"TOPIC1. ORIGIN OF CELLULAR LIFE? Origin of basic biological macromolecules; abiotic synthesis oforganic monomers and polymers? The first cells: evolution of prokaryotes; origin of eukaryotic cells;evolution of cellular eukaryotes.Organisms are composed of cells, which are arranged in tissues and organs. On the basis ofpresence or absence of nucleus, cells are classified mainly as, Prokaryotic cells and Eukaryoticcells. Genetic materials, in eukaryotic cells are separated from the cytoplasm, as geneticmaterials are present in nucleus. Nuclear envelope and cytoplasmic organelles are not seen inprokaryotic cells & these cells are smaller and simpler than eukaryotic cells. Regardless of thesedifferences, the cell is the basic unit of life and the basic molecular mechanism, whichgoverns the both types of cells, is same, indicating that cells are evolved from a singleprimordial ancestor.1.1 THE DISCOVERY OF CELLSAn English man, Robert Hooke (in 1665) used the word “cell” to describe the fine structure ofFigure 1.1 THIN SLICE OF CORKHoneycomb structure was observed byRobert Hooke, due to the cell walls of thosecells which were once living. cork. In the thin slice of cork, he observed the honeycomb like structure, due to the cell walls ofdead plant tissues. Meanwhile, a Dutchman, Anton van Leeuwenhoek, examined a drop ofpond water under the microscope and observed the “animalcules”. He also described variousforms of bacteria, which he obtained from scrapings of his teeth and from water. In 1938, aGerman Botanist, Matthias Schleiden, concluded that, in spite of the differences in structure andfunctions of various tissues, the plant embryo arose from a single cell. Meanwhile, a Germanzoologist, Theodor Schwann proposed that the cells of plants and animals share similarstructures. Hence, the concept of Cell theory is credited to these two Germans, involving twotenets of the cell theory: (1) All organisms are composed of one or more cells; (2) The cell isthe structural unit of life. The third tenet of the cell theory was proposed by a Germanpathologist, Rudolf Virchow, stating “Cells can arise only by division from a preexisting cell.”THE ORIGIN AND EVOLUTION OF CELLS1.2 ORIGIN OF BASIC BIOLOGICAL MOLECULES(A)ABIOTIC SYNTHESIS Many explanations have been proposed regarding the origin of lifeon Earth (at least 3.8 billion years ago). We know that organic molecules are the important partof the life and for a cell synthesis, organic molecule is required. Hence, to understand the originof life or evolution of cell, first we have to understand the origin of organic molecules.THEORIES1. Organic molecules were rained down on Earth from outer space:Astronomers have identified a number of organic molecules (methane, methanol, formaldehyde,cyanoacetylene) and some inorganic building blocks (carbon monoxide, carbon dioxide,hydrogen sulphide) in interstellar space using infrared spectroscopy. Many reports of the aminoacids and organic molecule production by taking mixture of those molecules, present ininterstellar space and then exposing these mixtures to high temperature or ultraviolet light maysupport this theory. But the fact of absence of many amino acids, which are essential part ofhuman life and the presence of isotopes in higher amount, in the interstellar space and meteorite,questions this theory. Whether or not the organic, inorganic molecules that formed life arrivedhere from interstellar space, there is doubt that organic matter rains down on the Earthcontinuously.2. Organic molecules were synthesized at hydrothermal vents on the ocean floor: At around 100 degree Celsius, some deep-sea hydrothermal vents discharge high amounts ofhydrogen sulphide, carbon dioxide and hydrogen. Formed gases bubbles are found to be rich iniron sulphides, which can catalyze the organic molecule synthesis and also, the fact that manyenzymes have iron and Sulphur atoms at their active sites lead to this theory.3. Organic molecules were synthesized when comets or asteroid struck the Earth:In 2014, some researchers in the Czeck republic reported the synthesis of the four bases foundin RNA in the abiotic conditions by bombarding the mixture of formamide (thought to bepresent in high amount on the early Earth) with laser pulses. This condition mimicked thepressure-temperature combination, expected when a meteorites strikes the Earth.4. Organic molecules were synthesized from inorganic compound in the atmosphere: The most acceptable proposal (in 1920) suggests that the life was originated in the sea followinga chemical evolutionary process. It is said that initially atmosphere of Earth contained carbondioxide and nitrogen in high amount and hydrogen, hydrogen sulphide and carbon monoxide insmall amounts; whereas little or no free oxygen was present in Earth’s atmosphere. Thisatmosphere resulted in the reducing conditions. In 1950s, Stanley Miller%demonstratedexperimentally the spontaneous formation of organic molecules. In this experiment, a variety oforganic molecules, such as amino acids were formed in the presence of water, when the electricsparks were passed into a mixture of hydrogen, methane and ammonia gases. At the end of aweek, to show the presence of amino acids and other organic molecules, Miller performed paperchromatography.(B) ASSEMBLING POLYMERS: HOW POLYMER-the basis of life itself- could beASSEMBLED?: Abiotic synthesis produces a mixture of enantiomers and each type inhibits thepolymerization of other. Hence, it is believed that early polymers were used to assemble on solidor mineral surfaces that prevented their degradation. In the laboratory, RNA molecules havebeen synthesized on mineral surfaces. Figure1.2SPOMTANEOUSFORMATION OF ORGANICMOLECULES: Water vapor wasrefluxed through an atmosphere consistingof methane, ammonia and hydrogen, intowhich electric sparks were discharged.Analysis of the reaction products revealedthe formation of a variety of organicmolecules, including the amino acidsalanine, aspartic acid, glutamic acid andglycine.Most acceptable hypothesis (1.2 A.4) suggests that the inorganic compounds, present in thisreducing atmosphere, interacted to form organic molecules in the presence of a source ofenergy (sunlight, electrical discharges or high temperatures). These organic compoundsaccumulated in the sea and then subjected to physical effects of the environment, combined andformed amino acids. These amino acids formed peptides, macromolecules and other complexorganic substances, which served as the precursors of the life.The ability to replicate itself is thecritical criteria of precursors, resulting in the capability of reproduction and further evolution.(C) AN RNA BEGINNING: In present-day cells, the nucleic acids have the capability to directtheir own self-replication, where as another informational macromolecule protein lacks thischaracter. In understanding molecular evolution, Sid Altman and Tom Cech has an importantcontribution. In 19802s, they discovered the capability of RNA to catalyze a number ofreactions, such as nucleotide polymerization and the description of RNA molecules, where RNA directs the new RNA strand synthesis from an RNA template. Thus RNA is unique, as it notonly serves as a template but also catalyze its own replication.It is also believed that RNA is the initial genetic system and above mentioned Chemicalevolution is thought to be based on self replicating RNA molecules. Hence, that period ofevolution is also known as the RNA world. Ordered interactions have been occurred betweenRNA and amino acids, resulting in the present day evolved genetic code and RNA had beenreplaced by DNA as the genetic material.1.3 THE FIRST CELLIt is believed that the first cell was formed, when membrane composed of phospholipidsenclosed the RNA. The amphipathic nature of the phospholipids is an key character. Thehydrophobic (water insoluble) chain is joined to hydrophilic (water soluble) chain head groups,which contain phosphate. The phospholipid bi-layer results in a stable barrier, separating theFigure 1.3ENCLOSUREOF SELFREPLICATING RNAIN A PHOSPHOLIPIDMEMBRANE Eachphospholipid has twohydrophobic tails attachedto a hydrophilic headgroup. The hydrophobictails are buried in the lipidbilayer: the hydrophilicheads are exposed towater on both sides of themembrane. interior of the cell from external environment. It maintained the enclosed RNA and associatedmolecules as a unit. During this period, evolution of RNA-directed protein synthesis may haveoccurred, resulting the first cell consisting of RNA, which has self-replication ability and itsencoded protein and cell had capability of self-reproduction and further evolution.THE EVOLUTION OF METABOLISMAs cells were originated in a sea, they were able to obtain energy and food from the environmentdirectly due to the accumulation of organic compounds. But,as these situations are self- limiting, cells had to evolve their mechanisms to generate energy and synthesize themolecules, required for replication. The synthesis, utilization and principal pathways formetabolism of energy are conserved in present day cells. In spite of the differences, every cellsutilize Adenosine 5- triphosphate as the source of energy for various purpose (synthesis of cellcomponents, movement etc.). The mechanisms,to synthesize ATP have been evolved due to thedevelopment of three important metabolic pathways, which are glycolysis, photosynthesis andoxidative metabolism.As it is already mentioned that during the initial phase, the Earth’s atmosphere contained no freest oxygen (1.2 A.4),the 1 reactions which generate energy is believed to be the anaerobicbreakdown of organic molecule, Glycolysis pathway in which glucose is converted intolactate. This pathway has the net energy gain of 2 molecules of ATP, which is the source ofchemical energy. Regardless of the fact that this pathway arose very early during evolution, allpresent day cells still carry out glycolysis. The next evolutionary step is thought to be the photosynthesis development, as it allowed thecell to convert light energy into chemical energy and provided independence to the cell fromst the preformed organic compound utilization. Hydrogen sulphide was utilized by 1photosynthetic bacteria to convert carbon dioxide into organic molecule. Due to the developmentin photosynthesis, use of water as electron donor and hydrogen to convert carbon dioxide intoorganic molecules has been evolved, producing free oxygen as by product. This is considered tobe the responsible for the abundance of oxygen in the atmosphere of Earth, which led to theoxidative metabolism developmentOxidative metabolism provided a mechanism, which is more efficient than anaerobic pathwayto generate energy from organic compound utilizing high reactivity of oxygen (the completeaerobic breakdown of glucose into carbon dioxide and water yields energy of 36-38 molecules ofATP).1.4 EVOLUTION OF PROKARYOTESProkaryotes are simple unicellular organisms lacking a nucleus and other complex organelles,which are found in eukaryotes. In 1990, Cart Woese divided prokaryotes into archaebacteria, asmall group of primitive anaerobes found in extreme environments (high salt, acidity,temperature) and bacteria, live in wide range of environment, at various temperatures andacidity, with or without oxygen.Prokaryotes are mostly bacteria, and their advancements led to more complex living organisms.As bacteria modified structures to expand their tolerance and territory, they changed into newer "