. Best NEET Coaching in Dimapur. They then duplicate their own, killing their hosts, killing them. Do you consider viruses to be either living or dead? Virus means venom or poisonous fluid. Dmitri Ivanowsky (1892) identified certain microbes as the causal agent of the mosaic disease tobacco (Figure 2.6a). They were also found to be smaller than the bacteria due to the fact that they were passed through filters that were resistant to bacteria. M.W. Beijerinek (1898) proved that the extract of tobacco plants infected by the disease may cause infection in healthy plants. The pathogen has been named the pathogen of the future ”virus” and refers to the fluid Contagium Vivo Fluidum (infectious living fluid).
W.M. Stanley (1935) proved that viruses can be crystallized The crystals and the granules are made up of and are mostly composed of. They are inert beyond their own specific host cell. Viral parasites are the only obligate pathogens. Along with proteins, viruses also have genetic material that can It could be DNA or RNA. There is no virus that contains DNA and RNA. A virus is A nucleoprotein, as well as the genetic material that it contains is infective. In general, viruses The viruses that infect plants are single-stranded RNA, as do the viruses infecting animals can be single or double Stranded RNA, or double DNA. Best NEET Coaching in Dimapur. Bacterial infections or bacteriophages (viruses which infect bacteria) are typically, double-stranded DNA viruses are usually double stranded (Figure 2.6b). The coat of protein known as capsid, is composed of tiny subunits known as capsomeres. It protects thenucleic acid.
The capsomeres are organized in polyhedral or helical geometric shapes. Viral infections cause illnesses like small pox, mumps and herpes and influenza. AIDS in humans is caused by an infection. In plants the signs could include mosaic formation or leaf curling and rolling symptoms can be yellowing, leaf rolling and rolling along with vein clearing which can cause dwarfing and slowing growth.Viroids in 1971: T.O. Diener discovered a brand new infectious agent that It was smaller than viruses, and it was responsible for potato tuber diseases. It was It was found to be a non-free RNA, but it was not able to produce the coat of protein typically found in viruses. which is why it was named hence the name. The viroid’s RNA was of low molecular mass. Prions: In modern medicine certain neurological disorders that are infectious The results showed that they were passed on by an agent made up of folded abnormally protein.
The agent was comparable in size to viruses. They were also known as prions. The most prominent ailments caused by prions include bovine spongiform Encephalopathy (BSE) often referred to as mad cow disease, which affects cattle, and the variant that is similar to Cr-Jacob Disease (CJD) in humans. Lichens : Lichens are symbiotic associations i.e. mutually beneficial connections, between fungi and algae. This algal part is referred to as Fungal and phycobiont as mycobiont, both of which are autotrophic and heterotrophic and heterotrophic. Best NEET Coaching in Dimapur. Algae make food for fungi and heterotrophic It provides shelter and absorbs mineral elements and water to its companion. They are so close in their connection that when one sees an lichen in the wild, one would You would never think that they contained two distinct organisms inside them. Lichens are excellent pollution indicators They don’t grow in areas of pollution.
The biological classification of animals and plants was first thought of by Aristotle on the the basis for morphological characters that are simple. Linnaeus later classified the living creatures on the basis of simple morphological characters. divided into two kingdoms: Plantae into two kingdoms – Plantae and Animalia. Whittaker suggested a complex five kingdom classification : Monera, Protista, Fungi, Plantae and Animalia. The most important The criteria for the five kingdom classification included the body structure, cell structure, way of food and reproduction, as well as the phylogenetic relationship. In the classification of five kingdoms bacteria are part of the Kingdom of Monera. Bacteria are cosmopolitan in distribution. Best NEET Coaching in Dimapur. Bacteria are among the organisms that show the greatest extent metabolic diversity. Bacteria can be autotrophic or heterotrophic in their manner of nutrition.
Kingdom Protista includes all single-celled Eukaryotes, such as Chrysophytes, Dinoflagellates, Euglenoids Slime-moulds, Chrysophytes, Dinoflagell Protozoans. Protists are nucleus-specific and have other membrane-bound organelles. They reproduce sexually as well as asexually. Kingdom Fungi members are sexually and asexually asexual. Kingdom Fungi show a great range of sexual and asexual characteristics. in habitats and structures. Most fungi are saprophytic , in their diet. They exhibit sexual and asexual reproduction. Phycomycetes, Ascomycetes, Basidiomycetes as well as Deuteromycetes constitute the two classifications within this kingdom. The plantae includes all eukaryotic chlorophyll-containing organisms. Algae, gymnosperms, bryophytes and angiosperms are also included in this group. The life cycles of plants show gamestophytic and the sporophytic generation.
The heterotrophic multicellular eukaryotic Cell wallless organisms are part of the Kingdom Animalia. The method of Diet for these species is the holozoic. They reproduce most often through sexual mode. Certain acellular organisms such as viruses and viroids aswell in lichens can be found in this is not part of the five kingdom classification system. Viroids : In 1971, T.O. Diener discovered a novel virus that could be transmitted to other people. was less harmful than viruses, and it was responsible for potato tuber illness. Best NEET Coaching in Dimapur. It was The RNA was identified as a free RNA, but it was not able to produce the coat of protein that is typically found in viruses. which is why it was named hence the name. The RNA in the viroid was low in molecular weight. Prions : In modern medical science, certain neurodegenerative diseases that are infectious They were discovered to be carried by an agent comprised of folds that are abnormally folded protein.
The agent was comparable in size to viruses. The agents were referred to as prions. The most well-known ailments caused by prions include bovine spongiform Encephalopathy (BSE) often referred to as mad cow disease, which affects cattle, and Its corresponding variant, Cr-Jacob disease (CJD) in humans. Lichens : Lichens are symbiotic associations i.e. mutually beneficial interactions between fungi and algae. Algal components are referred to as fungal component and phycobiont as mycobiont, both of which are autotrophic and heterotrophic and heterotrophic. Best NEET Coaching in Dimapur. Algae make food for fungi and heterotrophic Shelter and absorb minerals substances and water from its companion. It is so close that when one sees the lichens in the wild, one would Never would you imagine having two different species of organisms in them.
Lichens They are excellent indicators of pollution They don’t grow in areas that are polluted. Are you aware that all organisms, even the largest, start their life from a single cell? You may wonder how a single cell then goes on to form such large organisms. Growth and reproduction are characteristics of cells, indeed of all living organisms. All cells reproduce by dividing into two, with each parental cell giving rise to two daughter cells each time they divide. These newly formed daughter cells can themselves grow and divide, giving rise to a new cell population that is formed by the growth and division of a single parental cell and its progeny. In other words, such cycles of growth and division allow a single cell to form a structure consisting of millions of cells.
10.1 CELL CYCLE Cell division is a very important process in all living organisms. During the division of a cell, DNA replication and cell growth also take place. All these processes, i.e., cell division, DNA replication, and cell growth, hence, have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes. Best NEET Coaching in Dimapur. The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. 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.
CELL CYCLE AND CELL DIVISION Phases of Cell Cycle A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours (Figure 10.1). However, this duration of cell cycle can vary from organism to organism and also from cell type to cell type. Yeast for example, can progress through the cell cycle in only about 90 minutes. Best NEET Coaching in Dimapur.
The cell cycle is divided into two basic phases: l Interphase l M Phase (Mitosis phase) The M Phase represents the phase when the actual cell division or mitosis occurs and the interphase represents the phase between two successive M phases. It is significant to note that in the 24 hour average duration of cell cycle of a human cell, cell division proper lasts for only about an hour. The interphase lasts more than 95% of the duration of cell cycle. The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis).
The interphase, though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. The interphase is divided into three further phases: l G1 phase (Gap 1) l S phase (Synthesis) l G2 phase (Gap 2) G1 phase corresponds to the interval between mitosis and initiation of DNA replication. During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA. S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1 , even after S phase the number of chromosomes remains the same, i.e., 2n.
In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. During the G2 phase, proteins are synthesised in preparation for mitosis while cell growth continues. How do plants and animals continue to grow all their lives? Do all cells in a plant divide all the time? Do you think all cells continue to divide in all plants and animals? Can you tell the name and the location of tissues having cells that divide all their life in higher plants? Do animals have similar meristematic tissues? A diagrammatic view of cell cycle indicating formation of two cellsfrom one cell M Phase. Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. Best NEET Coaching in Dimapur. These cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle.
Cells in this stage remainmetabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism. In animals, mitotic cell division is only seen in the diploid somatic cells. However, there are few exceptions to this where haploid cells divide by mitosis, for example, male honey bees. Against this, the plants can show mitotic divisions in both haploid and diploid cells. From your recollection of examples of alternation of generations in plants (Chapter 3) identify plant species and stages at which mitosis is seen in haploid cells. 10.2 M PHASE This is the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell. Best NEET Coaching in Dimapur. Since the number of chromosomes in the parent and progeny cells is the same, it is also called as equational division. Though for convenience mitosis has been divided into four stages of nuclear division (karyokinesis), it is very essential to understand that cell division is a progressive process and very clear-cut
lines cannot be drawn between various stages.
Karyokinesis involves following four stages: l Prophase l Metaphase l Anaphase l Telophase 10.2.1 Prophase Prophase which is the first stage of karyokinesis of mitosis follows the S and G2 phases of interphase. In the S and G2 phases the new DNA molecules formed are not distinct but intertwined. Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation (Figure 10.2 a). The centrosome, which had undergone duplication during S phase of interphase, now begins to move towards opposite poles of the cell. The completion of prophase can thus be marked by the following characteristic events: l Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere. l Centrosome which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters.
The two asters together with spindle fibres forms mitotic apparatus. You have studied mitosis in onion root tip cells. It has 16
chromosomes in each cell. Can you tell how many chromosomes will the cell have at G1 phase, after S phase, and after M phase?
Also, what will be the DNA content of the cells at G1, after S and at G2, if the content after M phase is 2C? CELL CYCLE AND CELL DIVISION 165 Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic
reticulum, nucleolus and the nuclear envelope. Best NEET Coaching in Dimapur. 10.2.2 Metaphase The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell. By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope. This then, is the stage at which morphology of chromosomes is most easily studied.
At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere (Figure 10.2 b). Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the centre of the cell. Hence, the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole (Figure 10.2 b). The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate. Best NEET Coaching in Dimapur. The key features of metaphase are: l Spindle fibres attach to kinetochores of chromosomes.
l Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles. 10.2.3 Anaphase At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter
chromatids, now referred to as daughter chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles. As each chromosome moves away from the equatorial plate, the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind (Figure 10.2 c). Thus, anaphase stage is characterised by Figure 10.2 a and b : A diagrammatic view of stages in mitosis the following key events: l Centromeres split and chromatids separate.
l Chromatids move to opposite poles. Telophase At the beginning of the final stage of karyokinesis, i.e., telophase, the chromosomes that have reached their respective poles decondense and lose their individuality. The individual chromosomes can no longer be seen and each set of chromatin material tends to collect at each of the two poles (Figure 10.2 d). This is the stage which shows the following l Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. l Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei. l Nucleolus, golgi complex and ER reform. 10.2.5 Best NEET Coaching in Dimapur. Cytokinesis Mitosis accomplishes not only the segregation of duplicated chromosomes into daughter nuclei (karyokinesis), but the cell itself is divided into two daughter cells by the separation of cytoplasm called cytokinesis at the end of which cell division gets completed (Figure 10.2 e). In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane.
The furrow gradually deepens and ultimatelyjoins in the centre dividing the cell cytoplasm into two. Plant cells however, are enclosed by a relatively inextensible cell wall, thererfore they undergo cytokinesis by a different mechanism. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the
formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells. At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells. Best NEET Coaching in Dimapur.
In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut). 10.3 Significance of Mitosis Mitosis or the equational division is usually restricted to the diploid cells only. However, in some lower plants and in some social insects haploid cells also divide by mitosis. It is very essential to understand the significance of this division in the life of an organism. Are you aware of some examples where you have studied about haploid and diploid insects? Mitosis usually results in the production of diploid daughter cells with identical genetic complement.
The growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio. A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced. Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life. Best NEET Coaching in Dimapur.
The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes. Gametes are formed from specialised diploid cells. This specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called meiosis. Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms whereas fertilisation restores the diploid phase. We come across meiosis during gametogenesis in plants and animals. This leads to the formation of haploid gametes. The key features
of meiosis are as follows: l Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication. Best NEET Coaching in Dimapur. Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase. l Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologouschromosomes.
l Four haploid cells are formed at the end of meiosis II. Meiotic events can be grouped under the following phases: Meiosis I Meiosis II Prophase I Prophase II Metaphase I Metaphase II Anaphase I Anaphase II Telophase I Telophase II 10.4.1 Meiosis I Prophase I: Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. It has been further
subdivided into the following five phases based on chromosomal behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. During leptotene stage the chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene. This is followed by the second stage of prophase I called zygotene. During this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
However, these are more clearly visible at the next stage. The first two stages of prophase I are relatively short-lived compared to the next stage that is pachytene. During this stage, the four chromatids of each bivalent chromosomes becomes distinct and clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase. Best NEET Coaching in Dimapur. Crossing over leads to recombination of genetic material on the two chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over. The beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers.
These X-shaped structures, are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years. The final stage of meiotic prophase I is diakinesis. This is marked by terminalisation of chiasmata. During this phase the chromosomes are
fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. Diakinesis represents transition to metaphase.
Metaphase I: The bivalent chromosomes align on the equatorial plate (Figure 10.3). The microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes. Anaphase I: The homologous chromosomes separate, while sister chromatids remain associated at their centromeres (Figure 10.3). Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as dyad of cells (Figure 10.3). Best NEET Coaching in Dimapur.
Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus. The stage between the two meiotic divisions is called interkinesis and is generally short lived. There is no replication of DNA during interkinesis. Interkinesis is followed by prophase II, a much simpler prophase than prophase I.
10.4.2 Meiosis II Prophase II: Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis. The nuclear membrane disappears by the end of prophase II (Figure 10.4). The chromosomes again becom compact. Metaphase II: At this stage the chromosomes align at the equator and
the microtubules from opposite poles of the spindle get attached to the kinetochores (Figure 10.4) of sister chromatids. Anaphase II: It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together),
allowing them to move toward opposite poles of the cell (Figure 10.4) by shortening of microtubules attached to kinetochores.
Stages of Meiosis I Telophase II: Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells (Figure 10.4). 10.5 SIGNIFICANCE OF MEIOSIS Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution. Figure 10.4 Stages of Meiosis II According to the cell theory, cells arise from preexisting cells. The process by which this occurs is called cell division. Any sexually reproducing organism starts its life cycle from a single-celled zygote. Best NEET Coaching in Dimapur.