Cells the living units
CELLS: THE LIVING UNITS
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- 16 BIOMEMBRANES Cells communicate with their environment by cell membranes.Biomembranes (= cell membranes = Biological membranes) are quasifluid film - like portions about 75Å thickness. They occur around the protoplast and eukaryotic cell organelles (mitochondria) as well as inside some organelles. Cell membrane when used alone refer to Plasmalemma of a cell only. Cell membranes refer to Biomembranes of all cells. Cell membrane is thin film-like, outermost, elastic, selectively Permeable , living, quasifluid, asymmetric, sheet like, trilaminar, repairable, lipoproteinaceous, hydrophilic layer.
Cells the living units Cell membrane is also called as Plasma membrane, plasma lemma or Biomembrane The term cell membrane was introduced by Nageli and Cramer (1885) for outer membrane covering of the Protoplast. It was replaced by plasmalemma or plasma membrane by Plowe (1931). Plasmamembrane occurs in Nucleus, mitochondria as Double membrane Lysosome , Golgi apparatus, microbodies and, Vacuoles as Single membrane. Cell Membranes are not visible under optical microscope it is visible under Transmission electron microscope (TEM) as Trilaminar i.e. Three layered with middle electron transparent layer and narrow electron layers on either sides. CHEMICAL COMPOSITION OF CELL MEMBRANE Chemically it consists of proteins (50 - 70%) lipids (20 - 50%) and carbohydrates (1- 5%) Nucleic acids like DNA RNA are absent. Proteins are globular belong to different types like structural, enzymatic, carrier, Permeases and receptor. Thousands of different types of proteins can occur in cell membranes. (a) Carriers for transporting specific molecules into or out of the cell (b) Receptors. For immediate flow of informations into the cells Lipids form the main bulk and exist in the form of phospholipids. Lipids and integral proteins are amphipathic in nature (i.e. have both polar hydrophilic and nonpolar hydrophobic) ends. The Hydrophobic ends are situated inside the bilayer while the Hydrophilic groups are directed outwards. Thus, the membrane is held together primarily by hydrophobic attraction. However, the lipids have links in their fatty acid tails these links prevent close packing of molecules and make the membrane structure more fluid. The fluidity increases with decreasing length of fatty acids. In Recognition mechanism the sugar portions of glycolipids and glycoproteins are involved. (a) Sugar - recognition sites two neighbouring cells may bind with each other causing cell - to cell adhesion. This enables cells to orientate themselves and to form tissues (b) Glycoproteins provide basis of immune response and various control systems, where they act as antigens. (c) Glycoproteins are important in the body's defence against foreign bodies phagocytes seem to distinguish foreign bodies (nonself) form the animal's own cells (self) in a similar manner. Carbohydrates occur only at the outer surface of the membrane Carbohydrates are mainly as branched or unbranched Oligosaccharides Eg :-Hexoamine, Sialic acid They are attached to external surface of phospholipids and Proteins forming glycolipids and glycoproteins which form cell coat called as Glycocalyx. The ratio of lipids to proteins is O.8 to 4.1
Cells the living units In Human RBC 40 : 52 Cell membrane 40 : 58 Muscles 35 : 65 - 17 Molecular organisation:- Important models are lamellar and mosaic (a) Lipid Model - (Overton 1900) - Proposed cell membranes be formed of a layer of lipids. Gorter & Grendel supported it. (b) Lattice Model - (by wolpers - 1941) - According to him lipids were thought to be distributed in the meshes of protein frame work. (c) Lamellar or Sandwitch model- Danielli (1935) and Davson, 1935 proposed a "trilaminar Model' According to this, the plasma membrane is formed of a bimolecular layer of Phospholipids (35Å thick) sandwitched between two layers of Proteins (each 20 Å thick). Thus the total thickness of plasma membrane, should be 20Å + 35Å + 20Å = 75Å (i.e. 75Å) Robertson's Model - J. D. Robertson (1959) Proposed unit membrane concept" According to this all biological membranes have the same basic structure (a) 75Å in thickness. (b) A characteristic trilaminar appearance when viewed with electron microscope (c) The three layers are a result of the same arrangement of Proteins and lipids as in sandwitch model. Greater membrane model of Lehninger (1968) : According to him, The inner surface of the membrane is covered by unconjugated proteins and outer surface by glycoproteins to which oligosaccharide side chains are attached. (d) Fluid Mosaic model (protein Mosaic Model):-By Singer and Nicolson (1972)It is the latest and most widely accepted model. According this the cell membrane consists of a highly viscous fluid matrix of two layers of phospholipid molecules. These molecules act as a relatively impermeable barrier to the passage of most water soluble molecules. Protein molecules or their complexes occur in the membrane, but not in continuous layer; instead, these occur as separate particles asymmetrically arranged in mosaic pattern. These Proteins are of two types (a) Peripheral or extrinsic proteins are loosely bound at the polar surfaces of lipid layers. (b) Integral or intrinsic proteins are deeply penetrate into the lipid layer Some of the integral proteins penetrate through the phospholipid layers and project on both the surfaces, These are called Trans membranes or Tunnel proteins. ORIGIN OF CELL MEMBRANE - arises as an independent structure during cell growth by self assembly of its components. ER forms membrances of organelles. Fluidity of membranes It was proved by Frye and Edidin (1970). They are quasifluid because their ability for instant repair and fusion and lateral movement of proteins. Fluidity decreases with lowering of Temperature and increases with rise in temperature Infoldings of plasmamembrane Three types of infoldings are there
Cells the living units (a) out pushings (Evaginations) like microvilli, flagellary or ciliary sheaths, stereocilia Microvillinumerous (upto 3000), fine plasmalemma evaginations found in intestinal epithelial cells as striated or brush border epithelium, increases area of absorption Flagellary sheaths- True Cilia and flagella are covered by plasmamembrane sheath Stereocilia- Non motile elongated evaginations of plasma membrane, Secretory or Sensory Eg:- Macula, crista, Epididymus. (b) Inpushings (Invaginations) like Pores. Pores- At places plasmamembrane is connected with ER forming pores leading to channels of ER Mesosomes (Fitz Jammes, 1960) - present in Gram (+) ve bacteria, Infoldings of cells membrane, help in respiration septum formation, seperation of replicated DNA and Synthesis of materials for extracellular transport (c) Junctional complexes (= Intracellular junctions = cell Junctions ) - These are modifications of cell membranes. Characteristic of animal cells helps to connect cells to a tissue together. - 18 Cell Junction Strip around the cells called Zonula (plural Zonulae) As spot called Macula (Plural Maculae) Fusions as Occludens (Plural occludentes) Cementing material as adherens (Plural adherentes)
Some Cell Junctions
Desmosomes- Common in stratified squamous Epithelium, consists of intercellular cementing material, intercellular thickened areas, linkers and fine tonofibrils to glue adhere cells together at places like spot welding. Hemidesmosomes Intercellular thickening is present on one side and replaced by collagen fibrils. Tight Junctions- Common in Epithelial cells of capillaries and brain cells, membranes of two cells appear to fuse in this region. Gap junctions - have hydrophilic intercellular channels for allowing movement of substances have protein cylinders called connexons common in cells of heart wall. Terminal Bars or Belt desmosomes - These are desmosomes without tonofibrils and discoid thickenings Interdigitations- formed by infoldings of two adjacent membranes at place of contact common in epithelial cells. Septate desmosomes- Here tonofibrils and intercellular cement both are absent. Instead a number of transverse septa are found Eg: - Invertebrates. Membrane Transport Passage of Metabolites , Bye-Products and Biochemicals across biomembrane is called membrane Transport. Size of particle passing through plasmalemma is 1 – 15Å Membrane Transport occurs through Four methods.
Cells the living units (a) Passive Transport (b) Facilitated, (c) Active (d) Bulk Passive Transport - Energy is not utilised. Occurs through diffusion and osmosis. Diffusion - The movements of particles from the region of their higher concentration or electrochemical potential to the region of their lower concentration. Osmosis - discovered by Abbe Nollet (1748) Diffusion of water across a semipermeable membrane that occurs under the influence of an osmotically active solution Osmotic Pressure(O.P) is the maximum pressure developed in osmotically active solution due to entry of water into it across a semipermeable membrane. Osmometer is useful for measuring O.P Direction of osmosis is governed by tonocity of external solution and DPD of cell. Tonocity is the tension developed by a system when it is placed in a solution. It is of three types. (a) Hypotonic - Less osmotic concentration or O. P as compared to another. (b) Isotonic - Same osmotic concentration or O.P Isotonic solution do not show osmotic movement. 0.9% NaCl and 0.1 Glucose solution are Isotonic to RBC. (c) Hypertonic - More osmotic concentration Endosmosis and Exosmosis are determined by Tonocity. If a cell kept in hypertonic solution, exosmosis occurs. Exosmosis is osmotic loss of water from a cell or system when the latter is placed in hypertonic solution. Eg:- Drops of water are seen oozing out of sliced. Cucumber if the same is sprinkled with salt. Continuous exosmosis due to outer concentrated hypertonic solution causes crenation in animal cells. Endosmosis is osmotic entry of water in an osmotic system or cell. Endosmosis causes plant cells to become turgid but animal cells burst in water/ hypotonic solution. - 19 Passive Transport Mechanism - Passive Transport can continue to occur if the absorbed solute is immobilised. Two methods are there. (a) Hydrophilic Membrane channels are Narrow channels formed in the membrane by tunnel Proteins water, CO2, O2 diffuse through these channels. Small ions and small water soluble solutes (b) Lipid Matrix permeability - Lipid soluble substances pass through the cell membrane according to their solubility and Concentration gradient. Eg: Triethyl citrate, Ethyl alcohol, Methane. (c) Facilitated Diffusion or Facilitated Transport is passage of substances along the concentration gradient without expenditure of energy and with the help of permeases. Permeases are the special Permeating substances.They form path ways for movement of certain substances.This transport occurs in case of some aminoacids, nucleotides and Sugars.
Cells the living units (c) Active Transport is the transport of materials across the membrane by utilising energy. Large sized molecules and particles move against concentration gradient through this transport. This transport is mediated by carriers and gated channels. Carriers are movable intrinsic proteins which have affinity for specific solute particle. Carrier combines with solute particle and forms carrier- solute complex. ATP is required for moving this carrier. Gated channels are opened by either in electrical potential or specific substances, Eg :- Calcium channels Pumps- Active transport systems are also called pumps E.g :- Na+ - K+ pump, K+ Pump, H+ Pump. Na+ – K+ Pump operates across many animal membranes. Three Na+ ions are passed out while two K+ ions are pumped in for every hydrolysis of ATP. E.g :-Sea gulls and Penguins Operate Na+ – K+ Pump for excreting NaCl through their Nasal glands. Secondary Active Transport is the active transport by permeation of other substances. It is of 2 types (a) Counter- transport -- Ca2+ and H+ movement outwardly as excess Na+ Passes inwardly. (b) Co transport- Transport of Glucose and some aminoacids along with inward pushing of excess Na+. Bulk Transport is Active Transport through Vesicles common in Secretory and Excretory cells macromolecules, liquid droplets, bacteria, large sized foreign particles are transported in Bulk by Vesicles. Vesicles are formed by infolding / unfolding of cell membranes using ATP. It is of 2 types Exocytosis (cell vomiting) - is bulk exit of materials from inside to the outside of the cells with the help of vesicles. It is also called as Ephagy, reverse pinocytosis or Emeiocytosis. E. g :- Secretion, Excretion and removal of undigestible food, release of neurotransmitter by nerve cell etc. (a) Endocytosis (cell ingestion) is bulk active intake of extra cellular useful material from outside to inside of cell by carriers Endocytosis, based on the nature of substance involved in transport it is of 2 types (b) Pinocytosis (cell drinking) is bulk intake of extracellular fluid with the help of Vesicle called Pinosome Digestion of food in Pinosome does not occur No lysosomal activity and exocytosis occur. (c) Phagocytosis (cell eating) is bulk intake (ingest) or engulfing of solid material with the help of vesicles called phagosome or food Vacuole. Undigested food is thrown out by ephagy (exocytosis) E. g:- Monocytes, Neutrophils (WBCs), Macrophages, Kuffer cells in liver, Reticular cells in spleen, histocytes in connective tissue. Pinocytosis reported by Lewis (1937) Phagocytosis reported by MetchniKoff (1883) Ingestion of minute colloidal particles by WBC or Macrophage is called ultraphagocytosis . Role of membranes in cellular movements by two methods, Psuedopodia and undulations. - 20 -
Cells the living units (a) Undulations are regular waves of membrane protrusions or rufflings which occur in the forward region of the cells. (b) Pseudopodia are temporary out growths of the cell which are used for locomotion. e.g. :- WBC, Macrophages. STRUCTURE OF CELL The cell is the basic structural and functional unit of life. The study of cell is called as cytology or cell Biology. Term cell (L. Cella = hollow space), coined by Robert Hooke (1665) is misnomer as cell is not a hollow structure. Robert Hooke (1665) discovered cells in a section of bottle cork (Quercus suber = oak plant). His observations were published in his book, Micrographia. Free living cells (Bacteria, Protozoa) were discovered by - Leewenhoek (1674) Dutrochet (1824) Suggested that all plants and animals are made of cells. Father of cytology - Robert Hooke. Father of Modern cytology - C. P. Swanson. Term cytology was coined by Hertwig (1893) who wrote a book entitled "cells & Tissues" Malphigi, the father of microscopic anatomy (1661), called cells as Saccules (Utricles). Cell theory is not applicable to viruses Since they are acellular. Prokaryotes - The organisms which lack true nucleus are called prokaryotes. Eukaryotes - The organism with true Nucleus. PROTOPLAST Unit mass of Protoplasm contained in a cell, term coined by Hanstein. Protoplast has four important components - Plasmalemma + cytoplasm + Nucleus + Vacuoles. PLASMA MEMBRANE The outer living limit of the cell is plasma membrane. Plasma membrane is also called as - Plasmalemma, cytoplasmic membrane, cell membrane, Ectoplasm. Thickness of Plasma membrane is 75 Å. It is made up of Phospholipids, Proteins & carbohydrates. Trilamellar model/sandwitch model of plasma membrane was proposed by - Danielli & Davson (1935) In Sandwitch model of Plasmamembrane the bimolecular lipid Zone (35Å thick ) is sandwiched, between two protein layers (20Å thick each). Unit membrane concept states that all the membranes in the cell have same trilamellar structure. Fluid mosaic model is the widely accepted model of plasma membrane proposed by "Singer and Nicolson (1972)".
Cells the living units
Plasma membrane is dynamic and Semipermeable. It regulates Endocytosis and Exocytosis. Eydocytosis - Entry of materials into the cell. Exocytosis - Exit of materials out of the cell. Phagocytosis - Engulfing of solids through plasmamembrane. Pinocytosis or cell drinking - Ingestion of liquids through Plasmamembrane Cell vomiting or Ameocytosis - expulsion of liquids through plasma membrane Desmosomes - Localized thickenings in the Plasmamembrane. Functions of plasma membrane - Osmosis, Passive and Active absorption. Plasma lemma - Nageli and cramer (1855) gave the name cellmembrane to the outer boundary of protoplasm. Overton (1899) proved its existence. J. Q - Plowe (1931) later on called it plasmalemma. PROTOPLASM The viscous fluid in a living cell is Protoplasm. Dujardin coined the term 'Sarcode' for Protoplasm. - 21 The term protoplasm was coined by J.E. Punkinje "Protoplasm Theory of Inheritance" was proposed by Max Schultze Huxley stated that 'The physical basis of life is Protoplasm' Book written by Huxley about protoplasm is "The physical basis of life Protoplasm is a polyphasic colloid. It exists in the form of sol- gel complex Sol - Quasi - fluid state of a colloidal system. Gel - Quasi - Solid state of a colloidal system. Protoplasm is divided into Nucleoplasm (Nuclear protoplasm) and cytoplasm (extranuclear protoplasm). PH of protoplasm around 6.8. Protoplasm contains 85 - 90% water and constitutes 95% weight of an organism. The main constituents of Protoplasm are oxygen (62%) carbon (20%), hydrogen (10%) and nitrogen (3%) Besides, It also contains Ca, P, Cl, S, K, Na, Mg, I and Fe. Major component in Protoplasm is water, main organic component is protein ( 7 - 10%), main element is oxygen. The Specific gravity of protoplasm - Slightly greater than one. Protoplasm Coagulates and dies on heating above 60°C. Protoplasm exhibits Brownian movement and Tyndall effect (These properties indicate colloidal nature of protoplasm.)
Cells the living units Important theories on the organization of protoplasm are Reticular Hypothesis - Heitzmann, Fibrillar Hypothesis – Flemming Granular Hypothesis – Altmann The Most accepted Theory is colloidal Hypothesis proposed by Wilson (1925). Colloidal particles in the protoplasm also shows solution, gelation, Imbibition and possess electric charge proteins can be separated from crystalloid by dialysis. Protoplasm, a living substance shows properties like Nutrition, Metabolism and Irritability. CYTOPLASM Cytoplasm extends from plasmamembrane to Nuclear membrane. The extra nuclear portion of Protoplasm is called cytoplasm. It is composed by cytoplasmic matrix and cytoplasmic structures. Cytoplasmic matrix is an amorphous/transclucent homogenous colloidal liquid called Hyaloplasm or cytosol It consists of various inorganic molecules like water, Salts of Na and other metals and various organic compounds like carbohydrates lipids, proteins, Nucleoproteins, Nucleic acids (RNA, DNA) and a variety of enzymes. In the cytoplasmic matrix certain living and non- living structures are found. The Non living structures are paraplasm, deutoplasm or inclusions while the living structures are membrane bound, are called as cell organelles. The major cytoplasmic cellorganelles are - ER, Golgi complex , Mitochondria, Ribosomes. The Minor or micro cell organelles or micro bodies are - Lysosomes, Glyoxysomes, etc. The fabric of microtubules and microfilaments present in the cytosol is called - cytoskeleton. Cytoplasm exhibits constant streaming movements called cyclosis CELL ORGANELLES (ORGANOIDS) These are of four types on the basis of membranes (a) Organelle bounded by Single unit membrane Eg :- ER, golgi bodies, Microbodies (Peroxisomes, lysosomes) (b) Organelle bounded by double membrane Eg :- Mitochondria, and Nucleus. (c) Organelle bounded by Triple membrane Eg :- Transosomes. (d) Organelle without any membrane Eg :- Ribosomes, centriole, spindle, Microfilaments and Microtubules, Nucleolus, ENDOPLASMIC RECTICULUM ER is noticeable only in Electron Microscope.
Cells the living units
ER is a major component in the Endomembrane system. ER is Extensive membrane bound network of channels which acts as intracellular transport system. Garnier for the first time had described them as filamentous structure, ergastoplasm. They were first discovered by Porter (1945),Claude and Fullman. Endoplasmic reticulum name was coined by porter (1953). ER extends from outer nuclear membrane upto plasma membrane. ER is found in all Eukaryotic cells except mature erythrocytes. It is abundant in liver, Pancreas and other actively synthesizing cells. ER is absent in RBC, ova, Embryonic cells and prokaryotes. In muscles, It is called Sarcoplasmic Reticulum, in eyes called Myeloid bodies and in Nerves as Nissilgranules. Important components of ER are (1) Cisternae (2) Tubules (3) Vesicles. Cisternae are parallel inter connected flattened sacs of 40 – 50 nm thickness). Tubules are often branched, network, 50 – 100 nm in diameter Vesicles are round or oval, 25 – 500 nm in diameter ER contains two surfaces cytoplasmic surface and luminar surface. Enzymes occur both on the cytoplasmic surface (Eg: - P - 450, Cyt b5, some Reductases, Nucleotidase) and luminar surface (Eg :- Glucose 6- phosphatase, peptidases, β- glucoronidase). ER Originates from the ground substance (hyaloplasm) or infoldings of plasma membrane or evaginations of Nuclear membrane. However, exact nature of Origin is unknown. ER is of two types - Smooth Endoplasmic Reticulum (SER) and Rough Endoplasmic Reticulum (SER) RER- ER studded with Ribosomes also called as Granular ER (GER) It is mainly made up of cisternae. Well developed in Cells actively engaged in Protein synthesis like Plasma cells and Goblet cells Functions or RER - (1) Protein Synthesis (2) Reorganisation of Nuclear Envelope SER Ribosome Free part of ER is called SER or Agranular(Ag. ER). It consists of Tubules only. Abundant in the cells engaged in Glycogen and lipid metabolism. Functions of SER - (1) Detoxification in liver (2) Synthesis of Vitamins, carbohydrates fats and sterols. (3) Glycogenolysis (4) Origin of Golgi complex Common features of ER - (1) Intra cellular transport (2) Mechanical support (3) Increases the surface area for metabolism. Microsomes (Claude - 1941) are the fragments of ER containing Ribosomes. They contain large amount of RNA and phospholipids. Structure not found in Intact cells – Microsomes Transitional ER is E.R without Ribosomes Annulated ER - ER having pores (Mecullo, 1972) ER acts as a circulatory or Transporting system. The transport of various secretory products of granular ER may be as follows
Cells the living units
Granular ER → Agranular ER → Golgimembrane → Lysosomes or Secretory granules. GOLGI COMPLEX An Italian physician Camillo Golgi (1898) first recognized this structure in nerve cells of barn owl by Silver metallic Impregnation Technique. Also called as Golgi bodies, Golgi apparatus, Golgisome, lipochondria, Dalton complex, Idiosome, Baker's body or middle man of cell. Golgi complex is found in all the eukaryotic cells except Red Blood cells. Units of Golgicomplex are called – Dictyosomes The Components of Dictyosomes - (a) Cisternae (3 - 7) (b) Vacuoles (3) vesicles. A dictyosome has a stack of Usually 3 - 12 cisternae with swollen ends tubules and vesicles Golgi complex shows Polarity. Having two faces (a) Maturing face (m) or Trans face or Distal face or Concave face - Near cell membrane (b) Forming face (F) or Cis or convex face or proximal face- towards Nuclear membrane From 'm' face lysosomes and Secretory vesicles are formed. Golgi complex is abundant in glandular cells. The main function of Golgi complex – Secretion FUNCTIONS OF GOLGI COMPLEX (a) Seat of origin of Primary lysosomes (b) Seat of Formation of glycolipids & Glycoproteins (c) It Packs and transports certain materials like proteins and polysaccharides out of the cell. (d) Secretion of Hormones such as Insulin. (f) It helps in storage of Secretory products (g) It forms acrosome in sperms, Golgi complex also forms yolk and cortical granules in Eggs, lactoprotein in mammary glands, Golgi complex regulate fluid balance of cell. All secretory cells are rich in Golgi bodies. Origin - Elements of Golgi complex may arise from (a) ER (b) Nuclear Envelope (c) Pre-existing Golgi complex. MYTOCHONDRIA Cell organelle concerned with aerobic respiration – Mitochondrion They were first described by Kolliker (1880) in the muscle cells. (1882) He called them as Sarcosomes. Fleming called them as Fila.
Cells the living units Altmann in 1894 described them as 'Bioplasts' The term 'mitochondria' was coined by Benda in 1897 Synonyms for Mitochondria are - Bioplasts, Chondriosomes, chondriomites, plastochondria, Plastosomes, filavermicules, cellular furnaces, Fuchsinophilic granules etc. Mitochondria are seat for cellular Respiration - Kingsbury. Vital Stain for Mitochondria Janus green B or gentian violet. In a normal liver cell, 1000 to 1600 mitochondria per cell. Shapes of mitochondria are generally rod-shaped (filamentous), Thread, Spherical (yeast), oval Size of mitochondrion - 1 - 10μm × 0.2μ. STRUCTURE OF MITOCHONDRIA Mitochondria is surrounded by two membranes called outer and inner membrane which are lipoproteinaceous ( lipids 25 - 35%) , Proteins 60 - 70%) The fluid filled space between membranes of the envelope - Perimitochondrial space or outer space. The inner compartment , also called mitochondrial chamber, is bound by the inner membrane. The mitochondrial chamber filled with dense proteinaceous material called matrix contains most of the enzymes of Kreb's cycle. Mitochondrion without outer membrane – Mitoplast Matrix contains Enzymes, Circular DNA, RNA, 70s Ribosomes etc. The inner membrane is convoluted to form finger like or plate like infoldings called cristae or Mitochondrial crests. The inner membrane is having electron carriers, stalked particles, Enzymes. Particles are of 8.5 nm in size called elementary or F1 particles or oxysomes or Respiratory assemblies or Fernandez - Moran particles Oxysomes were discovered by Fernandez -moran (1962) They contain a special ATPase Enzyme involved in Oxidative phosphorylation in oxysome head region. Main Function of oxysome- generation of ATP by oxidative phosphorylation Mitochondria produce energy in form of ATP also called as cellular currency also called 'power houses of the cell (by Seekevitz) The mitochondrial System of cell is called as chondriome. Matrix face of Mitochondria is called m – face Cytoplasmic face of Mitochondria is called C – face Each oxysome having a base piece (F0 particle) is 110Å in diameter the stalk (F5 - F6 particle) is 50Å and head (F1 particle) is 80 – 100Å in diameter.
Cells the living units
The F1 particle is made up of 5 types of subunits and capped by ATPase inhibitor protein. The mitochondrial DNA (M - DNA) has a melting temperature (Tm) different from between nuclear DNA. Molecular weight between 9 - 11 million, This DNA is 1% of total DNA of cell discovered by Nasbs (1966). Mitochondrial RNA (M - RNA) is resistant to the action of ribonuclease. Several Enzymes of krebs cycle are found in the hyaloplasm (outside the mitochondria) as well but there are two Enzymes α-ketoglutaricidehydrogenase and Succinic dehydrogenase which occurs inside the mitochondria only. Oxysomes were earlier termed as electron transport particles (ETP). The base (F0 particle) contains the proton channel. Origin of Mitochondrian. There are several views regarding the origin of mitochondrion. (a) They are formed by autoreplication. (b) They originate from tiny particles called promitochondria. (c) They arise by division or budding from the existing mitochondria. (d) Some believe that they originate from nuclear envelope through the process of evagination. Molecules that come out frequently from mitochondrion – ATP. 70% of cellular enzymes are found in mitochondria. Electron transport occurs on inner Mitochondrial Membrane. Life Span of Mitochondria 5 - 10 days. Best method of biochemical analysis of mitochondria - cell fractionation. Semiautonomous cell organelles – Mitochondria. Cell within a cell – Mitochondria. Cell organelles concerned with energy transformation reactions – Mitochondria. Mitochondria convert potential energy into Biochemical energy. RIBOSOMES Universal cell organelles - Ribosomes. Cell organelles concerned with protein synthesis - Ribosomes (hence called protein factories). Ribosomes were discovered by Palade (Hence called Palade granules) in animal cell called them as microsomes. Claude called these structures as ribosomes. Membrane less cell organelles - Ribosomes. Ribosomes are the smallest cell organelles (230Å in diameter). Eukaryotic ribosomes are - 80S & 70S ribosomes.
Cells the living units 80S ribosomes are found in cytoplasm. 70S ribosomes are found in Mitochondria. Ribosomes have two unequal sub units. 80S Ribosomes -> 40 S and 60 S subunits. 70S Ribosomes -> 30 S and 50 S subunits. Sub units of ribosomes are composed of - proteins & RNA (Hence called Ribonucleo protein particles). In eukaryotes, ribosomes are synthesized in Nucleolus (Ribosome factory). 60S sub units are attached to ER by means of proteins called - Ribophorins. 60S sub unit consists of - 40 proteins, 3 bits of rRNA (28S, 5.8S, & 5S). 40S sub units consists of - 30 proteins & a bit of (18S) rRNA. Chemical composition of 80S ribosomes - 40-60% RNA + 60-40% proteins. Sub units of ribosomes are combined by - Mg²+. A group (6 -8) of ribosomes attached to the same mRNA is called- Polyribosomes/Polysomes (or) ergosomes. Ribosomes are concerned with translation phase of protein synthesis. Ribosomes can be isolated by - cell Fractionatlon (homogenization & ultra centrifugation). (Rich, 1963). Centres of protein synthesis; site of protein synthesis; site of translation - Ribosomes. Enzyme for peptide bond formation is - Peptidyl transferase. Peptidyl transferase is located in - Larger subunit of ribosome (60 S or 50 S). Distinct sites in the larger sub unit are A - site (aminoacyl tRNA site). P - site (peptidyl tRNA site). Protein factories or work benches of proteins Ribosomes. MICRO ORGANELLES Lysosomes, Peroxisomes, Glyoxysomes, Sphaerosomes are together called Micro-organelles or Microbodies. All the micro organelles are single membrane bound structures. Lysosomes originate from Golgi complex, other micro-organelles develop from ER. LYSOSOMES Lysosomes were discovered by Christian de Duve (1955) in rat liver cells. Lysosomes are called enzyme packets since they store several (50) types of hydrolysing enzymes to digest (autolysis) almost every type of organic matter except cellulose.
Cells the living units
Lysosomes are abundant in digestive glands. The most important enzyme in lysosome is acid phosphatase. Function of lysosome - digestion. Digestion of food, foreign bodies, etc., by lysosomes is called - heterophagy. Digestion of the own cell components by lysosomes is called - autophagy/autolysis. Lysosomes are called suicidal bags or atom bombs or disposal units or scavenger of cell. Lysosomes carry on Intracellular and extra cellular digestion. Lysosomes are polymorphic having different types. Lysosomes having only digestive enzymes - primary lysosomes. Lysosomes enclosing food vacuoles, foreign bodies or cell organelles are called - secondary lysosomes. Lysosomes with undigested matter are called Residual bodies/Tertiary Lysosomes. Size of lysosomes 0.2 -0.8m in diameter. PEROXISOMES Peroxisomes are microbodies which are also known as small lysosome which are mostly found in liver cells and help in detoxification of toxic substances inside the liver like – H2O2, alcohol etc. MICROTUBULES & MICRO FILAMENTS Micro tubules were discovered by - Ledbetter & Porter. Micro tubules are made up of a protein called Tubulin. Structures organized by microtubules - Spindle fibres, flagella, cilia, centrioles, cytoskeleton. Microtubules as spindle fibres determine the plane of new cell wall formation. Microtubules, intermediate filaments and microfilaments together form a new work called Cytoskeleton. 26 CENTROSOME AND CENTRIOLES Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous peri centriolar materials. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cart wheel. They are mad up of nine evenly spaced peripheral fibrils, of tabulin protein. FUNCTION Centrioles form the basal body of cilia or flagella and spindle fibres that give rise to spindle apparatus during cell division in animal cell. CILIA AND FLAGELLA
Cells the living units
Cilia and flagella are hair like out growths of cell membrane. Cilia are small structure with work like oars causing the movement of either the cell or the surrounding fluid. Flagella or cilia have three parts : (i) Root (ii) Basal plate (iii) Shaft NUCLEUS Dynamic centre of the cells is - Nucleus. Nucleus was discovered by- Robert Brown (1831) in orchid cells. Organisms with a true nucleus are called - Eukaryotes. Cells are usually uninucleate (with a single nucleus). Cell without nucleus - Enucleate cell. The study of nucleus is Karyology. Fusion of nucleus karyogamy and division is karyokinesis. Cellular activities and morphology are regulated by - nucleus. The vehicle of heredity is - nucleus. In higher organisms somatic cells have diploid nucleus (definitive nucleus) and sex cells have haploid nucleus Interphase nucleus is convenient for the study of its structure. Nucleus consists of i) Nuclear envelope, ii) Nucleoplasm iii) Nucleolus & iv) Chromatin reticulum. Nuclear envelope (karyotheca) is a double membraned envelope. The fluid filled space between the nuclear membranes perinuclear space (100-3000A°). Nuclear envelope in eukaryotes disappears during prophase, reappears during telophase. Nuclear envelope originates from ER (annulated RER). Transport of materials between cytoplasm and nucleoplasm occurs through nuclear pores. Nuclear pore +annulus (Protein cylinder) are together called nuclear pore complex. Intranuclear part of protoplasm is called - nucleoplasm. Nucleoplasm is also called nuclear sap, karyolymph, Karyoplasm, etc. Spherical membrane less structure found in the nucleus is called nucleolus. Nucleolus was discovered by Fontana.(1781) Nucleolus consists of DNA, RNA and proteins. Nucleolus is concerned with biogenesis of Ribosomes (Hence called Ribosome factory). Secondary constriction associated with nucleolus formation is called nucleolar organizer region.
Cells the living units
DNA and Histones together form chromatin. Chromatin fibres together appear as a net work called - Chromatin reticulum.Nucleus consists of a constant number of Chromatin fibres. Chromatin fibres condense to form chromosomes during prophase of cell division. The tightly coiled and deeply stained chromatin of interphase nucleus is called heterochromatin. The less stained and uncoiled part of chromatin is called Euchromatin.Euchromatin has active genes and heterochromatin has inactive genes. - 27 The terms Euchromatin & Heterochromatin were coined by Heitz. Lumps of heterochromatin are called Chromocentres, Karyosomes, pseudonucleoli. CHROMOSOMES Rod shaped, deeply stained nucleoprotein bodies found in a dividing cell - Chromosomes. Chromosomes were discovered by Hofmeister in pollen mother cells of Tradescantia. The term ' Chromosome' was coined by Waldeyer. Chromosomes are visible during Mitosis and Meiosis. Best phase to observe chromosome morphology - Metaphase. Number of chromosomes is constant for each species. Best stage to count the chromosome number Diakinesis. Shape of the chromosome can be observed in Anaphase. STRUCTURE OF THE CHROMOSOME Longitudinal halves of a metaphase chromosome are called - Chromatids. Chromatids of the same chromosome (sister chromatids) remain attached at a point called primary constriction or centromere. Colourless constriction in the chromosome at which spindle fibres attach with the chromosome is called – primary constriction/centromere/Kinetochore/Kinomere. Centromere is useful for orientation of chromosomes at equitorial plane of the cell and their movement to opposite poles. Protein discs present on either side of centromere are also called - Kinetochores. Chromatic portions on either side of centromere are called - arms. Chromosome morphology is distinct with regard to relative lengths of arms, and position of centromere. Chromosome classification is based on the number of centromeres. Acentric - without centromere Monocentric - with one centromere Dicentric - with two centromeres
Cells the living units
Polycentric - with many centromeres Holocentric - with diffuse centromere Neocentric - with newly acquired centromere Usually chromosomes are monocentric. Chromosomes with genetically identical arms are isochromosomes (Darlington 1940). Constriction in the chromosome at which spindle fibres do not attach - Secondary constriction. Secondary constriction involved in nucleolus formation - nucleolar organizer region. Nucleolar organizer region contains - r RNA genes. Small spherical portion of chromosome distal to secondary constriction is called - Satellite/Trebent. Chromosome with satellite - SAT - Chromosome. The ends of chromosomes are called - Telomeres. Telomeres exhibit polarity and provide stability for chromosomes. Folded fibre model of chromosome was proposed by - Du Praw. De Robertis (1956) and Ris (1957) proved that pellicle and matrix are absent in the chromosome. Each chromatid has single Chromonema (mononemic). CHEMICAL COMPOSITION OF CHROMOSOME Chromosomes contain DNA, Histones, RNA and non histone proteins. Histones are basic proteins. 5 types (H1, H1A, H2B, H3 and H4). Repeated units of chromatin - Nucleosomes. The term 'nucleosome' was coined by Wood-cock(1973). Cells the living units - 28 Nucleosome model was proposed by R.Kornberg. Chromosomes concerned with determination of body characters - Autosomes. Chromosomes concerned with chromosomes/allosomes/heterosomes Eg. X, Y Chromosomes Sex chromosomes were discovered by C.E.Mc Clung. X - linked inheritance is called Sex linked inheritance. Characteristic chromosome complement of a species observed at metaphase of somatic cell is called Karyotype. Diagrammatic representation of Karyotype is called - Idiogram. Chromosomes of abnormal size are called giant chromosomes. FUNCTIONS sex of the organism are called Sex
Cells the living units
Chromosomes are the Physical carriers of heredity or Vehicles of heredity. Chromosomal theory of inheritance was proposed by - W.S. Sutton (1902) and Theodore Boveri. Hereditory units are genes. Term 'gene' was coined by Johannsen. Gene theory (that genes are arranged in a linear order in the chromosome) was proposed by T.H.Morgan. Specific position of a gene in the chromosome is called - Locus. Genes which occupy the same locus in homologous chromosomes are called - alleles. Alternative forms of the same gene are called alleles. Genes found attached together in the same chromosome are called - Linkage group. Number of linkage groups in the organism is equal to the haploid number of chromosomes. All genes found in a haploid set of chromosomes are together called - genome. Genetic constitution of an organism is called - genotype. NUCLEIC ACIDS Nucleic acids are of two types - DNA & RNA (names were coined on the basis of sugar). Nucleic acids were discovered by - Friedrick Miescher from the nuclei of pus cells. Miescher called them nucleins and Altmann called them nucleic acids. Role of nucleins in heredity was suggested by Hertwig. Nucleic acids are unbranced, linear polymers of nucleotides. Structural units or building blocks of nucleic acids - nucleotides. Elements found in nucleic acids -C, H, 0 N, & P. Nucleotides are made up of a pentose sugar + N2, base + Phosphate radicals. Nucleoside consists of a pentose sugar and N2 base. The bond between sugar and N2 base is called glycosidic bond. The bond between sugar and phosphate phosphodiester bond. Nitrogen bases are of two types - purines & pyrimidines. Dicyclic N2, bases with two carbon - nitrogen rings are called purines (Adenine & Guanine). Monocylic N2 Bases with a single carbon - nitrogen ring are called - pyrimidines (Thymine, Cytosine & Uracil). In the polynucleotide chain, nucleosides are joined by Phosphodiester bonds. DNA
Cells the living units
DNA means Deoxyribonucleic acid. Biochemical with highest molecular weight and utmost importance - DNA. DNA is the genetic material. It is proved by transformation experiments on Pneumococcus conducted by Avery et al and by experiments on T2 phage by Hershey and Chase. DNA double helix model was proposed by J.D.Watson & F.H.C. Crick (1953). Scientists awarded Nobel Prize in 1962 for DNA structural model are Watson, Crick and Wilkins. Sugar in DNA is deoxyribose sugar. N2 bases in DNA are - Adenine, & Guanine (purines) Thymine and Cytosine (Pyrimidines). DNA is a right handed double helix. The two polynucleotide strands are coiled around a common axis in clockwise direction. The two polynucleotide strands of DNA are quite complementary and antiparallel. The polynucleotide strands are held together by hydrogen bonds. Purines & pyrimidines exist in 1:1 ratio which is due to complete complimentarity between polynucleotide strands. Length of DNA molecule is variable and diameter is constant (20 Å). Length of one coil of DNA is 34 Å. No. of nucleotides in one coil of DNA is 10 pairs or 20 nucleotides. Distance between successive nucleotide pairs is 3.4 Å and angle between them is 36°. DNA self replication takes place by Semiconservative method. Semiconservative method of DNA self replication was proposed by Watson & Crick and proved experimentally by Messelson and Stahl using N15 isotope in E.coli. Enzyme useful for DNA self replication is DNA polymerase. Synthesis of m RNA from DNA template is the heterocatalytic function of DNA. Proteins associated with DNA of eukaryotes are histones. Single stranded DNA is found in φ × 174 phage. Artificial DNA synthesis was carried by Arthur Kornberg. Artificial gene synthesis was carried by H.G. Khorana. Circular naked DNA is found in Mitochondria. RNA RNA means Ribonucleic acid. RNA is a single stranded polymer of ribonucleotides. Double stranded RNA is found in certain viruses - Reo virus, Wound tumour virus. RNA is nongenetic material.
Cells the living units
But in viruses it is the genetic material eg. TMV. N2 bases in RNA are - Adenine guanine, uracil and cytosine. In RNA Uracil replaces Thymine. Chargaff's rules are not applicable to RNA since it is single stranded. RNA is synthesized from DNA. The function of nongenetic RNA is protein synthesis. Non genetic RNA is of 3 types - m RNA, r RNA & tRNA. Largest type of RNA is ribosomal RNA (rRNA)and the smallest one is transfer RNA (t RNA). RNAs are produced from template DNA as complementary strands. Enzyme catalysing RNA synthesis is RNA Polymerase. mRNA Formation of m RNAfrorn DNA template is called Transcription. Direct template for protein synthesis is m RNA. m RNA was discovered by Jacob & Monod. Genetic information for protein synthesis is found in m RNA in the form of triplet codes or codons. Codon is a sequence of 3 successive nucleotides. m RNA constitutes 2-5% of total cellular RNA. m RNA is unstable. tRNA tRNA is also called soluble RNA, adopter RNA or interpreter of genetic code. - 30 tRNA consists of about 75-85 nucleotides (smallest type of RNA). tRNA is stable and constitutes about 15% of total RNA. tRNA is useful to bring suitable aminoacids into ribosomes during protein synthesis. Clover leaf model for RNA was proposed by R. Holley. tRNA with attached amino acid is called charged t RNA/aminoacyl t RNA. tRNA recognises the codon in m RNA by its anticodon. Three dimentional (3D) L – shaped model for t RNA was proposed by S.H. Kim, et al. rRNA Ribosomal RNA (r RNA) is a component part of ribosomal sub units. rRNA is the largest type of RNA and has several thousands of nucleotides. rRNA constitutes about 80% of total RNA.
Cells the living units
rRNA is in the form of an irregular double helix. Enzymes which destroy nucleic acids are nucleases (DNase, RNase).
CELL CYCLE
1. Reproduction at cellular level is called cell division. Cell division was first studied by Strasburger in plants.W. Fleming (1882) in animal cells and Prevost and Dumas (1824) in frog egg. 2. All organisms start their life from a single Primordial cell called Zygote. 3. In Multicellular Animals growth and Development take place due to cell division. Mitogens are the agents which stimulate cell division. Mitotic poisons are inhibitors of cell division. Azides and cyanides inhibit prophase,Colchicine checks spindle formation ,Chalones inhibit cell division in vivo and in vitro both ,Ribo nuclease blocks prophase, Heat shock prevent cell division and Mustard gas agglutinate all chromosomes. 4. Cell division is divided mainly into two phases (a) Karyokinesis (b) cytokinesis 5. Division of nucleus is Karyokinesis Division of cytoplasm is cytokinesis. Animal cytokinesis is centripetal. 6. Karyokinesis is of two types : (1) Mitosis (2) Meiosis MITOSIS OSIMitosis is called Equational Division because it results in the formation of two identical daughter Nuclei It is also called Indirect Nuclear division because of the formation of spindle and visible changes in chromosomes.It is also called Somatic Nuclear division because it occurs in the Nucleus of Somatic cells.The term Mitosis was used by Fleming in 1882 Mitosis is divided into two phases (1) Inter phase (2) Division phase (M - phase) INTERPHASE The interval between two division phases is called Interphase It is the longest phase in Mitotic cycle. It is also called as Resting phase Metabolically active stage of Mitosis is Interphase. Period of Intense biosynthetic activity is interphase Interphase is divided into 3 phases G1 phase, S phase, G2 phase G1phase is called First growth phase, gap period, Presynthetic Period ) S phase is synthetic phase, Replication of DNA (Chromosomes) occurs during S phase. G2 phase is Second growth phase or Post synthetic Period. Metabolic activities like growth of cytoplasm and its constituents occur during this phase.
Cells the living units Interphase takes one or two days for its completion. CHANGES IN INTERPHASE Nucleus is large and distinct Nucleus has got nuclear membrane Chromatin network is not distinguished into chromosomes. Nucleolus is also prominently visible . DIVISION PHASE (M - PHASE) Time taken for completion of a division is called division phase Time interval for completing a division as well as for its preliminary preparation is called cell cycle. Cell cycle includes Interphase and a Division phase. For convenience of study division phase is divided into 4 phases. 7. M- Phase:it is having following stages PROPHASE Prophase The chromatin network begins to coil and appear as longthread-like structures called as chromosomes. Each chromosome consists of two chromatids. They lie side by side and are join at a point called as centromere. The nucleolus gradually disappears from early prophase to late prophase The nuclear membrane also starts disappearing in the late prophase. METAPHASEe It starts with the complete disappearance of Nuclear Membrane The chromatids become shorter and thicker due to dehydration and condensation Chromatids acquire a specific shape & size. The chromatids move to the centre of the cell with their centromeres this is called metakinesis Five Fibrils appear in the cell cytoplasm and are organised to form spindle fibres. Asters are formed. The centromere of each chromosome lies on the equator and is attached to the spindle fibres. The best stage of Mitosis for analysing the chromosomes is Metaphase ANAPHASE The centromere of each chromosome is divided into two . So each chromatid get its own centromere. Chromatid with its own centromere is regarded as a Daughter Chromosome. The spindle fibres get attached to the centromeres of their side
Cells the living units The spindle fibres gradually shortens, by this each chromatid with its centromere is pulled apart towards its respective pole. At the poles, each chromatid now behaves as Independent chromosome so the chromosome number of daughter nuclei is maintained the same The centromeres of the chromosomes lie towards the poles of the spindle, where as the arms are directed towards the Equator, At the End of Anaphase, a constriction appears in the middle periphery of the cytoplasm. TELOPHASE Last phase of Karyokinesis is Telophase Changes occuring during Telophase are reverse to Changes occured during prophase. At each pole, the chromatids (now the Daughter chromosomes) become uncoiled , thin and invisible. Chromosomes are again reorganised into chromatin Network The Nucleolus and nuclear membrane reappears again Thus two daughter nuclei are formed at the two poles of a cell. The formed two daughter nuclei are qualitatively and quantitatively similar to the parent Nucleus. Finally the cell is simply Pinched off into two daughter cells by a constriction 8. Meiosis: The cell division in which chromosome number is reduced / to half is called Meiosis. Meiosis was discovered by Strasburger. The Term Meiosis was used by Farmer and Moore Meiosis must take place in an organism to maintain the chromosome number constant. In animal cells Meiosis is termed as “Terminal or gametic meiosis” The cell which undergoes Meiosis is called a Meiocyte In meiosis one Replication of chromosome is followed by two divisions in the nucleus and cytoplasm MEIOSIS – (FIRST NUCLEAR DIVISION) In this division, the chromosome number is reduced to half. New daughter cells produced are dissimilar to the parent cell. It is also called Reductional division or Heterotypic division
IT IS DIVIDED INTO THE FOLLOWING STAGES.
32 Prophase I It takes long duration and differs from prophase of mitosis. It is divided into 5 subphases. 9. (a) Leptotene or Leptonema :The nucleus increases in size and volume by absorbing water The chromatin network shows condensation and forms a number of long thread like structures called as chromosomes.
Cells the living units Each chromosome shows characteristic beaded appearance because of the presence of chromomeres along the entire length Nuclear membrane and Nucleolus are distinct. 10. (b) Zygotene or Zygonema :The two homologous chromosomes (one paternal and one maternal ) lie side by side. This is known as pairing or Synapsis of Homologous chromosomes. Each pair of chromosomes is known as Bivalent Further shortening of chromosomes is present due to the coiling and chromosomes become more distinct Nuclear membrane and nucleolus are present 11. (c) Pachytene or pachynema Each chromosome of a bivalent splits longitudinally into two sister chromatids by this Bivalent becomes a tetrad, The two chromatids one from each bivalent partially coil around each other of exchange their genetic material This is known as crossing over. The points of crossing over are known as chiasmata. Nucelolus and Nuclear membrane are present. 12. (d) Diplotene or Diplonema Homologous chromosomes which had paired during Zygotene now start separating from one another As the chromosomes are showing gradual condensation, so there is a tendency that chiasmata tend to slip out of the chromosomes This is known as Terminalisation of chiasmata. Chromosomes start seperating out but the separation is not complete Nuclear membrane and Nucleolus start degenerating. 13 . (E) Diakinesis The Bivalents condense further and get randomly distributed in the cytoplasm. The separation of paired chromosomes is almost complete Terminalisation of chiasmata is almost complete. Nuclear membrane and Nucleolus disappear 14. Metaphase I The bivalents come to lie in the Equatorial plane in such a way that one member faces one pole of the spindle apparatus, and the other member on the other pole. Thus two members of the pair lie on opposite side of the equator. Nuclear membrane and nucleolus completely disappear. Centromeres of the chromosomes are attached to spindle fibres 15. Anaphase – I Each chromosome, consisting of two chromatids, separate and move to opposite poles as they are pulled apart by the shortening of spindle fibre. The number of chromosomes moving towards the opposite poles is reduced to half. The chromosomes on the poles are genetically different from the parent chromosomes due to crossing over. 16. Telophase –I At the opposite poles, the chromosomes loose their identity and Nuclear membrane is formed around the nuclei Nucleolus also reappears Thus each nucleus formed has half the number of chromosomes as compared to the parent cell Meiosis –I results in the formation of two haploid nuclei from a diploid nucleus Meiosis – II is also called Homotypic Division or Equational division It is similar to mitosis and divided into following four stages.
Cells the living units
17. Prophase II The chromosomes reappear Each chromosome has two chromatids Nucleus membrane and nucleolus disappear The chromosome become short by coiling and condensation. 18. Metaphase II The chromosomes get arranged on the equator of the spindle. Centromere divides into two The microtubules of the spindle are attached with the centromere of the chromosomes. 19. Anaphase II The centromere of each chromosome divides into two so that now each chromatid gets its centromere Each centromere gets attached to the spindle fiber of its side Shortening of spindle fibers occurs so that the chromatids are pulled apart towards their respective poles Nucleolus and Nuclearmembrane are absent. 20. Telophase II The chromatids (called Daughter chromosomes ) on the respecitve poles uncoil and form the chromatin network again. The Nuclear membrane and Nucleolus are formed Each Haploid nuclei are seen in each cell Finally cytoplasm is constricted into four parts to gives four daughter cells So after Meiosis - II 4 haploid daughter cells are formed. PROTEIN SYNTHESIS Protein synthesis take place with the help of DNA, RNA and ATP molecules. Protein synthesis takes place in two steps. (i) Transcription (ii) Translation (i) Transcription : Formation of RNA by the DNA is known as transcription. In this process two strands of DNA uncoil. In which one of them function like template for the synthesis of RNA strand. In the formation of new RNA strand N2 Base arranged according to the sequence of DNA template at the place of thymine, uracil attached at the RNA strand. In this manner new RNA form which comes in to cytoplasm and bring the coded information in the form of genetic code which are triplet i.e. (AAG, AUG, AUC, etc.) (ii) Translation : Formation of the chain of aminoacid on the ribosome with the help of mRNA , transfer RNA and ATP molecules is known as translation. Triplet code of mRNA which brings from the DNA functions like the code for amino acid. Process of Translation :The m-RNA is attached to the groove of two sub unit of ribosome. On the basis of triplet code transfer RNA bring aminoacid with the help of ATP to the ribosome. On the ribosome amino acid attached one by one from initiation codon to terminal codon and form a polypeptide chain. Ultimately one or more than one polypeptide chain attain their 3-D structure and form protein molecules.
Cells the living units
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