Plant Cytology PDF

‫بسم الله الرحمن الرحيم‬ Plant Cytology Biotechnology students Level II  Cytology The internal ultrastructure of plant cell Cytology It is the biological science which deals with the study of the fine detailed structure, function, molecular organization, morphology, physiol...

‫بسم الله الرحمن الرحيم‬ Plant Cytology Biotechnology students Level II  Cytology The internal ultrastructure of plant cell Cytology It is the biological science which deals with the study of the fine detailed structure, function, molecular organization, morphology, physiology growth, reproduction and genetics of the cells. The term cytology is derived from the greek word (Kytos) or (Cytos) which means a hollow space or cell.  Robert Hooke, an English physicist and microscopist of the 1600s, made significant discoveries with the compound microscope (a microscope with two lenses, the eyepiece and the objective). In 1665 Hooke used a microscope to examine a sliver of cork from the bark of a certain tree. Hooke saw that cork was composed of tiny boxes or compartments, which he named cells. Hooke recognized that he was looking at dead cells, of which all that remained were the cell walls. As Hooke and later biologists observed the contents of living cells, they began to realize that the cell’s interior was also an important part of the cell. Methods to Study Cells The invention of the microscope proved to be one of the most important contributions to expanding biological knowledge. The first microscopes, invented in the late 1500s, were not useful instruments. Although they could magnify specimens, the magnified images were blurry and details were indiscernible. In the 1600s, Anton van Leeuwenhoek (1632– 1723) perfected the art of grinding lenses (pieces of glass with curved surfaces) and used them in microscopes of his own design to produce clear, magnified images. Leeuwenhoek made many microscopes and peered through them to discover a world of organisms that were previously unknown.  Microscopes continued to improve during the 19th century so that biologists were able to observe tiny structures within cells, which they called organelles (little organs). The biologists envisioned these structures carrying out special functions for the cell much as organs, such as the heart and stomach or leaves and roots, carry out specific jobs for a multicellular organism. In 1830 Robert Brown, a Scottish botanist, first identified and named the cell’s nucleus (an organelle that serves as its control center).  By the late 1830s, biologists had examined enough different kinds of tissues to conclude that all organisms are composed of cells. Two German biologists, Matthias Schleiden, a botanist, and Theodor Schwann, a zoologist, published separate papers in 1838 and 1839, respectively that clearly stated that cells are the structural units of life. This statement has come to be known as the cell theory (The theory that the cell is the basic unit of life, of which all living things are composed, and that all cells are derived from preexisting cells).  CELL THEORY 1. The basic unit of life is the cell. (Hooke) In 1665, an English scientist named Robert Hooke made an improved microscope and viewed thin slices of cork viewing plant cell walls Hooke named what he saw "cells"  CELL THEORY 2. All living things are made of 1 or more cells. Matthias Schleiden (botanist studying plants) Theodore Schwann (zoologist studying animals) stated that all living things were made of cells Schwann Schleiden The important features of Cell Principles are: 1. All organisms are made up of cells. 2. New cells are produced from the pre-existing cells. 3. Cell is a structural and functional unit of all living organisms. 4. A cell contains hereditary information which is passed on from cell to cell during cell division. 5. All the cells are basically the same in chemical composition and metabolic activities. 6. The structure and function of the cell are controlled by DNA. 7. Sometimes the dead cells may remain functional as tracheids and vessels in plants. Exception to cell Theory Cell theory does not have universal application, i.e., there are certain living organisms which do not have true cells. Viruses are biologists’ puzzle. They are an exception to cell theory. They lack protoplasm, the essential part of the cell and a metabolic machinery for energy production and for the synthesis of proteins.  There are certain other organisms such as the protozoan Paramecium, the fungus Rhizopus and the alga Vaucheria which do not fit into the purview of the cell theory. All of these organisms have bodies containing undivided mass of protoplasm which lacks cell-like organization and has more than one nucleus. They tend to raise the question that whether cell is a basic unit of structure in them. Vaucheria Rhizopus  Today, scientists use a variety of microscopes in their studies of various organisms. The light microscope, which is much improved since Leeuwenhoek’s time, focuses a beam of visible light through a transparent sample. Light microscopes provide magnification, an increase in the apparent size of an object, of up to about 1000 times; they also provide resolving power, the ability to reveal fine detail, up to 500 times that of the human eye.  transmission electron microscope (TEM), has much greater resolving power than a light scope, passes a beam of electrons rather than hrough the sample being studied. The electron scope can magnify an object 250,000 times or and has resolving power up to 500,000 times the human eye. canning electron microscope (SEM), the electron beam does not pass gh the specimen. Instead, the specimen is coated with a thin film of gold me other metal. When the electron beam strikes various points on the ce of the specimen, secondary electrons are emitted whose intensity with the contour of the surface. Thus, an SEM pro vides information the shape and external features of the specimen that cannot be obtained he TEM.  All living things found on the planet earth are divided into two major groups namely, prokaryotes and Eukaryotes based on the types of cells these organisms possess. Prokaryotic cells lack a well-defined nucleus and have a simplified internal organization. Eukaryotic cells have a more complicated internal structure including a well-defined, membrane- limited nucleus. Bacteria and Cyanobacteria are prokaryotes. Fungi, plants and animals are eukaryotes. Size Most of them are very small (≈ 1-5 Most are large cells (≈ 10-100µm). some µm. are larger than 1mm. Genetic material is not enclosed in a nuclear envelope and is present All possess a membrane- bound nucleus suspended in the cytoplasm in a region (Genetic material is enclosed within the Genetic material called nucleoid nucleus by a nuclear envelope). Nucleolus is absent Nucleolus is present Mainly, the DNA forms a single large They have multiple linear chromosomes circle that coils up on itself Cell Division No mitosis or meiosis. Mainly by Mitosis and meiosis types of cell division binary fission or budding occur. Cell wall Made up of peptidoglycan Cell wall is made up of cellulose in plants (mucopeptide). Cellulose is absent. and chitin in fungi Membrane bound organelles such as endoplasmic reticulum, golgi Organelles usually absent. complex, mitochondria, chloroplasts and vacuoles are present. Ribosomes are small made of 70 s Ribosomes Ribosomes are large made of 80 s units. units. prokaryotes often contain several plasmids (extrachromosomal stored DNA molecule). Unlike chromosomal DNA, plasmid Plasmids DNA is typically smaller and encode Absent  Two Major Cell Types Prokaryotic cells – (Streptococcus, E.coli, etc.) NO NUCLEUS Eukaryotic cells. These include: plants, animals, fungi, protists HAVE A NUCLEUS 4 PLANT CELL 6 Prokaryotic vs. Eukaryotic no nucleus nucleus no membrane enclosed organelles membrane enclosed organelle single chromosome chromosomes in pairs no streaming in the cytoplasm streaming in the cytoplasm cell division without mitosis simple flagella cell division by mitosis smaller ribosomes complex flagella larger ribosomes simple cytoskeleton complex cytoskeleton no cellulose in cell walls  Comparison: Plant cells Animal cells  Large, central vacuole  No large, central vacuole  Chloroplasts  No chloroplasts  Rigid cell wall outside  No rigid cell wall of cell membrane Components of plant cell: A- Cell wall B-The protoplast Components of plant cell  ‫صورة بالمجهر األلكتروني توضح‬ ‫تركيب الخلية النباتية‬ ‫فجوة‬ ‫عصارية‬ ‫غالف‬ ‫بالستيدات‬ ‫نواة النواة‬ ‫جدار خلوي‬ ‫نوية‬ ‫ريبوزوم‬ ‫فراغ بين‬ ‫خلوي‬ ‫مادة بين‬ ‫خلوية‬ ‫ميتوكوندري‬ ‫شبكة‬ ‫ا‬ ‫إندوبالزمية‬ ‫بالستيدة‬ ‫بها نشا‬ The protoplast The protoplast is defined as all of the plant cell enclosed by the cell wall. It consists of:- 1. The living (protoplasmic) components include the nucleus, mitochondria Plastids (chloroplasts), Golgi bodies or Golgi Apparatus (GA) (Dictyosomes) spherosomes, lysosomes, endopasmic reticulum, ribosomes, …..etc. 2. The non-living (non-protoplasmic) components include the vacuoles and the ergastic substances (food products and metabolic by-products). Cell membrane:- The outermost layer of the protoplast is the plasma membrane A thin, non-rigid structure made of protein and lipids that encloses cell contents It regulates interactions between the cell and its external environment. It is selectively permeable since it allows only certain substances to enter or leave the cell through it. Nothing can enter or leave the cell without passing through this membrane. Ultra structure of the cell membrane Cell membranes are about 75A0 thick. Under the electron microscope they appear to consist of 3 layers:- 1. An outer electron dense layer of about 20Ao thick 2. An inner electron dense layer of about 20Ao thick 3. A middle pale coloured layer about 35Ao thick. The outer and inner layers are formed of protein molecules whereas the middle one is composed of two layers of phospholipid molecules. Such a trilaminar Structure is called “Unit membrane” which is a basic concept of all membranes. Fluid mosaic Model Many models have been proposed to explain the molecular structure of plasma membrane. Fluid mosaic model was proposed by Singer and Nicholson (1972) and it is widely accepted by all. According to this model the cell membrane has fluid structure. All cellular membranes line closed compartments formed of lipids and proteins. According to this model the membrane is viewed as a two dimensional mosaic of phospholipids and protein molecules in which membrane proteins move about like icebergs in a sea. Karp 2010 Lipids The lipid molecules form a continuous bilayer. The protein molecules are arrang as extrinsic proteins on the surface of lipid bilayer and as intrinsic proteins t penetrate the lipid bilayer either wholly or partially. The lipid bilayer is formed of a double layer of phospholipid molecules. They are amphipathic molecules i.e. they have a hydrophilic and hydrophobic pa The arrangement of phospholipids forms a water resistant barrier. So that only lip soluble substances can pass through readily but not water soluble substances. The phospholipid bilayer forms the basic structure of all biomembranes which a contain proteins, glycoproteins, cholesterol and other steroids and glycolipids. T  A membrane is a fluid structure with a “mosaic” or mixture of various proteins embedded in it when viewed from the top Phospholipids can move laterally a small amount and can “flex” their tails Membrane proteins also move side to side or laterally making the membrane fluid 35  Phospholipids in the plasma membrane Can move within the bilayer two ways Lateral movement Flip-flop (~107 times per second) (~ once per month) 1- Most commonly they interact in lateral diffusion where they switch places with the lipid to the left or right of them. 2-They go through transverse diffusion where they flip flop ‫ يتقلب‬with the ones whose tails they are facing. This is due to the weak, Van der Waals interaction of the lipid molecules  The type of hydrocarbon tails in phospholipids Affects the fluidity of the plasma membrane Fluid Viscous Unsaturated hydrocarbon Saturated hydro- tails with kinks or curves Carbon tails The more the double bonds, the greater the amount of curve in the lipid and therefore more free space. The length of the lipids also plays a role Proteins Proteins are arranged in two forms:- 1- Extrinsic or peripheral proteins: These are superficially attached to either face of lipid bimolecular membrane and are easily removable by physical methods. 2. Intrinsic or Integral proteins: These proteins penetrate the lipid either wholly or partially and are tightly held by strong bonds. In order to remove them, the whole membrane has to be disrupted. The integral proteins occur in various forms and perform many functions.  Membrane Proteins and Their Functions A membrane is different proteins embedded in the fluid matrix of the lipid bilayer Integral proteins „Peripheral proteins Fibers of extracellular matrix (ECM)  Types of Membrane Proteins Integral proteins Penetrate the hydrophobic core of the lipid bilayer Are often transmembrane proteins, completely spanning the membrane EXTRACELLULAR SIDE  Six Major Functions of Membrane Proteins Transport. Membrane protein may provide a hydrophilic channel across the membrane that is selective for a particular solute. Or transport from one side to the other by changing shape. ATP Enzymes Enzymatic activity. Membrane protein may be an enzyme with its active site exposed to substances in the adjacent solution.. Signal Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a Receptor hormone. 41  Membrane proteins and lipids are made in the ER and Golgi apparatus ER 42 sma membrane nctions of plasma membrane all cells the plasma membrane has several essential functions to perform. These includ ansporting nutrients into and metabolic wastes out of the cell preventing unwante aterials from entering the cell. he plasma membrane maintains the proper ionic composition pH (~7.2) and osmot essure of the cytosol. To carry out all these functions, the plasma membrane contains specific transpo oteins that permit the passage of certain small molecule but not others. Several of thes oteins use the energy released by ATP hydrolysis to pump ions and other molecules in out of the cell against concentration gradients.  In addition to these universal function, the plasma membrane has other important functions to perform. Enzymes bound to the plasma membrane catalyze reactions that would occur with difficulty in an aqueous environment. The plasma membranes of many types of eukaryotic cells also contain receptor proteins that bind specific signaling molecules like hormones, growth factors, neurotransmitters etc. leading to various cellular responses. All the biological membranes are selectively permeable. Its permeability properties ensure that essential molecules such as glucose, amino acids and lipids readily enter the cell, metabolic intermediates remain in the cell and waste compounds leave the cell. In short it allows the cell to maintain a constant internalSubstances are transported across the membrane either by:.Passive Transport Diffusion Diffusion is the movement of molecules of any material along its own concentration gradient from a location of higher to a region of lower concentration in order to spread uniformly in the dispersion medium due to their random kinetic motion. Osmosis It is the special type of diffusion where the water or solven diffuses through a selectively permeable membrane from a region Active transport It is a vital process. It is the movement of molecules or ions against the concentration gradient. i.e the molecules or ions move from the region of lower concentration towards the region of higher concentration. The movement of molecules can be compared with the uphill movement of water. Energy is required to counteract the force of diffusion and the energy comes from ATP produced by oxidative phosphorylation or by concentration gradient of ions. Thus active transport is defined as the energy dependent transport of molecules or ions across a semi permeable membrane against the concentration gradient. Activ2-Active transport e Transport Uses energy to move solutes against their concentration gradients Requires energy, usually in the form of ATP 49 Inside Cell: High K+ / Low Na+ relative to extracellular medium The sodium potassium pump uses energy to generate and maintain these concentration gradients  The Sodium-Potassium Pump Extracellular fluid with high concentration Na+ K+ of Na+ Na+ P ADP P K+ Cytoplasm ATP P with high concentration of K+ 1 2 3 4 5 Three Na+ bind to Phosphate is Phosphorylation K+ binds to Release of the transferred from changes the the protein, causing phosphate cytoplasmic side of ATP to the protein. shape of the phosphate changes the the protein. protein, moving release. shape of the Na+ across the protein, membrane. moving K+ to the cytoplasm. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Sodium-potassium pumps are important for muscle contractions, the transmission of nerve impulses, and the absorption of nutrients.  sodium-potassium pumps in animal cells pump sodium ions out, and potassium in, up their concentration gradient  Facilitated Diffusion Facilitated diffusion Is a type of Passive Transport aided by Proteins Transport proteins speed the movement of molecules across the plasma membrane 54  Comparison of Passive & Active Transport Passive transport. Substances diffuse spontaneously Active transport. Some transport proteins act as down their concentration gradients, crossing a pumps, moving substances across a membrane membrane with no expenditure of energy by the cell. against their concentration gradients. Energy for The rate of diffusion can be greatly increased by transport this work is usually supplied by ATP. proteins in the membrane. ATP Diffusion. Hydrophobic Facilitated diffusion. Many hydrophilic substances molecules and (at a slow diffuse through membranes with the assistance of rate) very small uncharged transport proteins, either channel or carrier proteins. polar molecules can diffuse through the lipid bilayer. 55  electrochemical gradient combines the membrane potential d the concentration gradient  Proton Pump – + EXTRACELLULAR FLUID – ATP + H+ H+ Proton pump H+ + – H+ + H+ – CYTOPLASM + H+ + – 57  3- Bulk transport Bulk Transport Bulk transport across the plasma membrane occurs by exocytosis and endocytosis In endocytosis The cell takes in macromolecules by forming new vesicles from the plasma membrane 58 Endocytosis can occur in three ways Phagocytosis ("cell eating") Pinocytosis ("cell drinking") Receptor-mediated endocytosis Endocytosis Plasma membrane sinks inward, pinches off and forms a vesicle Vesicle often merges with Golgi for processing and sorting of its contents Endocytosis Vesicle forming Endocytosis - terms Phagocytosis – cell eating Membrane sinks in and captures solid particles for transport into the cell Examples: Solid particles often include: bacteria, cell debris, or food  Phagocytosis EXTRACELLULAR FLUID Pseudopodium Solutes of amoeba Pseudopodium Bacterium 1 m Food vacuole “Food” or other An amoeba engulfing a bacterium particle via phagocytosis (TEM) Food vacuole CYTOPLASM Pinocytosis – cell drinking Cell brings in a liquid. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet.  Pinocytosis Plasma membrane 0.25 m Coat protein Pinocytotic vesicles forming (TEMs) Coated pit Coated vesicle Fig. 5-9c Receptor-mediated endocytosis Plasma membrane Coat protein Receptor Coated vesicle Coated pit Coated pit Specific molecule Material bound to receptor proteins more bound molecules (purple) inside the vesicle, other molecules (green) are also present. The cytoplasm The cytoplasm is a transparent liquid, gel like substance and contains several types of organelles and contain granules, vesicles, organic compounds, inorganic substances and membranes. The young cells are filled with the cytoplasm, while the mature cells have large vacuoles lacking the cytoplasm. Cytoplasm is always in a motion and consists of organic and inorganic substances (85-90 % of tile cytoplasm is water).  The protoplasmic components Mitochondria Mitochondria are the organelles that carry out cell respiration. A Mitochondrion is also called as the “Power house of the cell” because it stores and releases the energy of the cell. The energy released is used to form ATP (Adenosine Triphosphate). Mitochondria are the principal sites of ATP production in aerobic cells. Most eukaryotic cells contain many mitochondria,which occupy u to 25 percent of the volume of the cytoplasm. These complex organelles are among the largest organelles general exceeded in size only by the nucleus, vacuoles and chloroplasts. Typically the mitochondria are sausage shaped but these may granular, filamentous, rod-shaped, and spherical or thread like.  Many steps of respiration are mediated by enzymes bound to mitochondrial membranes. The enzymes are located adjacent to each other so that the product of one reaction (which becomes the reactant in the next reaction) is passed directly to the next enzyme; this controls the highly reactive intermediates. Mitochondrial membranes are folded, forming large sheets or tubes known as cristae. This folding provides room for large numbers of enzymes. Reactions that do not involve highly reactive intermediates take place in the liquid matrix between the cristae.  Around this complex of cristae and matrix is a second membrane, the outer mitochondrial membrane, which probably just gives shape and a little rigidity to the mitochondrion. The outer membrane is rather freely permeable, but the inner mitochondrial membrane, which forms the cristae, is selectively permeable and has numerous pumps and channels. Mitochondria have their own DNA and ribosomes, both of which are different from those of the rest of the cell. Mitochondrial DNA is a circular molecule and lacks histones; the ribosomes are small and resemble those found in prokaryotes.  Mitochondria are dynamic organelles; they can grow larger, either as the cell do or as the cell's need for respiration increases. They are often about 1 µm in diameter but up to 5 µm in length, and even muc longer in some cases. Mitochondria can also divide into two, increasing the number of mitochondria p cell. Under rare conditions, the mitochondria in some species fuse together, forming single giant mitochondrion in each cell. Mitochondria occupy between 1% and 25% of the cell volume; the higher valu are similar to those for animal cells. Mitochondria as semi-autonomous organelles Mitochondria are self-perpetuating semi-autonomous bodies. These arise new by the division of existing mitochondria. These are also regarded as intracellular parasitic prokaryotes that have established symbiotic relationship with the cell. The mitochondrial matrix contains DNA molecules which are circular and 70s ribosomes, tRNA and enzymes for functioning of mitochondrial genes. Plastids Plastids are a group of dynamic organelles able to perform many functions. One prominent activity is photosynthesis, carried out by the green plastids, chloroplasts. Diverse types of metabolism occur in other classes of plastid: synthesis, storage, and export of specialized lipid molecules; storage of carbohydrates and iron; and formation of colors in some flowers and fruits.  Plastids are found in most of the plant cells and in some photosynthetic protists. These are absent in prokaryotes and in animal cells. It originates from the pro-plastids which is found in the embryo cells. Plastids are of three types: 1. chloroplasts 2. Chromoplasts 3. leucoplasts  Chromoplasts are coloured plastids other than green. They are found in coloured parts of plants such as petals of the flower, pericarp of the fruits etc. Crystalline Chromoplasts in a cell of a red paprika Chromoplasts in the cells of mandarin chromoplasts in the cells of the pulp and  Leucoplasts are the colourless plastids. These colourless plastids are involved in the storage of carbohydrates, fats and oils and proteins. The plastids which store carbohydrates are called amyloplasts. The plastids storing fats and oils are called elaioplasts. The plastids storing protein are called proteinoplasts.  loroplast Chloroplasts can be as long as 10mm and are typically 0.5 - 2.0mm thick, ut they vary in size and shape in different cells, especially among the lgae. Like mitochondrion, the chloroplast is surrounded by an outer and inner membrane. Chloroplasts contain an internal system of extensive inter connected membrane- limited sacs called thylakoids which are flattened to form disks. These are often grouped in stakes of 20-50 thylakoids to form what are  Stroma, a semi fluid, colourless, colloidal complex contain DNA, RNA, ribosomes and several enzymes. The DNA of chloroplast is circular. The ribosomes are 70s type. The matrix of higher plant’s chloroplasts may contain starch as storage product. Thylakoids may occur attached to the inner membrane of the chloroplast envelop.  About 40-100 grana may occur in a chloroplast. Many membranous tubules called stroma lamellae (intergranal thylak inter connect thylakoids of different grana. Thylakoid membrane contains photosynthetic pigments. The thyla membrane contains green pigments (Chlorophylls) and other pigments enzymes that absorb light and generate ATP during photosynthesis. Part of this ATP is used by enzymes located in stroma to convert CO2 carbon intermediates which are then exported to the cytosol and conv to sugars.  The molecular mechanism by which ATP is formed is very similar in mitochondria and chloroplasts. Chloroplasts and mitochondria have other features also in common. Both migrate often from place to place within cells and both contain their own DNA which code for some of the key organella proteins. These proteins are synthesized in the ribosomes within the organelle. However, most of the proteins in each of these organelles are encoded in the nuclear DNA and are synthesized in the cytosol. These proteins are then incorporated into the organelle. Ribosomes Ribosomes are small subspherical granular organelles, not enclosed by any membrane. They are composed of ribonucleoproteins and they are the site of protein synthesis.  They occur in large number. Each ribosome is 150-250A in diameter and consists of two unequal subunits, a larger dome shaped and a smaller ovoid one. The smaller sub unit fits over the larger one like a cap. These two sub units occur separately in the cytoplasm and join to form ribosomes only at the time of protein synthesis. At the time of protein synthesis many ribosomes line up and join an mRNA chain to synthesise many copies of a particular polypeptide. Such a string of ribosomes is called  Ribosomes occur in cytoplasmic matrix and in some cell organelles. Accordingly, they are called cytoplasmic ribosomes or organelle ribosomes. The organelle ribosomes are found in plastids and mitochondria. The cytoplasmic ribosomes may remain free in the cytoplasmic matrix or attached to the surface of the endoplasmic reticulum. The attached ribosomes generally transfer their proteins to cisternae of endoplasmic  Based on size or sedimentation coefficient(s), ribosomes are of two types. 70s and 80s. 70s type of ribosomes are found in all prokaryotic cells and 80s type are found in eukaryotic cells. S is Svedberg unit which is a measure of particle size with which the particle sediments in a centrifuge. In eukaryotic cells, synthesis of ribosomes occurs inside the nucleolus. Ribosomal RNA are  The ribosomal proteins are synthesized in the cytoplasm and shift to the nucleolus for the formation of ribosomal subunits by complexing with rRNA. The subunits pass out into the cytoplasm through the nuclear pores. In prokaryotic cells, both ribosomal RNAs and proteins are synthesized in the cytoplasm. Thus the ribosomes act as the protein factories of the cell.  Endoplasmic reticulum It is a system of narrow tubes and sheets of membrane that form a network throughout the cytoplasm. There are two types of Endoplasmic reticulum: 1- Rough ER, or RER 2- Smooth ER, or SER Endoplasmic reticulum Rough ER, or RER A large proportion of a cell's ribosomes are attached to the ER, giving it a rough appearance. As an attached ribosome synthesizes a protein, it passes through the membrane and collects in the lumen. If the protein is a storage product, as in seeds of legumes, it merely remains in the ER, which may become quite swollen. But if the protein is to be secreted, ER form vesicles, detach, move to the plasma membrane, and fuse with it, releasing their contents to the cell's exterior by exocytosis. In many cases the protein must be modified through Golgi apparatus.  Endoplasmic reticulum Smooth ER, or SER Endoplasmic reticulum that lacks ribosomes. it is involved in lipid synthesis and membrane assembly. SER is abundant only in cells that produce large amounts of fatty acids (cutin and wax on epidermal cells), oils (palm oil, coconut oil, safflower oil), and fragrances of many flowers. The Golgi apparatus It is a factory for processing and packaging proteins and polysaccharides. Golgi apparatus (or Golgi complex) is made of one or more dictysomes (or Golgi bodies) which are stacks of 3-10 flattened sacs (cisternae), The edges of the sacs often bulge out and detach as vesicles, sacs that contain cellular products. The vesicles transport materials to the plasma membrane, to the outside of the cell, or to other organelles within the cell. The Golgi apparatus The Golgi apparatus collects and processes materials that are to be exported from the cell. In plant cells, for example, the Golgi apparatus produces and transports some of the polysaccharides that make up the cell wall. The Golgi apparatus also collects materials that are stored inside large, membrane-bounded sacs called vacuoles.  Microbodies Spherical bodies about 0.5 to 1.5 µm in diameter. They are bound by a single membrane. Found in eukaryotic cell. It is not easy to tell exactly what they are but by using special chemical reactions and stains, it has been possible to determine the contents and even the types of reactions in these bodies. Peroxiso mes Glyoxyso  Peroxiso Peroxisomes contain enzymes that oxidize mes certain molecules normally found in the cell, notably fatty acids and amino acids. Those oxidation reactions produce hydrogen peroxide, which is the basis of the name peroxisome. Hydrogen peroxide is potentially toxic to the cell, so peroxisomes also contain enzymes such as catalase, which detoxifies peroxide by converting it to water and oxygen and. neutralizing the toxicity.  Glyoxyso (are called special mes Occur onlyperoxisomes in plants ) Involved in converting stored fats into sugars. They are important during the germination of fat-rich, oily seeds such as peanut, sunflower, and coconut.  Spherosomes : also known as (plant lysosomes). Occur only in plants They take part in the storage and synthesis of lipids. They originate from the endoplasmic reticulum and 98% of them are lipid (the remaining 2% are protein) Some tissues, such as tobacco endosperm and maize root-tip have spherosomes that contain hydrolytic enzymes and hence have lysosomal activity that: which breakdown molecules. Protect the cell from the forging bodies and prevent cell wall eakdown Breakdown and digest the dead organelles. Nucleus & Nucleolus  Main Characteristics  The nucleus is found in all the eukaryotic cells of the plants and animals  Roughly spherically ‫كروي‬shaped  Largest and most easily seen organelle under light microscopy  The nucleus is the heart of the cell. Why? - It is almost all of the cell’s DNA is kept, replicated, transcribed and heredity information. - The nucleus, controls different metabolic as well as genetic activities of the cell, and controls the cell's growth and reproduction.  Nucleus functions as the main distinguishing feature of eukaryotic cells  Some of erythrocytes contain no nucleus, such cells human red blood rather than Cells  Nucleus It is the heart of the cell. It is here that almost all of the cell’s DNA is confined, replicated and transcribed. It controls different metabolic as well as hereditary activities of the cell. It serves as the main distinguishing feature of eukaryotic cells, i.e., this is the true nucleus as opposed to the nuclear region, prokaryon or nucleoid of the prokaryotic cells.  Nucleus Nucleus is the largest organelle in eukaryotic cells. It is surrounded by two membranes. Each one is a phospholipid bilayer containing many different types of proteins. In many cells the outer nuclear membrane is continuous with the rough ER. The two nuclear membranes appear to fuse at the nuclear pores. These ring like pores are constructed of a specific set of membrane proteins and these act like channels that regulate the movement of substances between the nucleus and the cytosol.  Nucleus The nucleus is found in all the eukaryotic cells of the plants and animals. However, certain eukaryotic cells such as the mature sieve tubes of higher plants contain no nucleus. The position of nucleus depend on the cell type and it is often variable. Usually the nucleus remains located in the center. But its position may change from time to time according to the metabolic states of the cell.  Nucleus In a growing or differentiating cell, the nucleus is metabolically active, producing DNA and RNA. The RNA is exported through nuclear pores to the cytoplasm for use in protein synthesis. The material filled in the nucleus is called nucleoplasm (or nuclear sap). The nucleoplasm contains many thread-like, coiled and much elongated structures known as chromatin fibers, which observed only in the interphase nucleus.  Nucleus During the cell division (mitosis and meiosis) chromatin fibers become thick ribbon-like structures which are known as the chromosomes which form the physical basis of heredity. Genes, the chemical basis of heredity, are arranged in linear fashion on the chromosomes. Chemically, chromatin consists of DNA and proteins. Small quantity of RNA may also be present but the RNA rarely accounts for more than about 5 percent of the total chromatin present.  Two types of chromatin material have been recognised, e.g., heterochromatin and euchromatin. A.Heterochromatin. The darkly stained, condensed region of the chromatin is known as heterochromatin. The condensed portions of the nucleus are known as chromocenters or karyosomes or false nucleoli. The heterochromatin occurs around the nucleolus and at the periphery. It is supposed to be metabolically and genetically inactive because it contains comparatively small amout of the DNA and large amount of the RNA.  B. Euchromatin. The light stained and diffused region of the chromatin is known as the euchromatin. The euchromatin contains comparatively large amount of DNA. It is the area of the chromosome which is rich in gene concentration and actively participates in the transcription process.  Usually the cells contain single nucleus but the number of the nucleus may vary from cell to cell. According to the number of the nuclei following types of cells have been recognized: 1. Mononucleate cells. Most plant and animal cells contain single nucleus. 2. Binucleate cells. The cells which contain two nuclei are known as binucleate cells. Such cells occur in certain protozoans such as Paramecium and cells of cartilage and liver. 3. Polynucleate cells. The cells which contain many (from 3 to 100) nuclei are known as polynucleate cells. The polynucleate cells of the plants are known as coenocytes. The siphonal algae Vaucheria contains hundreds of nuclei and certain Nucleolus Most cells contain in their nuclei one or more prominent spherical colloidal acidophilic bodies, called nucleoli. However, cells of bacteria and yeast lack nucleolus. The number of the nucleoli in the cells may be one, two or four. The position of the nucleolus in the nucleus is eccentric. Most of the ribosomal RNA of a cell is synthesized in the nucleolus. The finished or partly finished ribosomal subunits pass through a nuclear pore into the cytosol.  The nucleolus: - Contained within the nucleus is a dense structure composed of RNA and proteins called the nucleolus - The nucleolus helps to synthesize ribosomes by transcribing and assembling ribosomal RNA The non-protoplasmic components Vacuole A vacuole is a membrane-bounded sac filled with a liquid that contains a variety of materials in addition to water— dissolved salts, ions, pigments, and waste products. In certain mature plant cells, the vacuole may occupy as much as 90 percent of the volume of the cell. The vacuolar membrane (also called the tonoplast) surrounds each vacuole and is similar in many respects to the plasma membrane.  Vacuole The vacuole performs several important functions for a plant cell. One of the most significant is that the vacuole helps the cell maintain its shape by making it turgid. The vacuole swells and presses against the cytoplasm, which in turn presses against the cell’s plasma membrane and cell wall. So, vacuoles provide strength for non woody plants. Without the collective strength of their turgid vacuoles, plants would wilt. Vacuole The vacuole also serves as a temporary storage area; excess materials such as calcium ions are stored in and water soluble pigments such as anthocyanins, waste products, malformed proteins, and the like also enter the vacuole, where they may be disassembled so that their component parts can be used again. Some wastes accumulate and form small crystals that are visible under the light microscope.  CHROMOSOMES Chromosomes were discovered by von in 1842. The term chromosome, which W. Waldeyer coined in 1888, means “colored body.”Von discovered chromosomes after staining techniques were developed that made them visible. The nucleoprotein material of the chromosomes is referred to as chromatin. When diffuse, chromatin is referred to as euchromatin; when condensed and readily visible, as heterochromatin. Although all eukaryotes have chromosomes, in the interphase between divisions, they are spread out or diffused throughout the nucleus and are usually not identifiable. Each chromosome, with very few exceptions, has a distinct attachment point for fibers (microtubules) that make up the mitotic and meiotic spindle apparatus. The present name chromosome (Gr., chrom= colour, soma=body) was coined by W. Waldeyer (1888) to darkly stained bodies of nucleus. Sutton (1902) observed that the chromosome pair in synapsis is made up of one maternal and one paternal member. He believed that chromosomes, acting in this way, may be the physical basis for the Mendelian laws of heredity. He is credited as the originator of the theory of the chromosomal basis for heredity. The karyotype The karyotype of a human male stained to reveal bands on each of the chromosomes. The autosomes are numbered from 1 to 22. The X and Y are the sex chromosomes CHROMOSOME NUMBER The mumber of the chromosomes is constant for a particular species. Therefore, these are of great importance in the determination of the phylogeny and taxonomy of the species. The number or set of the chromosomes of the gametic cells such as sperms and ova is known as the gametic, reduced or haploid sets of chromosomes. The haploid set of the chromosomes is also known as the genome. The somatic or body cells of most organisms contain two haploid set or genomes and are knows as the diploid cells. The number of chromosomes in each somatic cell is the same for all members of a given species.  The organism with the lowest number of the chromosomes is the nematode, Ascaris megalocephalus univalens which has only two chromosomes in the somatic cells (i.e., 2n =2). Among plants, chromosome number varies from2n = 4 in Haplopappus gracilic (Compositae) to 2n = >1200 in some pteridophytes. The diploid number of a species bears no direct relationship to the species position in the phylogenetic scheme of classification. Lastly, while ‘n’ normally signifies the gametic or haploid chromosome number, ‘2n’ is the somatic or diploid chromosome number in an individual.  In polyploid individuals, however, it becomes necessary to establish an ancestral primitive number, which is represented as ‘x’ and is called the base number.  For example, in wheat Triticum aestivum 2n = 42 ; n = 21 and x = 7, showing that common wheat is a hexaploid (2n = 6x). Autosomes and Sex chromosomes In a diploid cell, there are two of each kind of chromosome (these are termed homologous chromosomes), except for the sex chromosomes. One sex has two of the same kind of sex chromosome and the other has one of each kind.  For example, in human, there are 23 pairs of homologous chromosomes (i.e., 2n = 46 ; a chromosome number which was established by Tijo and Levan in 1956). The human female has 44 non-sex chromosomes, termed autosomes and one pair of homomorphic (morphologically similar) sex chromosomes given the designation XX. The human male has 44 autosomes and one pair of heteromorphic or morphologically dissimilar sex chromosomes, i.e., one X chromosome and one Y chromosome. Size The size of chromosome is normally measured at mitotic metaphase and may be as short as 0.25um in fungi and birds, or as long as 30 um in some plants such as Trillium.  However, most metaphase chromosomes fall within a range of 3 um in fruitfly (Drosophila), to 5um in man and 8um to 12um in maize. The organisms with less number of chromosome contain comparatively large-sized chromosomes than the chromosomes of the organisms having many chromosomes. The monocotyledon plants contain large-sized chromosomes than the dicotyledon plants. The plants in general have large-sized chromosomes in comparison to the animals.  Further, the chromosomes in a cell are never alike in size, some may be exceptionally large and others may be too small.  The largest chromosomes are lampbrush chromosomes of certain vertebrate oocytes and polytene chromosomes of certain dipteran insects. Chromosome Structure  Four Classes of Chromosomes Types based on the position of the centromere during anaphase Midterm exam Answer the following questions Write short notes on the following 1- membrane proteins structure and function 2- endocytosis 3-classes of chromosomes in karyotype structure , draw different types  Telomeres Located at the ends of the chromosomes Contain hundreds to thousands of six nucleotide repeats Most cells lose 50-200 repeats after each cell division After about 50 divisions, shortened telomeres signal the cell to stop dividing Sperm, eggs, bone marrow, and cancer cells produce telomerase that prevent shortening of telomere  Chromosome Structure P q Chemical nature of chromosome If you stain the nucleus of a cell in interphase, you cannot see “chromosomes”. The chromatin is not condensed. Nuclosomes= Histones, H2a,H2b, H3,H4, MATERIAL OF THE CHROMOSOMES 1. Euchromatin: Portions of chromosomes that stain lightly are only partially condensed; this chromatin is termed euchromatin. It represents most of the chromatin that disperse after mitosis has completed. Euchromatin contains structural genes which replicate and transcribe during G1 and S phase of interphase. The euchromatin is considered genetically active chromatin, since it has a role in the phenotype expression of the genes. 2. Heterochromatin: In the dark-staining regions, the chromatin remains in the condensed state and is called heterochromatin. Heterochromatin is characterized by its especially high content of repititive DNA sequences and contains very few, if any, structural genes (i.e., genes that encode proteins). It is late replicating (i.e., it is replicated when the bulk of DNA has already been replicated) and is not transcribed. CHEMICAL COMPOSITION Chromatin which has been isolated from rat liver contains DNA, RNA and protein. The protein of chromatin is of two types : the histones and the non-histones.  Rat liver chromatin has been used as a model for chromatin.  It possesses a histone to DNA ratio near 1 : 1, a non-histone protein to DNA ratio of 0.6 : 1 and a RNA/DNA ratio of 1 : 1. 2.Histones Histones are very basic proteins, basic because they are enriched in the amino acids arginine and lysine to a level of about 24 mole present.  Arginine and lysine at physiological pH are cationic and can interact electrostatically with anionic nucleic acids. Thus, being basic, histones bind tightly to DNA which is an acid. There are five types of histones in the eukaryotic chromosomes, namely H1, H2A, H2B, H3 and H4. 3.Non-histones In contrast to the modest population of histones in chromatin, non-histone proteins display more diversity. In various organisms, number of non-histones can vary from 12 to 20. Heterogeneity of these proteins is not conserved in evolution as the histones.  These non-histones differ even between different tissues of the same organism suggesting that they regulate the activity of specific genes.  ULTRASTRUCTURE The field of ultrastracture of the chromatin is still the area where electron microscope had failed to provide us a clear picture of the organization of DNA in the chromatin. For the study of chromosomes with the help of electron microscope, whole chromosome mounts as well as sections of chromosomes were studied. Such studies had demonstrated that chromosomes have very fine fibrils having a thickness of 2nm—4nm. Since DNA is 2nm wide, there is a possibility that a single fibril corresponds to a single DNA molecule. Nucleosome If we presume that a single chromatid has a single long DNA molecule, we have no choice but to believe that DNA should be present in a coiled or folded manner. nuclesome model which was proposed by R.D. Kornberg (1974) (Fig. 13.7) and confirmed and christened by P. Oudet et al., (1975). In eukaryotes, DNA is tightly bound to an equal mass of histones, which serve to form a repeating array of DNA-protein particles, called nucleosomes. If it was stretched out, the DNA double-helix in each human chromosome would span the cell nucleus thousands of time. Histones play a crucial role in packing this very long DNA molecule in an orderly way (i.e., nucleosome) into nucleus only a few micrometres in diameter.  Thus, nucleosomes are the fundamental packing unit particles of the chromatin and give chromatin a “beadson- a-string” appearance in electron micorgraphs taken after treatments that unfold higher- order packing (Olins and Olins, 1974). Each nucleosome is a disc-shaped particle with a diameter of about 11 nm and 5.7 nm in height containing 2 copies of each 4 nucleosome histones–H2A, H2B, H3 and H4. This histone octamer forms a protein core [(i.e., a core of histone tetramer (H3, H4)2 and the apolar regions of 2(H2A and H2B)] around which the double- stranded DNA helix is wound 1¾ time containing 146 base pairs. In undigested chromatin the DNA extends as a continuous thread from nucleosome to nucleosome.  Each nucleosome bead is separated from the next by a region of linker DNA which is generally 54 base pair long and contains single H1 histone protein molecule. Generally, DNA makes two complete turns around the histone octamers and these two turns (200 bp long) are sealed off by H1 molecules Nucleosome structure A typical human cell has enough “DNA to wrap around the cell more than 15,000 times”. Therefore, DNA packaging is crucial because it makes sure that those excessive DNA are able to fit nicely in a cell that is many times smaller. Histones are proteins that allow DNA to be tightly packaged into units called nucleosomes. There are five main types of histone, H1, H2A, H2B, H3 and H4 More proteins are therefore required for DNA packaging with histone because histone only is not enough to do this process. Nucleosome is DNA wraps around the histones. Each nucleosome is connected by a DNA linker of 50 bp to form a fiber like structure called chromatin. Chromatin fibers can be further compacted to form higher order of structures called heterochromatin or euchromatin Levels of DNA Packaging in Eukaryotes Cell Cycle All cells are produced by divisions of pre-existing cell. Continuity of life depends on cell division. A cell born after a division, proceeds to grow by macromolecular synthesis, reaches a species-determined division size and divides. This cycle acts as a unit of biological time and defines life history of a cell. Cell cycle can be defined as the entire sequence of events happening from the end of one nuclear division to the beginning of the next. The cell cycle involves the following three cycles 1. Chromosome cycle: In it DNA synthesis alternates with mitosis (or karyokinesis or nuclear division). During DNA synthesis, each double- helical DNA molecule is replicated into two identical daughter DNA molecules and during mitosis the duplicated copies of the genome are ultimately separated. 2. Cytoplasmic cycle: In it cell growth alternates with cytokinesis (or cytoplasmic division). During cell growth many other components of the cell (RNA, proteins and membranes) become double in quantity and during cytokinesis cell as a whole divides into two. Usually the karyokinesis is followed by the cytokinesis but sometimes the cytokinesis does not follow the karyokinesis and results into the multinucleate cell 3. Centrosome cycle: Both of the above cycles require that the centrosome be inherited reliably and duplicated precisely in order to form the two poles of the mitotic spindle ; thus, centrosome cycle forms the third component of cell cycle. 1. G1 Phase: After the M phase of previous cell cycle, the daughter cells begin G1 of interphase of new cell cycle. G1 is a resting phase. It is called first gap phase, since no DNA synthesis takes place during this stage; currently, G1 is also called first growth phase, since it involves synthesis of RNA, proteins and membranes which leads to the growth of nucleus and cytoplasm of each daughter cell towards their mature size G1 involves transcription of three types of RNAs, namely rRNA, tRNA and mRNA ; rRNA synthesis is indicated by the appearance of nucleolus in the interphase (G1 phase) nucleus. Proteins synthesized during G1 phase (1) regulatory proteins which control various events of mitosis ; (2) enzymes (e.g., DNA polymerase) necessary for DNA synthesis of the next stage ; and (3) tubulin and other mitotic apparatus proteins. 2. S phase: During the S phase or synthetic phase of interphase, replication of DNA and synthesis of histone proteins occur. New histones are required in massive amounts immediately at the beginning of the S period of DNA synthesis to provide the new DNA with nucleosomes. Thus, at the end of S phase, each chromosome has two DNA molecules and a duplicate set of genes 3. G2 phase: This is a second gap or growth phase or resting phase of interphase. During G2 phase, synthesis of RNA and proteins continues which is required for cell growth. It may occupy 10 to 20 per cent time of cell cycle. As the G2 phase draws to a close, the cell enters the M phase. 4. M phase or Mitotic phase: The mitosis (Gr., mitos=thread) occurs in the somatic cells and it is meant for the multiplication of cell number during embryogenesis Mitosis starts at the culmination point of interphase (i.e., G2 phase). It is a short period of chromosome condensation, segregation and cytoplasmic division. Mitosis is important for replacement of cells lost  Mitosis As a process, mitosis is remarkably similar in all animals and plants. It is a smoothly continuous process and is divided arbitrarily into following stages or phases. 1. Prophase: The appearance of thin-thread like condensing chromosomes marks the first phase of mitosis, called prophase (Gr., pro=before ; phasis=appearance). Each prophase chromosome is composed of two coiled filaments, the chromatids, which are the result of the replication of DNA during the S phase. As prophase progresses, the chromatids become shorter and thicker and two sister chromatids of each chromosome are held together by a special DNA-containing region, called the centromere or primary constriction 2. Metaphase: The breakdown of nuclear envelope signals enables the mitotic spindle to interact with the chromosomes. This stage is characterized by a period of frantic activity during which the spindle appears to be trying to contain and align the chromosomes at the metaphase plate During metaphase (Gr.,meta=after; phasis =appearance) the chromosomes are shortest and thickest. Their centromeres occupy the plane of the equator of the mitotic apparatus (a region known as the equatorial or metaphase plate), although the chromosomal arms may extend in any direction. At this stage the sister chromatids are still held together by centromere and the kinetochores of the two sister chromatids face opposite poles ; this would permit proper separation in the next phase 3.The anaphase: (Gr., ana=up ; phasis=appearance) begins abruptly with thesynchronous splitting of each chromosome into its sister chromatids, called daughter chromosomes,each with one kinetochore. 4. Telophase: The end of the polar migration of the daughter chromosomes marks the beginning of the telophase ; which in turn is terminated by the reorganization of two new nuclei and their entry into the G1 phase of interphase. In general terms, the events of prophase Cytokinesis Both DNA synthesis and mitosis are coupled to cytoplasmic divison, or cytokinesis—the constriction of cytoplasm into two separate cells.  During cytokinesis, the cytoplasm divides by a process, called cleavage. The mitotic spindle plays an important role in determining where and when cleavage occurs. Cytokinesis usually begins in anaphase and continues through telophase and into interphase. Significance of Mitosis The mitosis has the following singificance for living organisms : 1. The mitosis helps the cell in maintaining proper size. 2. It helps in the maintenance of an equilibrium in the amount of DNA and RNA in the cell. 3. The mitosis provides the opportunity for the growth and development to organs and the bodyof the organisms. 4. The old decaying and dead cells of body are replaced by the help of mitosis. 5. In certain organisms, the mitosis is involved in asexual reproduction. 6. The gonads and the sex cells depend on the mitosis for the increase in their number. 7. The cleavage of egg during embryogenesis and division of blastema during blastogenesis, both involve mitosis.  MEIOSIS Meiosis is a nuclear division mechanism that precedes cytoplasmic division of immature reproductive cells.  It occurs only in sexually reproducing eukaryotic species. The term meiosis (Gr., meioum=to reduce.  Meiosis produces a total of four haploid cells from each original diploid cell. These haploid cells either become or give rise to gametes, which through union (fertilization) support sexual reproduction and a new generation of diploid organisms. Thus, meiosis is required to run the reproduc-tive cycle of eukaryotes. Meiosis superficially resembles two mitotic divisions without an intervening period of DNA replication. The first meiotic division includes a long prophase in which the homologous chromosomes become closely associated to each other and interchange of hereditary material takes place between them. Further, in the first meiotic division the reduction of chromosome number takes place and, thus, two haploid cells are resulted by this division. In the second meiotic division the haploid cell divides mitotically and results into four haploid cells. Both the meiotic divisions occur continuously and each includes the usual stages of the meiosis, viz., prophase, metaphase, anaphase and telophase.  First meiotic division The prophase of first meiotic division is very significant phase because the most cytogenetical events such as synapsis, crossing over, etc., occur during this phase. The prophase is the longest meiotic phase, therefore, for the sake of convenience it is divided into six substages, viz., proleptonema (proleptotene), leptonema (leptotene), zygonema (zygotene), pachynema (pachytene), diplonema (diplotene) and diakinesis. The successive meiotic substages can be represented as follows : Meiosis starts after an interphase which is not very different from that of an intermitotic interphase.  During the premeiotic interphase DNA duplication has occurred at the S phase. In the G2 phase of interphase apparently there is a decisive change that directs the cell toward meiosis Prophase I The first prophase is the longest stage of the meiotic division. It includes following substages: 1-Leptotene In the leptotene stage the chromosomes become more uncoiled and assume a long thread-like shape. The chromosomes at this stage take up a specific orientation inside the nucleus. 2- Zygotene (Gr., zygon=adjoining). In the zygotene stage, the pairing of homologous chromosomes takes place. The homologous chromosomes which come from the mother (by ova) and father (by sperm) are attracted towards each other and their pairing takes place. The pairing of the homologous chromo-somes is known as synapsis (Gr.,synapsis=union).  The synapsis begins at one or more points along the length of the homolo-gous chromosomes. The pairing of the homologous chromosome is very exact and specific. The paired homologous chromosomes are joined by a roughly 0.2-µm thick, protein.  This complex extends along the whole length of the paired chromosomes and is usually anchored at either end to the nuclear envelope. 3- Pachytene (Gr., pachus=thick). In the pachynema stage the pair of chro-mosomes become twisted spirally around each other and cannot be distinguished separately. In the middle of the pachynema stage each homologous chromosome spilts lengthwise to form two chromatids. During pachynema stage an important genetic phenomenon called “ crossing over” takes place. The crossing over involves reshuffling, redistribution and mutual exchange of hereditary material of two parents between two homologous chromosomes. The process of interchange of chromatin material between one non-sister chromatid of each homologous chromosome is known as the crossing over which is accompanied by the chiasmata formation. The nucleolus remains prominent up to this stage and it is found to be associated with the nucleolar organizer region of the chromosome. 4-Diplotene In diplonema, unpairing or desynapsisof homologous chromosomes is started and chiasmata are first seen. At this phase the chromatids of each tetrad are usually clearly visible, but the synaptonemal complex appears to be dissolved, leaving participating chromatids of the paired homologous chromosome physically joined at one or more discrete points called chiasmata (singular, chiasma; Gr., chiasma=cross piece). These points are where crossing over took place. Often there is some unfolding of the chromatids at this stage, allowing for RNA synthesis and cellular growth. 5-Diakinesis In the diakinesis stage the bivalent chromosomes become more condensed and evenly distributed in the nucleus. The nucleolus detaches from the nucleolar organizer and ultimately disappers.  The nuclear envelope breaks down. During diakinesis the chiasma moves from the centromere towards the end of the chromosomes and the intermediate chiasmata diminish. This type of movement of the chiasmata is known as terminalization.  The chromatids still remain connected by the terminal chiasmata and these exist up to the metaphase. Metaphase I Metaphase I consists of spindle fibre attachment to chromosomes and chromosomal alignment at the equator. During metaphase I, the microtubules of the spindle are attached with the centromeres of the homologous chromosomes of each tetrad. Centromere of each chromosome is directed towards the opposite poles. Anaphase I At anaphase I homologues are freed from each other and due to the shortening of chromosomal fibres or microtubules each homologous chromosome with its two chromatids and undivided centromere move towards the opposite poles of the cell.  The chromosomes with single or few terminal chiasma usually separate more frequently than the longer chromosomes containing many chiasmata. The actual reduction and disjunctionoccurs at this stage. Here it should be carefully noted that the homologous chromosomes which move towards the opposite poles are the chromosomes of either paternal or maternal origin. Telophase I The arrival of a haploid set of chromosomes at each pole defines the onset of telophase I, during which nuclei are reassembled. The endoplasmic reticulum forms the nuclear envelope around the chromosomes and the chromosomes become uncoil. The nucleolus reappears and, thus, two daughter chromosomes are formed. After the karyokinesis, cytokinesis occurs and two haploid cells are formed. Both cells pass through a short resting phase of interphase. During interphase, no DNA replication occurs, so that chromosomes at the second prophase are the same double-stranded structures that disappeared at the first telophase. In case of Trilliumtelophase I and interphase I do not occur and the anaphase I is followed by prophase II directly.  Second Meiotic Division Second meiotic division is actually the mitotic division which divides each haploid meiotic cell into two haploid cells. The second meiotic division includes following four stages: Prophase II In the prophase second, each centriole divides into two and, thus, two pairs of centrioles are formed. Each pair of centrioles migrates to the opposite pole. The microtubules get arranged in the form of spindle at the right angle of the spindle of first meiosis.  The nuclear membrane and the nucleolus disappear. The chromosomes with two chromatids become short and thick Metaphase II During metaphse II, the chromosomes get arranged on the equator of the spindle. The centromere divides into two and, thus, each chromosome produces two monads or daughter chromosomes. The microtubules of the spindle are attached with the centromere of the chromosomes. Anaphase II The daughter chromosomes move towards the opposite poles due to the shortening of chromosomal microtubules and stretching of interzonal microtubules of the spindle. Telophase II The chromatids migrate to the opposite poles and now known as chromosomes.  The endoplasmic reticulum forms the nuclear envelope around the chromosomes and the nucleolus reappears due to synthesis of ribosomal RNA (rRNA) by DNA and also due to accumulation of ribosomal proteins.  After the karyokinesis, in each haploid meiotic cell, the cytokinesis occurs and, thus, four haploid cells are resulted. These cells have different types of chromosomes due to the crossing over in the prophase I. Meiosis 1: main stages of first meiotic division Meiosis II: stages of second meiotic division  Note the following genetic phenomenon in prophase 1 synapsis Chiasmata Cossing over Terminlization process All occurs at prophase 1 substages SIGNIFICANCE OF MEIOSIS The meiosis has the greatest significance for the biological world because of its following uses: 1. The meiosis maintains a definite and constant number of the chromosomes in the organisms. 2. By crossing over, the meiosis provides an opportunity for the exchange of the genes and, thus, causes the genetical variations among the species. The variations are the raw materials of the evolutionary process. Thus the meiosis is a peculiar taxonomic, genetical and evolutionary process  STRUCTURAL CHANGES Structural changes in chromosome may be of the following types: 1. deficiency or deletion which involves loss of a broken part of a chromosome; 2. duplication involves addition of a part of chromosome (i.e., broken segment becomes attached to a homolog which, thus, bears one block of genes in duplicate); 3. inversion in which broken segment reattached to original chromosome in reverse order, 4. translocation in which the broken segment becomes attached to a nonhomologous chromosome resulting in new linkage relations.  Changes in Structure of chromosome a b c d Diagrammatic representations illustrating the four major types of chromosomal structural changes;  Changes in Chromosomes Number Each species has a characteristic number of chromo somes in the nuclei of its gametes and somatic cells. The gametic chromosome number constitutes a basic set of chromosomes called genome. A gamete cell contains single genome and is called haploid. When haploid gametes of both sexes (male and female) unite in the process of fertilization a diploid zygote with two genomes is formed chromosomal aberrations may include whole genomes and entire single chromosomes. Polyploidy is Common in Plants Phenotypic Effects of Polyploidy (i) Morphological effect of polyploidy. The polyploid plants have been found to contain large-sized pollen grains, cells, leaves, stomata, xylem, etc. The polyploid plants are more vigorous than diploids (ii) Effect on fertility of polyploidy. The most important effect of polyploidy is that it reduces the fertility of polyploid plants in variable degrees. Many polyploid plants are larger than their diploid counterparts.A comparison of octaploid (left) and diploid (right) strawberries The Origin of Hexaploid (6n) Wheat Allopolyploids vs. Autopolyploids Allopolyploids are created by hybridization between different species. Autopolyploids are created by chromosome duplication within a species. Chromosome doubling is a key event in the formation of polyploids.  Aneuploidy The under- or overrepresentation of a chromosome or a chromosome segment can affect a phenotype.  Aneuploidy—a numerical change in part of the genome Trisomy—triplication of one chromosome Hypoploid—an organism in which a chromosome or chromosome segment is underrepresented Hyperploid—an organism in which a chromosome or chromosome segment is overrepresented Monosomy—the absence of one Datura stramonium Trisomics Down Syndrome:A Human Trisomy XX, +21 ,47 Down Syndrome:A Human Trisomy Klinefelter's Syndrome (XXY, Male) 1 in every 500 to 1000 male births. Small Firm Testicles Low Testosterone Infertility Incomplete Masculinization Female Body Hair Distribution (Sparse facial, armpit, and pubic hair) Decreased Libido  Human Nondisjunction:Aneuploidy Turner Syndrome (XO)