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Structure of prokaryotic cell Structure of prokaryotic cell Structure of prokaryotic cells There are four main structures shared by all prokaryotic cells 1. The plasma membrane 2. Cytoplasm 3. Ribosomes 4. Genetic material 1. Plasma membrane • Prokaryotic cells can have multiple plasma membranes. • The plasma membrane in prokaryotic cells is responsible for controlling what gets into and out of the cell. A series of proteins stuck in the membrane also aid prokaryotic cells in communicating with the surrounding environment. • Among other things, this communication can include sending and receiving chemical signals from other bacteria and interacting with the cells of eukaryotic organisms during the process of infection. 2. Cytoplasm • The cytoplasm in prokaryotic cells is a gel-like, yet fluid, substance in which all of the other cellular components are suspended. • It is very similar to the eukaryotic cytoplasm, except that it does not contain organelles. • The cytoskeleton helps prokaryotic cells divide and helps the cell maintain its plump, round shape. 3. Ribosomes Prokaryotic ribosomes are smaller and have a slightly different shape and composition than those found in eukaryotic cells. Bacterial ribosomes have about 1/2 of the amount of ribosomal RNA (rRNA) and 1/3 fewer ribosomal proteins (53 vs. ~83) than eukaryotic ribosomes have. Just like in eukaryotic cells, prokaryotic ribosomes build proteins by translating messages sent from DNA. Genetic material • All prokaryotic cells contain large quantities of genetic material in the form of DNA and RNA. Because prokaryotic cells, by definition, do not have a nucleus, the single large circular strand of DNA containing most of the genes needed for cell growth, survival, and reproduction is found in the cytoplasm. The DNA tends to look like a mess of string in the middle of the cell Pili, flagella • Some prokaryotic cells also have other structures like the cell wall, pili (singular pillus), and flagella (singular flagellum). • Each of these structures and cellular components plays a critical role in the growth, survival, and reproduction of prokaryotic cells. Summary of differences! Prokaryotic Cells Eukaryotic cells small cel larger cells (> 10 mm) ls (< 5 mm) always unicellular often multicellular no nucleus or any membrane-bound organelles always have nucleus and other membrane-bound organelles DNA is circular, without proteins DNA is linear and associated with proteins to form chromatin ribosomes are small (70S) ribosomes are large (80S) no cytoskeleton always has a cytoskeleton cell division is by binary fission cell division is by mitosis or meiosis reproduction is always asexual reproduction is asexual or sexual Eukaryotic cell As in eukaryotic cells, the prokaryotic chromosome is intimately associated with special proteins involved in maintaining the chromosomal structure and regulating gene expression. In addition to a single large piece of chromosomal DNA, many prokaryotic cells also contain small pieces of DNA called plasmids. These circular rings of DNA are replicated independently of the chromosome and can be transferred from one prokaryotic cell to another through pili, which are small projections of the cell membrane that can form physical channels with the pili of cells. Eukaryotic cell A cell is defined as eukaryotic if it has a membrane-bound nucleus. What do eukaryotic cells have All eukaryotic cells have A nucleus Genetic material A plasma membrane Ribosomes Cytoplasm, including the cytoskeleton What others? Most eukaryotic cells also have other membrane-bound internal structures called organelles. Organelles include Mitochondria Golgi bodies Lysosomes Endoplasmic reticulum Vesicles NUCLEUS The nucleus in the cell is analogous to the brain in the body. It is a control center for a cell. The nucleus stores all the information the cell needs to grow, reproduce, and function. This information is contained in long but thin molecules of deoxyribonucleic acid, or DNA. One of the functions of the nucleus is to protect the cell’s DNA from damage. The nucleus also contains a small round body called a nucleolus that holds nucleic acids and proteins. The nuclear membrane has pores through which the contents of the nucleus communicate with the rest of the cell. The nuclear membrane tightly controls what gets into the nucleus and what gets out. Chromosomes are also located in the nucleus and are basically organized structures of DNA and proteins. In eukaryotes chromosomal DNA is packaged and organized into a condensed structure called chromatin. Chromosomes are single pieces of DNA along with genes, proteins, and nucleotides, and chromatin is a condensed package of chromosomes that basically allows all the necessary DNA to fit inside the nucleus. In eukaryotic organisms, the DNA inside the nucleus is also closely associated with large protein complexes called histones. Along with the nuclear membrane, histones help control which messages get sent from the DNA to the rest of the cell. The information stored in DNA gets transferred to the rest of the cell. The central dogma of biology is: DNA → RNA → Protein and it all starts in the nucleus Eukaryotic plasma membrane The plasma membrane in eukaryotic cells is responsible for controlling what gets into and out of the cell. A series of proteins stuck in the membrane help the cell communicate with the surrounding environment. Communication can include Sending and receiving chemical signals from other eukaryotic cells Interacting with the cells of prokaryotic organisms during the process of infection. Eukaryotic ribosomes •Eukaryotic ribosomes are larger and have a slightly different shape and composition than those found in prokaryotic cells. •Eukaryotic ribosomes have twice the amount of ribosomal RNA (rRNA) and 1/3rd more ribosomal proteins than prokaryotic ribosomes. •Despite these differences, the function of the eukaryotic ribosome is virtually identical to the prokaryotic version. Eukaryotic Cytoplasm and Cytoskeleton The cytoplasm in eukaryotic cells is a gel-like, yet fluid, substance in which all of the other cellular components are suspended, including all of the organelles. The underlying structure and function of the cytoplasm, and of the cell itself, is largely determined by the cytoskeleton, a protein framework along which particles in the cell, including proteins, ribosomes, and organelles, move around. Mitochondria They serve as power house of cell, and are surrounded by two membranes. They have their own genome. They also divide independently of the cell in which they reside, meaning mitochondrial replication is not coupled to cell division. Mitochondria The inner membrane of mitochondria has restricted permeability like the plasma membrane of a cell. The inner membrane is loaded with proteins involved in electron transport and ATP synthesis. This membrane surrounds the mitochondrial matrix, where the citric acid cycle produces the electrons that travel from one protein complex to the next in the inner membrane. Mitochondrial genomes show a great deal of variation as a result of divergent evolution. Mitochondrial genes conserved across evolution include rRNA genes, tRNA genes, and small number of genes that encode proteins involved in electron transport and ATP synthesis. The mitochondrial genome retains similarity to its prokaryotic ancestor. Mitochondrial rRNAs more closely resemble bacterial rRNAs than the eukaryotic rRNAs found in cell cytoplasm. In addition, some of the codons that mitochondria use to specify amino acids differ from the standard eukaryotic codons. Cells have extensive sets of intracellular membranes, which together compose the endomembrane system. The endomembrane system was first discovered in the late 1800s when scientist Camillo Golgi noticed that a certain stain selectively marked only some internal cellular membranes. Golgi thought that these intracellular membranes were interconnected, but advances in microscopy and biochemical studies of the various membraneencased organelles later made it clear the organelles in the endomembrane system are separate compartments with specific functions. These structures do exchange membrane material, however, via a special type of transport. The endomembrane system includes the Endoplasmic reticulum(ER), Golgi apparatus, and Lysosomes, Vesicles also allow the exchange of membrane components with a cell's plasma membrane. The ER, Golgi apparatus, and lysosomes are all members of a network of membranes, but they are not continuous with one another. Therefore, the membrane lipids and proteins that are synthesized in the ER must be transported through the network to their final destination in membrane-bound vesicles. Golgi Apparatus The Golgi apparatus functions as a molecular assembly line in which membrane proteins undergo extensive post-translational modification. Many Golgi reactions involve the addition of sugar residues to membrane proteins and secreted proteins. The carbohydrates that the Golgi attaches to membrane proteins are often quite complex, and their synthesis requires multiple steps. Golgi apparatus In electron micrographs, the Golgi apparatus looks like a set of flattened sacs. Vesicles that bud off from the ER fuse with the closest Golgi membranes, called the cis-Golgi. Molecules then travel through the Golgi apparatus via vesicle transport until they reach the end of the assembly line at the farthest sacs from the ER — called the trans-Golgi. At each workstation along the assembly line, Golgi enzymes catalyze distinct reactions. Later, as vesicles of membrane lipids and proteins bud off from the trans-Golgi, they are directed to their appropriate destinations — either lysosomes, storage vesicles, or the plasma membrane. Cytoskeletal proteins Cytoskeletal proteins are proteins that make up the cytoskeleton, flagella or cilia of cells. Generally, cytoskeletal proteins are polymers, and include tubulin (the protein component of microtubules), actin (the component of microfilaments) and lamin (the component of intermediate filaments). Contractile proteins Contractile proteins are proteins that mediate sliding of contractile fibres (contraction) of a cell’s cytoskeleton, and of cardiac and skeletal muscle. Heart and muscle contractile fibres consist of bundles of actin polymers that slide alongside each other by the activity of the motor protein myosin and associated contractile proteins such as troponin and titin. Actin Actin is the most abundant protein in most eukaryotic cells. It is highly conserved and participates in more protein-protein interactions than any known protein. These properties, along with its ability to transition between monomeric (G-actin) and filamentous (F-actin) states under the control of nucleotide hydrolysis, ions, and a large number of actin-binding proteins, make actin a critical player in many cellular functions, ranging from cell motility and the maintenance of cell shape and polarity to the regulation of transcription. Moreover, the interaction of filamentous actin with myosin forms the basis of muscle contraction. Owing to its central role in the cell, the actin cytoskeleton is also disrupted or taken over by numerous pathogens. Structure of Membrane the Plasma Like all other cellular membranes, the plasma membrane consists of both lipids and proteins. The fundamental structure of the membrane is the phospholipid bilayer which forms a stable barrier between two aqueous compartments. In the case of the plasma membrane, these compartments are the inside and the outside of the cell. Proteins embedded within the phospholipid bilayer carry out the specific functions of the plasma membrane, including selective transport of molecules and cell-cell recognition. The plasma membranes of animal cells contain four major phospholipids(phosphatidylcholine, phosph atidylethanolamine, phosphatidylserine, and sphingomyelin), which together account for more than half of the lipid in most membranes. These phospholipids are asymmetrically distributed between the two halves of the membrane bilayer. The outer leaflet of the plasma membrane consists mainly of phosphatidylcholine and sphingomyelin, whereas phosphatidylethanolamine and phosphatidylserine are the predominant phospholipids of the inner leaflet. A fifth phospholipid, phosphatidylinositol, is also localized to the inner half of the plasma membrane. Although phosphatidylinositol is a quantitatively minor membrane component, it plays an important role in cell signaling. The head groups of both phosphatidylserine and phosphatidylinositol are negatively charged, so their predominance in the inner leaflet results in a net negative charge on the cytosolic face of the plasma membrane. Phospholipid molecule Head-Glycerol and phosphates, Hydrophilic Tail-Fatty acid chains, Hydrophhobic In 1925, two Dutch scientists (E. Gorter and R. Grendel) were instrumental in finding that membranes consist of double layers Active transport Active transport describes when a cell uses energy to transport something. It deals with the movement of individual molecules across the cell membrane. The liquids inside and outside of cells have different substances. Sometimes a cell has to work and use some energy to maintain a proper balance of ions and molecules. Active transport usually happens across the cell membrane. There are thousands of proteins embedded in the cell's lipid bilayer. Those proteins do much of the work in active transport. They are positioned to cross the membrane so one part is on the inside of the cell and one part is on the outside. Only when they cross the bilayer are they able to move molecules and ions in and out of the cell. The membrane proteins are very specific. One protein that moves glucose will not move calcium (Ca) ions. There are hundreds of types of these membrane proteins in the many cells of our body. Active transport requires energy and work, passive transport does not. The process of moving sodium and potassium ions across the cell membrance is an active transport process involving the hydrolysis of ATP to provide the necessary energy. Exocytosis and endocytosis The movement of macromolecules such as proteins or polysaccharides into or out of the cell is called bulk transport. There are two types of bulk transport: exocytosis and endocytosis, and both require the expenditure of energy (ATP). Exocytosis In exocytosis, materials are exported out of the cell via secretory vesicles. In this process, the Golgi complex packages macromolecules into transport vesicles that travel to and fuse with the plasma membrane. This fusion causes the vesicle to spill its contents out of the cell. Exocytosis is important in expulsion of waste materials out of the cell and in the secretion of cellular products such as digestive enzymes or hormones. Exocytosis Exocytosis is a process in which an intracellular vesicle (membrane bounded sphere) moves to the plasma membrane and subsequent fusion of the vesicular membrane and plasma membrane ensues. Many cellular processes involve exocytosis. Examples of few of the processes that use exocytosis are: secretion of proteins like enzymes, peptide hormones and antibodies from cells. release of neurotransmitter from presynaptic neurons placement of integral membrane proteins acrosome reaction during fertilization antigen presentation during the immune response recycling of plasma membrane bound receptors Endocytosis Endocytosis, on the other hand, is the process by which materials move into the cell. There are three types of endocytosis: 1. Phagocytosis, 2. Pinocytosis, and 3. Receptor-mediated endocytosis. In phagocytosis or “cellular eating,” the cell’s plasma membrane surrounds a macromolecule or even an entire cell from the extracellular environment and buds off to form a food vacuole or phagosome. The newly-formed phagosome then fuses with a lysosome whose hydrolytic enzymes digest the “food” inside. In pinocytosis or “cellular drinking,” the cell engulfs drops of fluid by pinching in and forming vesicles that are smaller than the phagosomes formed in phagocytosis. Like phagocytosis, pinocytosis is a non-specific process in which the cell takes in whatever solutes that are dissolved in the liquid it envelops. Unlike phagocytosis and pinocytosis, receptor-mediated endocytosis is an extremely selective process of importing materials into the cell. This specificity is mediated by receptor proteins located on depressed areas of the cell membrane called coated pits. The cytosolic surface of coated pits is covered by coat proteins. In receptormediated endocytosis, the cell will only take in an extracellular molecule if it binds to its specific receptor protein on the cell’s surface. Similar to the digestive process in non-specific phagocytosis, this coated vesicle then fuses with a lysosome to digest the engulfed material and release it into the cytosol. Mammalian cells use receptor-mediated endocytosis to take cholesterol into cells. Cholesterol in the blood is usually found in lipidprotein complexes called low-density lipoproteins (LDLs). LDLs bind to specific receptor proteins on the cell surface, thereby triggering their uptake by receptor-mediated endocytosis. Lysosomes Lysosomes are central, acidic and membrane bound organelles that contain hydrolase enzyme for the breakdown of all types of biological polymers - proteins, nucleic acids, carbohydrates and lipids. They are mostly found in animal cells, while in yeast and plants, it acts as lytic vacuoles. It is enclosed by membrane known as lysosomal membrane that maintains the digestive enzyme at pH 4.5. Functions of lysosomes: • Maintains pH by pumping protons from cytosol across the membrane via proton pumps and chloride ion channels. • Protects the cytosol and rest of the cells from degradative enzymes within the lysosome. • Acts as digestive system of the cell, serving both to degrade material taken up from the outside of the cell and to digest obsolete components of cell itself. • Sequestration of lysosomal enzymes. • Mediation of fusion events between lysosomes and other organelles. • Transport of degradation products to the cytoplasm Lysosomal membrane To perform its function with efficacy the lysosomal membrane needs some additional features in its membrane. It is slightly thicker than that of the plasma membrane. It contains substantial amounts of carbohydrate componen , particularly sialic acid. In fact, most lysosomal membrane proteins are highly glycosylated, which may help protect them from the lysosomal proteases in the lumen. The lysosomal membrane has another unique property of fusing with other membranes of the cell. The entire process of digestion is carried out within the lysosome. Most lysosomal enzymes act in an acid medium. Acidification of lysosomal contents depends on an ATP-dependent proton pump which is present in the membrane of the lysosome and accumulates H+ inside the organelle. Lysosomal membrane also contains transport proteins that allow the final products of digestion of macromolecules to escape so that they can be either excreted or reutilized by the cell. Lysosomal membrane composition: The V-class H+ ATPase pump is generally present in lysosomal membrane. This class of ATPase pump only transports H+ ions. Its main function is to acidify the lumen of the organelles. The proton gradient between the lysosomal lumen (pH ≈4.5–5.0) and the cytosol (pH ≈7.0) depends on ATP production by the cell. These V-class proton pumps contain two domains: a cytosolic hydrophilic domain (V1) and a transmembrane domain (V0) with multiple subunits in each domain. Binding and hydrolysis of ATP by the B subunits in V1 provides the energy for pumping of H+ ions through the proton-conducting channel formed by the c and a subunits in V0. These V-class proton pumps are not phosphorylated and dephosphorylated during proton transport. Figure 2 CELL CYCLE The cell cycle or cell-division cycle is the series of events that take place in a cell leading to its division and duplication (replication) that produces two daughter cells. In prokaryotes which lack a cell nucleus, the cell cycle occurs via a process termed binary fission. In cells with a nucleus, as in eukaryotes, the cell cycle can be divided into three phases: interphase, the mitotic (M) phase, and cytokinesis. During interphase, the cell grows, accumulating nutrients needed for mitosis, preparing it for cell division and duplicating its DNA. During the mitotic phase, the cell splits itself into two distinct daughter cells. During the final stage, cytokinesis, the new cell is completely divided. To ensure the proper division of the cell, there are control mechanisms known as cell cycle checkpoints. Various phases G0-A resting phase where the cell has left the cycle and has stopped dividing G1-Cells increase in size S (Synthesis)–DNA replication takes place G2-During the gap between DNA synthesis and mitosis cells continue to grow. The G2 check point ensures that it is ready to enter M-Phase M-Cell growth stops and cellular energy is focused on Orderly division into 2 daughter cells Cancer The fundamental abnormality resulting in the development of cancer is the continual unregulated proliferation of cancer cells. Rather than responding appropriately to the signals that control normal cell behavior, cancer cells grow and divide in an uncontrolled manner, invading normal tissues and organs and eventually spreading throughout the body. The generalized loss of growth control exhibited by cancer cells is the net result of accumulated abnormalities in multiple cell regulatory systems and is reflected in several aspects of cell behavior that distinguish cancer cells from their normal counterparts. Cancer Cancer can result from abnormal proliferation of any of the different kinds of cells in the body, so there are more than a hundred distinct types of cancer, which can vary substantially in their behavior and response to treatment. The most important issue in cancer pathology is the distinction between benign and malignant tumors . A tumor is any abnormal proliferation of cells, which may be either benign or malignant. A benign tumor, such as a common skin wart, remains confined to its original location, neither invading surrounding normal tissue nor spreading to distant body sites. A malignant tumor however, is capable of both invading surrounding normal tissue and spreading throughout the body via the circulatory or lymphatic systems (metastasis). Only malignant tumors are properly referred to as cancers, and it is their ability to invade and metastasize that makes cancer so dangerous. Whereas benign tumors can usually be removed surgically, the spread of malignant tumors to distant body sites frequently makes them resistant to such localized treatment. Benign and malignant tumors Both benign and malignant tumors are classified according to the type of cell from which they arise. Most cancers fall into one of three main groups: 1. carcinomas –Malignancies of epithelial cells (90%) 2. Sarcomas –solid tumors of connective tissues 3. leukemias or lymphomas- arise from the blood-forming cells and from cells of the immune system Carcinomas, which include approximately 90% of human cancers, are malignancies of epithelial cells. Sarcomas, which are rare in humans, are solid tumors of connective tissues, such as muscle, bone, cartilage, and fibrous tissue. Leukemias and lymphomas, which account for approximately 8% of human malignancies, arise from the blood-forming cells and from cells of the immune system, respectively. Tumors are further classified according to tissue of origin (e.g., lung or breast carcinomas) and the type of cell involved. For example, fibrosarcomas arise from fibroblasts, and erythroid leukemias from precursors of erythrocytes (red blood cells). Benign tumor Benign tumors can be serious if they press on vital structures such as blood vessels or nerves. Therefore, sometimes they require treatment and other times they do not. Causes of Benign Tumors Often the cause is unknown. But the growth of a benign tumor might be linked to: Environmental toxins, such as exposure to radiation Genetics Diet Stress Local trauma or injury Inflammation or infection Source: National Cancer Institute, USA Malignant tumors •Malignant tumours vary in size and shape. They grow in an uncontrolled, abnormal way and can grow into (invade) nearby tissues, blood vessels or lymphatic vessels. •They can interfere with body functions and become life-threatening. Source: Mayo clinic, USA Mutagenesis Mutagenesis is a process by which the genetic information of an organism is changed in a stable manner, resulting in a mutation. It may occur spontaneously in nature, or as a result of exposure to mutagens. It can also be achieved experimentally using laboratory procedures. In nature, mutagenesis can lead to cancer and various heritable diseases, but it is also a driving force of evolution. Tumor suppressor genes The activation of cellular oncogenes represents only one of two distinct types of genetic alterations involved in tumor development; the other is inactivation of tumor suppressor genes. Oncogenes drive abnormal cell proliferation as a consequence of genetic alterations that either increase gene expression or lead to uncontrolled activity of the oncogene encoded proteins. Tumor suppressor genes represent the opposite side of cell growth control, normally acting to inhibit cell proliferation and tumor development. In many tumors, these genes are lost or inactivated, thereby removing negative regulators of cell proliferation and contributing to the abnormal proliferation of tumor cells. An oncogene is a gene that has the potential to cause cancer. In tumor cells, they are often mutated or expressed at high levels. Most normal cells will undergo a programmed form of rapid cell death (apoptosis) when critical functions are altered. Activated oncogenes can cause those cells designated for apoptosis to survive and proliferate instead. Most oncogenes require an additional step, such as mutations in another gene, or environmental factors, such as viral infection, to cause cancer. Proto-oncogene A proto-oncogene is a normal gene that can become an oncogene due to mutations or increased expression. The resultant protein encoded by an oncogene is termed oncoprotein. Proto-oncogenes code for proteins that help to regulate cell growth and differentiation. Protooncogenes are often involved in signal transduction and execution of mitogenic signals, usually through their protein products. Upon activation, a proto-oncogene (or its product) becomes a tumor-inducing agent, an oncogene. Examples of proto-oncogenes include RAS, WNT, MYC, ERK, and TRK.