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.Mostsubcellular structures are too small to be resolved by the light micro-scope.That was the task reserved for the electron microscope in the1950 s and thereafter.Nowadays, imaging techniques have advancedeven further with the introduction of the scanning microscope comple-mented with signal processing methods.There are two types of structurally different cells: prokaryotes and eu-karyotes (from Greek, pro, before, karyon, kernel or nucleus, and eu, trueor correct).Only bacteria are prokaryotic cells, the rest (plants, animals)are eukaryotic.The prokaryotic cell has no nucleus.Its genetic material is concentratedin a region called nucleoid without membrane.The eukaryotic cell, in-stead, is surrounded by a membrane.The region between the nuclearmembrane and the cell membrane (or plasma membrane) is called cyto-plasm, where the organelles are located.The latter are largely absent inprokaryotic cells.Here, we are mainly concerned with eukaryotic cells.Their size is a gen-eral feature that strongly relates to function.Its plasma membrane acts asa selective barrier that allows sufficient passage of oxygen, nutrients, andwastes to service the entire cell.We have already referred to this mem-brane in the electrophysiology section and are familiar with the importantionic exchange that takes place across it.In addition to the externalmembrane, a eukaryotic cell has extensive and elaborately arranged in-ternal membranes, which partition the cell into compartments.They par-ticipate in the cell s metabolism.2.9.3.The NucleusThe nucleus (about 5µ in diameter), the most conspicuous organelle,contains the majority of the genes that control the eukaryotic cell.Thenuclear envelope is a double membrane separated by a space in the orderof 30×10 3µ.The envelope is perforated by pores of about 100×10 3µ.Atthe lip of each pore, the two membranes fuse.Let us remind that organ-elles are subcellular or intracellular structures.By and large, they are toosmall to be resolved by the light microscope.214 Understanding the Human MachineWithin the nucleus, the DNA (deoxyriboneucleic acid) is organizedalong with proteins into material called chromatin.As a cell prepares todivide (reproduce), chromatin coils up becoming thick enough to be dis-cerned as separate structures called chromosomes (chromos means color in Greek, and the name originates in the staining procedures tovisualize these structures when observed under the microscope).The nu-cleolus is the most prominent structure within the nucleus.The nucleus controls protein synthesis in the cytoplasm by sending mo-lecular messengers in the form of RNA (ribonucleic acid).This messen-ger RNA, or mRNA, is synthesized in the nucleus following instructionssupplied by the DNA.Once in the cytoplasm, mRNA attaches to ri-bosomes, where the genetic message is translated into the primary struc-ture of a specific protein.Ribosomes are particles whose initial compo-nents are synthesized within the nucleus, too.Such components traversethe nuclear envelope pores into the cytoplasm where they combine toform these small organelles.DNA is the molecule which contains genetic information and makes upour genes.It consists of two complementary nucleotide chains containingthe bases adenine (A), thymine (T), guanine (G) and cytosine (C), held ina double stranded helix by bonds between base-pairs.A with T and Gwith C.The chromosomes are structures made up of DNA and proteins.Theycontain the genetic information in a linear array.The order of nucleotidebases along a DNA strand is known as the sequence.The genetic infor-mation is encoded in the precise order of the base-pairs.DNA sequencingis the laboratory process designed to precisely determine the sequence ofbases in the DNA.During cell division, the entire DNA of the cell is cop-ied.Human cells have 23 pairs of chromosomes, one of each pair inheritedfrom each parent.The basic unit of heredity is the gene; they are orderedsequences of DNA base-pairs, located in specific positions on chromo-somes.Genes contain the information for producing proteins or func-tional RNA molecules, which make up the structure of cells, and controltheir functions.The complete genetic material of an organism conforms the genome.Thehaploid human genome contains 3 billion base-pairs of DNA organizedChapter 2.Source: Physiological Systems and Levels 215into 23 chromosomes.Diploid is the state of having two copies of eachchromosome.Most human cells, except the gametes, are diploid with 46chromosomes (23 pairs), including the sex chromosomes.Conversely,haploid is the state of having one copy of each chromosome.The repro-ductive cells are haploid, with 23 chromosomes in human egg and spermcells.In short, genes are sequences of base-pairs that encode information forproteins.They can range in size from less than 100 base-pairs to severalmillion base-pairs.A diploid genome of 6x109 bases, if stretched, wouldmeasure about 1.8 meters long.Chromosomes are relatively large struc-tures made up of DNA and proteins that contain genes, that is, the ge-netic information.The nucleus acts as a master station.2.9.4.Closing Remarks of Section 9Cell anatomy and cell physiology are inter-related disciplines of basicbiology standing on their own feet.This section is a mere refreshment ofsome concepts or just a bare preliminary set.We should briefly mentionthe remaining parts.The different membranes of an eukaryotic cell form the endomembranesystem.It includes the nuclear envelope, the endoplasmic reticulum, theGolgi apparatus (not related at all to the Golgi tendon organ already de-scribed before), lysosomes, and different types of vacuoles.Although theplasma membrane is the cellular boundary, it is part of the system be-cause it strongly interacts with the endoplasmic reticulum and other in-ternal membranes.Functions of this system are many, complex and im-portant
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