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INTRODUCTION TO CELL AND MOLECULAR BIOLOGY GENERAL PRINCIPLES OF CELLULAR ORGANIZATION Biology is a science that studies life. It is based on the fundamental laws of nature embodied in chemistry and physics. The field of biology today is so wide, that it has been divided into some separate disciplines. Molecular biology is one of these disciplines. The term molecular biology was first used in 1945 by William Astbury and was referred to the study of the chemical and physical structure of biological macromolecules (biopolymers). By that time biochemists had discovered many fundamental intracellular chemical reactions and explained the importance of proteins in cell activity. In 1953, scientists identified that DNA was the macromolecule containing the genetic information of a cell. Following this discovery, the new field of molecular genetics appeared. In the late 1970, a new method – recombinant DNA technology – was elaborated. This provided new tools and, in time, information about all cells became available at an extraordinary rate. As the molecular mechanisms of life have become clearer, the underlying similarities became more impressive than the differences. Biologists are confident that a limited number of general principles, summarizing common molecular mechanisms, will eventually explain even the most complex life processes in terms of chemistry and physics. COMMON CHARACTERISTICS OF LIVING CELLS There are certain common characteristics of all living organisms: growth, reproduction, homeostasis, metabolism, sensitivity, and energy acquisition. 1. Growth and development. Even single-celled (unicellular) organisms grow. When first formed by cell division, the cells are small, and must grow and develop into mature cells. Multicellular organisms pass through a more complicated process of differentiation and organogenesis. 2. Reproduction. All living things must be able to reproduce. Through reproduction the species continues to survive. All their hereditary characteristics that determine a species and make it suitable for a particular environment are transmitted to their offspring. 3. Homeostasis. It is the maintenance of a constant internal environment in terms of temperature, pH, water concentrations etc. An organism adjusts its metabolism to maintain stable internal conditions for an effective functioning of the organism. 4. Metabolism. All living beings must have a metabolism, the ability to carry out chemical reactions and exchange substances. 5. Sensitivity is the ability to respond to stimuli (both internal and external). 6. Energy acquisition and release. One view of life is that it is a struggle to acquire energy (from sunlight, inorganic chemicals or another organisms), and release it in the process of forming ATP. LEVELS OF ORGANIZATION OF BIOLOGICAL SYSTEMS Life on Earth is incredibly extensive and, to make it easier to study, biologists have broken living systems up into generalized hierarchical levels: molecules cells tissues organisms populations communities ecosystems biosphere Introduction to Molecular Biology. PL1 The focus of this course is on the fundamentals of life; that is, the properties that are held in common among all living things. We will concentrate almost exclusively on the molecular and cellular levels. CELL – MORPHOLOGICAL AND STRUCTURAL UNITY OF LIFE Cells are the structural units of all living beings (with the possible exception of viruses). All cells have the following characteristics in common. CELL MEMBRANE that separates the chaos outside a cell from the high degree of organization within the cell. A cell without a cell membrane is not a cell. CONTAINS DNA as its genetic material. All cells contain several varieties of RNA molecules and proteins; most of them are enzymes. All cells are composed of the same BASIC CHEMICALS: carbohydrates, proteins, nucleic acids, minerals, fats and vitamins. All cells REGULATE the flow of nutrients and wastes that enter and leave the cell. All cells REPRODUCE and are the result of reproduction. All cells require a SUPPLY OF ENERGY. All cells are HIGHLY REGULATED by ELABORATING SENSING SYSTEMS (chemical “noses”) that allow them to be aware of every reaction and many of the environmental conditions around them; this information is continually PROCESSED to make metabolic decisions. The above criteria are the minimal requirements of life. Two general cell types have evolved: prokaryotic and eukaryotic cells. Current data supports the theory that prokaryotes represent the initial or primitive (the simplest) cell type on earth and that eukaryotic cell types evolved from them. There is strong data to support the idea that eukaryotes evolved from aggregates of prokaryotic cells that became interdependent upon one another and eventually merged (fused) into a single larger cell. Eukaryotic cells are structurally and biochemically more complex than prokaryotes. They contain many membrane-bound organelles (cell structures with specific molecular organization and distinct functions), whereas prokaryotic cells contain no organelles. COMPARATIVE CHARACTERIZATION OF PROKARYOTES AND EUKARYOTES The both cell types do have DNA as genetic material (thus also possessing different RNAs, ribosomes and proteins), have an exterior membrane (plasma membrane), and are very diverse. Prokaryotes are bacteria and blue green algae (Kingdom Monera). Prokaryotes are cells without a nucleus and membrane-bound organelles. They have ribosomes genetic material, which is however not enclosed within a membrane. The genetic material is a single circular DNA situated in the cytoplasm. The prokaryotic DNA is associated with proteins, which can be easily separated. The reproduction of prokaryotes is through binary fission (no sexual process takes place). Nevertheless, there is a way for the exchange of genetic information through transfer of plasmids (short circles of DNA that pass from one bacterium to another). Often plasmids carry genes of resistance to antibiotics. The prokaryotes do not engulf Bacterium Structure solids nor do they have centrioles. The membrane of prokaryotes lacks cholesterol. Prokaryotes have a cell wall made up of peptidoglycan. The cytoplasm of 2 Introduction to Molecular Biology. PL1 prokaryotes is motionless. Any internal membranes are elaborations of the plasma membrane and are known as mesosomes. The mesosome is believed to play the role of ATP synthesis or energy center of prokaryotic cell. Eukaryotes form the remaining four kingdoms: Protista (ex.: Protozoa like Amoeba or Trypanosoma; Algal Protists such as Euglena or Chlamydomonas; and Fungus-like Protists, which include species of Myxomycota or slime molds), Fungi (yeasts, rusts, smuts, puffballs, truffles, molds), Plantae (plants), and Animalia (animals and humans). These are cells with a nucleus – an organelle where the genetic material is surrounded by an envelope (double membrane). The genetic material is encoded by linear DNA molecules that in complex with proteins form multiple chromosomes. Eukaryotes also contain membrane-bound organelles (Endoplasmic Reticulum (ER), Golgi apparatus (GA), lysosomes, peroxisomes, mitochondria, vesicles, endosomes) and some non-membranous organelles (ribosomes, nucleolus, cenrioles). The most complex eukaryotes are composed of plant and animal cells. Plants vary from animal cells in that they have large vacuoles, cell wall, chloroplasts, and a lack of lysosomes, centrioles, pseudopods, and flagella or cilia. Animal cells do not have the chloroplasts, and may or may not have cilia, pseudopods or flagella, depending on the type of cell. VIRUSES A virus is a submicroscopic infection particle composed of a protein coat and a nucleic acid core. The diameter of viral particles is 20-30 nm. Thus, they are much smaller than any prokaryotic cell. Viruses, like cells, carry genetic information encoded in their nucleic acid, and can undergo mutation and reproduce; however they cannot carry out metabolism. Viruses are obligated intracellular parasites, meaning that they require host cells to reproduce. In the viral life cycle, a virus infects a cell, allowing the viral genetic information to direct the synthesis of new virus particle by the cell. Outside the cell they can only be in nonreplicative state, as they lack enzymes necessary for complete reproduction of virus particle. There are many kinds of viruses. Viruses are classified by the type of nucleic acid they contain and the shape of their protein capsule. They can be: DNA – containing and RNAcontaining. DNA-containing viruses can be spiral, octahedral, complex without envelope, complex with envelope. RNA – containing viruses have RNA instead of DNA and the enzyme reverse transcriptase. Once inside the host cell, reverse transcription (making DNA from RNA) is accomplished by the reverse transcriptase. This new DNA is incorporated into the host DNA, where it transcribes new viral RNA genomes, as well as the RNA to synthesize new reverse transcriptase and protein capsules. Viruses cause a variety of diseases among all groups of living organisms. Viral diseases include the flu, common cold, herpes, measles, chicken pox and encephalitis. Bacteriophages (viruses that infect bacteria) invade the host cell and begin replicating viruses, eventually lysing or bursting the host cell, realizing the new viruses. 3