Saturday, 30 January 2016




Gregor Johann Mendel, born in 1822, is now remembered as the father of genetics  .

He grew up on a small farm in Hyncice (formerly Heinzendorf) in northern Moravia, which
was then a part of Austria and is now a part of the Czech Republic.

As a young boy, he worked with his father grafting trees to improve the family orchard. Undoubtedly, his success at grafting taught him that precision and attention to detail are important elements of success.

These qualities would later be important in his experiments as an adult scientist. Instead of farming, however,
Mendel was accepted into the Augustinian monastery of St. Thomas, completed his studies for the priesthood, and was ordained in 1847.

 Soon after becoming a priest, Mendel worked for a short time as a substitute teacher. To continue that role, he
needed to obtain a teaching license from the government. Surprisingly, he failed the licensing exam due to poor answers in the areas of physics and natural history.

Therefore, Mendel then enrolled at the University of Vienna to expand his knowledge in these two areas.

 Mendel’s training in physics and mathematics taught him to perceive the world as an orderly place, governed
by natural laws. In his studies, Mendel learned that these natural laws could be stated as simple mathematical relationships.

In 1856, Mendel began his historic studies on pea plants. For 8 years, he grew and crossed thousands of pea plants on a small 115- by 23-foot plot.

 He kept meticulously accurate records that included quantitative data concerning the outcome of his

He published his work, entitled “Experiments on Plant Hybrids,” in 1866. This paper was largely ignored by scientists at that time, possibly because of its title.

 Another reason his work went unrecognized could be tied to a lack of understanding of chromosomes and their transmission,   Nevertheless, Mendel’s ground-breaking work allowed  him to propose the natural laws that now provide a framework for our understanding of genetics.

Prior to his death in 1884, Mendel reflected, “My scientific work has brought me a great deal of satisfaction and I am convinced that it will be appreciated before long by the whole world.”

Sixteen years later, in 1900, the work of Mendel was independently rediscovered by three biologists with an interest in plant genetics: Hugo de Vries of Holland, Carl Correns of Germany, and Erich von Tschermak of Austria. Within a few years, the influence of Mendel’s studies was felt around the world.

In this section, we will examine Mendel’s experiments and consider their monumental significance in the field of genetics.

Mendel Chose Pea Plants as His Experimental Organism

Mendel’s study of genetics grew out of his interest in ornamental flowers.

Prior to his work with pea plants, many plant breeders had conducted experiments aimed at obtaining flowers with new varieties of colors.

When two distinct individuals with different characteristics are mated, or crossed, to each other, this is called
a hybridization experiment, and the offspring are referred to as hybrids. For example, a hybridization experiment could involve a cross between a purple-flowered plant and a white-flowered plant. Mendel was particularly intrigued, in such experiments, by the consistency with which offspring of subsequent generations showed characteristics of one or the other parent. His intellectual foundation in physics and the natural sciences led him to

Structure of a pea flower

Pollination and fertilization in angiosperms

Pollination and fertilization in angiosperms

Flower structure and pollination in pea plants.  The pea flower can produce both pollen and egg cells. The pollen grains are produced within the anthers, and the egg cells are produced within the ovules that are contained within the ovary. A modified petal called a keel encloses the anthers and ovaries. (b) Photograph of a flowering pea plant.

 A pollen grain must first land on the stigma. After this occurs, the pollen sends out a long tube through which two sperm cells travel toward an ovule to reach an egg cell.

The fusion between a sperm and an egg cell results in fertilization and creates a zygote. A second sperm fuses with a central cell containing two polar nuclei to create the endosperm. The endosperm provides a nutritive material for the developing embryo.

consider that this regularity might be rooted in natural laws that could be expressed mathematically. To uncover these laws, he realized that he would need to carry out quantitative experiments in which the numbers of offspring carrying certain traits were carefully recorded and analyzed. Mendel chose the garden pea, Pisum sativum, to investigate the natural laws that govern plant hybrids. The morphological features of this plant are shown in Figure 

Several properties of this species were particularly advantageous for studying plant hybridization. First, the species was available in several varieties that had decisively different physical characteristics.

Many strains of the garden pea were available that varied in the appearance of their height, flowers, seeds, and pods. A second important issue is the ease of making crosses. In flowering plants, reproduction occurs by a pollination event

 Male gametes (sperm) are produced within pollen grains formed in the anthers, and the female gametes (eggs) are contained within ovules that form in the ovaries. For fertilization to occur, a pollen grain lands on the stigma, which stimulates the growth of a pollen tube.

This enables sperm cells to enter the stigma and migrate toward an ovule. Fertilization occurs when a sperm enters the micropyle, an opening in the ovule wall, and fuses with an egg cell. The term gamete is used to describe haploid reproductive cells that can unite to form a zygote.

 It should be emphasized, however, that the process that produces gametes in animals is quite different from the way that gametes are produced in plants and fungi.

In some experiments, Mendel wanted to carry out self fertilization, which means that the pollen and egg are derived from the same plant. In peas, a modified petal known as the keel covers the reproductive structures of the plant.

Because of this covering, pea plants naturally reproduce by self-fertilization. Usually, pollination occurs even before the flower opens. In other experiments, however, Mendel wanted to make crosses between different plants.

 How did he accomplish this goal? Fortunately, pea plants contain relatively large flowers that are easy to manipulate, making it possible to make crosses between two particular plants and study their outcomes.

 This process, known as cross-fertilization, requires that the pollen from one plant be placed on the stigma of another plant.

 This procedure is shown in Figure  Mendel was able to pry open immature flowers and remove the anthers before they produced pollen. Therefore, these flowers could not self- fertilize.

He would then obtain pollen from another plant by gently touching its mature anthers with a paintbrush. Mendel applied this pollen to the stigma of the flower that already had its anthers removed.

In this way, he was able to cross-fertilize his pea plants and thereby obtain any type of hybrid he wanted.

cross-fertilized two different pea plants

How Mendel cross-fertilized two different pea plants.

 This illustration depicts a cross between a plant with purple flowers and another plant with white flowers. The offspring from this cross are the result of pollination of the purple flower using pollen from a white flower.

Mendel Studied Seven Characteristics That Bred True

When he initiated his studies, Mendel obtained several varieties of peas that were considered to be distinct. These plants were different with regard to many morphological characteristics.

 The general characteristics of an organism are called characters. The terms trait and variant are typically used to describe the specific properties of a character.

 For example, eye color is a character of humans and blue eyes is a trait (or variant) found in some people.
Over the course of 2 years, Mendel tested his pea strains to determine if their characteristics bred true.

 This means that a trait did not vary in appearance from generation to generation. For example,
if the seeds from a pea plant were yellow, the next generation would also produce yellow seeds. Likewise, if these offspring were allowed to self-fertilize, all of their offspring would also produce yellow seeds, and so on.

 A variety that continues to produce the same trait after several generations of self-fertilization is called a
true-breeding line, or strain.

Mendel next concentrated his efforts on the analysis of characteristics that were clearly distinguishable between different true-breeding lines. Figure  illustrates the seven character that Mendel eventually chose to follow in his breeding experiments.

All seven were found in two variants. A variant (or trait) may be found in two or more versions within a single species. For example, one character he followed was height, which was found in two variants: tall and dwarf plants. Mendel studied this character by crossing the variants to each other.

A cross in which an experimenter is observing only one character is called a monohybrid
cross, also called a single-factor cross.

 When the two parents are different variants for a given character, this type of cross produces single-character hybrids, also known as monohybrids.

An illustration of the seven characters that Mendel studied. Each character was found as two variants that were decisively different from each other.



·         An appreciation for the concept of heredity can be traced far back in human history. Hippocrates, a famous Greek physician, was the first person to provide an explanation for hereditary traits (ca. 400 b.c.e.).

·         He suggested that “seeds” are produced by all parts of the body, which are then collected and transmitted to the offspring at the time of conception. Furthermore, he hypothesized that these seeds cause certain traits of the offspring to resemble those of the parents.

This idea, known as pangenesis, was the first attempt to explain the transmission of hereditary traits from generation to generation

·         For the next 2000 years, the ideas of Hippocrates were accepted by some and rejected by many.

·         After the invention of the microscope in the late seventeenth century, some people observed sperm and thought they could see a tiny creature inside, which they termed a homunculus (little man). This homunculus was hypothesized to be a miniature human waiting to develop within the womb of  its mother.
·         Those who held that thought, known as spermists, suggested that only the father was responsible for creating future generations and that any resemblance between mother and offspring was due to influences “within the womb.”

·         During the same time, an opposite school of thought also developed.

·         According to the ovists, the egg was solely responsible for human characteristics The only role of the sperm was to stimulate the egg onto its path of development.

·         Of course, neither of these ideas was correct. The first systematic studies of genetic crosses were carried out by Joseph Kölreuter from 1761 to 1766. In crosses between different strains of tobacco plants, he found that the offspring were usually intermediate in appearance between the two parents.

·         This led Kölreuter to conclude that both parents make equal genetic contributions to their offspring. Furthermore, his observations were consistent with blending inheritance.

·         According to this view, the factors that dictate hereditary traits can blend together from generation to generation.

·         The blended traits would then be passed to the next generation. The popular view before the 1860s, which combined the notions of pangenesis and blending inheritance, was that hereditary traits were rather malleable and could change and blend over the course of one or two generations.

·         However, the pioneering work of Gregor Mendel would prove instrumental in refuting this viewpoint.

·         In Chapter 2, we will first examine the outcome of Mendel’s crosses in pea plants. We begin our inquiry into genetics here because the inheritance patterns observed in peas are fundamentally related to inheritance patterns found in other eukaryotic species, such as humans, mice, fruit flies, and corn.

·         We will discover how Mendel’s insights into the patterns of inheritance in pea plants revealed some simple rules that govern the process of inheritance.

·         In Chapters 3 through 8 , we will explore more complex patterns of inheritance and also consider the role that chromosomes play as the carriers of the genetic material. In the second part of this chapter, we will become familiar
·         with general concepts in probability and statistics. How are statistical methods useful? First, probability calculations allow us to predict the outcomes of simple genetic crosses, as well as the outcomes of more complicated crosses described in later chapters.

·         In addition, we will learn how to use statistics to test the validity of genetic hypotheses that attempt to explain the inheritance patterns of traits.

Friday, 29 January 2016



Hardly a week goes by without a major news story involving a genetic breakthrough. The increasing pace of genetic discoveries has become staggering.

The Human Genome Project is a case in point. This project began in the United States in 1990, when the National Institutes of Health and the Department of Energy joined forces with international partners to decipher the massive amount of information contained in our genome—the DNA found within all of our chromosomes.

Working collectively, a large group of scientists from around the world has produced a detailed series of maps that help geneticists navigate through human DNA. Remarkably, in only a decade, they determined the DNA sequence (read in the bases of A, T, G, and C) covering over 90% of the human genome.

The first draft of this sequence, published in 2001, is nearly 3 billion nucleotide base pairs in length.

The completed sequence, published in 2003, has an accuracy greater than 99.99%; fewer than one mistake was
made in every 10,000 base pairs (bp)! Studying the human genome allows us to explore fundamental details about ourselves at the molecular level.

The results of the Human Genome Project are expected to shed considerable light on basic questions, like how many genes we have, how genes direct the activities of living cells, how species evolve, how single cells develop into complex tissues, and how defective genes cause disease.

Furthermore, such understanding may lend itself to improvements in modern medicine by leading to better diagnoses of diseases and the development of new treatments for them .

As scientists have attempted to unravel the mysteries within our genes, this journey has involved the invention of many new technologies.

For example, new technologies have made it possible to produce medicines that would otherwise be difficult or impossible to make.

 An example is human recombinant insulin, sold under the brand name Humulin.

This medicine is synthesized in strains of Escherichia coli bacteria that have been genetically altered by the addition of genes that encode the polypeptides that form human insulin. The bacteria are grown in a laboratory and make large amounts of human insulin.

 As discussed in Chapter 19 , the insulin is purified and administered to many people with insulin dependent
Diabetes .
DNA, the molecule of life

 Trillions of cells Each cell contains:
• 46 human chromosomes, found in 23 pairs
• 2 meters of DNA
• Approximately 3 billion DNA base pairs per set of chromosomes, containing the bases A, T, G, and C
• Approximately 20,000 to 25,000 genes coding for proteins that perform most life functions

Tumour immunology animated lecture

Tumour immunology animated lecture

Review about immunology

Review about immunology

Wednesday, 27 January 2016

the Specificity of the Antibody Antigen Interaction

Early Theories Attempted to Explain
the Specificity of the Antibody
Antigen Interaction

  • One of the greatest enigmas facing early immunologists was the specificity of the antibody molecule for foreign material, or antigen (the general term for a substance that binds with a specific antibody).
  • Around 1900, Jules Bordet at the Pasteur Institute expanded the concept of immunity by demonstrating specific immune reactivity to nonpathogenic substances, such as red blood cells from other species.
  • Serum from an animal inoculated previously with material that did not cause infection would react with this material in a specific manner, and this reactivity could be passed to other animals by transferring serum from the first. 
  • The work of Karl Landsteiner and those who followed him showed that injecting an animal
  • with almost any organic chemical could induce production of antibodies that would bind specifically to the chemical.
  • These studies demonstrated that antibodies have a capacity for an almost unlimited range of reactivity, including responses to compounds that had only recently been synthesized
  • in the laboratory and had not previously existed in nature.
  •  In addition, it was shown that molecules differing in the smallest detail could be distinguished by their reactivity with different antibodies. Two major theories were proposed to account for this specificity: the selective theory and the instructional theory.
  • The earliest conception of the selective theory dates to Paul Ehrlich in 1900. In an attempt to explain the origin of serum antibody, Ehrlich proposed that cells in the blood expressed a variety of receptors, which he called “side-chain receptors,” that could react with infectious agents and inactivate them.
  • Borrowing a concept used by Emil Fischer in 1894 to explain the interaction between an enzyme and its substrate, Ehrlich proposed that binding of the receptor to an infectious agent was like the fit between a lock and key.
  •  Ehrlich suggested that interaction between an infectious agent and a cell-bound receptor would induce the cell to produce and release more receptors with the same specificity.
  • According to Ehrlich’s theory, the specificity of the receptor was determined before
  • its exposure to antigen, and the antigen selected the appropriate receptor. 
  • Ultimately all aspects of Ehrlich’s theory would be proven correct with the minor exception that the
  • “receptor” exists as both a soluble antibody molecule and as a cell-bound receptor; it is the soluble form that is secreted rather than the bound form released.
  • In the 1930s and 1940s, the selective theory was challenged by various instructional theories, in which antigen played a central role in determining the specificity of the antibody molecule. 
  • According to the instructional theories, a particular antigen would serve as a template around which
  • antibody would fold. The antibody molecule would thereby assume a configuration complementary to that of the antigen template. 
  • This concept was first postulated by Friedrich Breinl and Felix Haurowitz about 1930 and redefined in the 1940s in terms of protein folding by Linus Pauling. 
  • The instructional theories were formally disproved in the 1960s, by which time information was emerging about the structure of DNA, RNA, and protein that would offer new insights into the vexing problem of how an individual could make antibodies against almost anything.
  • In the 1950s, selective theories resurfaced as a result of new experimental data and, through the insights of Niels Jerne, David Talmadge, and F. Macfarlane Burnet, were refined
  • into a theory that came to be known as the clonal selection theory. 
  • According to this theory, an individual lymphocyte expresses membrane receptors that are specific
  • for a distinct antigen. 
  • This unique receptor specificity is determined before the lymphocyte is exposed to the antigen.
  • Binding of antigen to its specific receptor activates the cell, causing it to proliferate into a clone of cells that have the same immunologic specificity as the parent cell. The clonal selection theory has been further refined and is now accepted as the underlying paradigm of modern immunology.

Saturday, 23 January 2016

Overview of the Immune System

Overview of the Immune System

  • The immune system is remarkable defense system that has evolved to protect animals
  • from invading pathogenic microorganisms and cancer.
  •  It is able to generate an enormous variety of cells and molecules capable of specifically recognizing and eliminating an apparently limitless variety of foreign invaders. 
  •  These cells and molecules act together in a dynamic network whose complexity rivals that of the nervous system.
  • Functionally, an immune response can be divided into two related activities—recognition and response.
  •  Immune recognition is remarkable for its specificity.
  •  The immune system is able to recognize subtle chemical differences that distinguish one foreign pathogen from another. 
  • Furthermore, the system is able to discriminate between foreign molecules and the body’s own cells and proteins.
  •  Once a foreign organism has been recognized, the immune system recruits a variety of cells and molecules to mount an appropriate response, called an effector response, to eliminate or
  • neutralize the organism. 
  • In this way the system is able to convert the initial recognition event into a variety of effector responses, each uniquely suited for eliminating a particular type of pathogen.
  •  Later exposure to the same foreign organism induces a memory response, characterized by a more
  • rapid and heightened immune reaction that serves to eliminate the pathogen and prevent disease.
  • This chapter introduces the study of immunology from an historical perspective and presents a broad overview of the cells and molecules that compose the immune system, along with the mechanisms they use to protect the body against foreign invaders.
  •  Evidence for the presence of very simple immune systems in certain invertebrate organisms then gives an evolutionary perspective on the mammalian immune system, which is the major subject of this book. 
  • Elements of the primitive immune system persist in vertebrates as innate immunity along with a more highly evolved system of specific responses termed adaptive immunity. These two systems work in concert to provide a high degree of protection for vertebrate species. Finally, in some circumstances,
  • the immune system fails to act as protector because of some deficiency in its components; at other times, it becomes an aggressor and turns its awesome powers against its own host.
  •   In this introductory chapter, our description of immunity is simplified to reveal the essential structures and function of the immune system. Substantive discussions, experimental approaches, and in-depth definitions are left to the chapters that follow and follow my blog...
  • Like the later chapters covering basic topics in immunology, this one includes a section called “Clinical Focus” that describes human disease and its relation to immunity.
  • These sections investigate the causes, consequences, or treatments of diseases rooted in impaired or hyperactive immune function.

Thursday, 21 January 2016

Physiologic Barriers to Infection Include General Conditions and Specific Molecules

Physiologic Barriers to Infection Include General Conditions and Specific Molecules

  •        The physiologic barriers that contribute to innate immunity include temperature, pH, and various soluble and cellassociated molecules.Many species are not susceptible to certain diseases simply because their normal body temperature inhibits growth of the pathogens. Chickens, for example,
  • have innate immunity to anthrax because their high body temperature inhibits the growth of the bacteria. Gastric acidity is an innate physiologic barrier to infection because very few ingested microorganisms can survive the low pH of the stomach contents. One reason newborns are susceptible to some diseases that do not afflict adults is that their stomach contents are less acid than those of adults. A variety of soluble factors contribute to innate immunity, among them the soluble proteins lysozyme, interferon, and complement. Lysozyme, a hydrolytic enzyme found in mucous secretions and in tears, is able to cleave the peptidoglycan layer of the bacterial cell wall. Interferon comprises a group of proteins produced by virus-infected cells. Among the many functions of the interferons is the ability to bind to nearby cells and induce a generalized antiviral state.Complement, examined in detail in Chapter 13, is a group of serum proteins that circulate in an inactive state. A variety of specific and nonspecific immunologic mechanisms can convert the inactive forms of complement proteins into an active state with the ability to damage the membranes of pathogenic
  • organisms, either destroying the pathogens or facilitating their clearance. Complement may function as an effector system that is triggered by binding of antibodies to certain cell surfaces, or it may be activated by reactions between complement molecules and certain components of microbial
  • cell walls. Reactions between complement molecules or fragments of complement molecules and cellular receptors trigger activation of cells of the innate or adaptive immune systems. Recent studies on collectins indicate that these surfactant proteins may kill certain bacteria directly by disrupting
  • their lipid membranes or, alternatively, by aggregating the bacteria to enhance their susceptibility to phagocytosis. Many of the molecules involved in innate immunity have the property of pattern recognition, the ability to recognize a given class of molecules.Because there are certain types of molecules that are unique to microbes and never found in multicellular organisms, the ability to immediately recognize and combat invaders displaying such molecules is a strong feature
  • of innate immunity.Molecules with pattern recognition ability may be soluble, like lysozyme and the complement components described above, or they may be cell-associated receptors.
  • Among the class of receptors designated the toll-like receptors (TLRs), TLR2 recognizes the lipopolysaccharide (LPS) found on Gram-negative bacteria. It has long been recognized that
  •  (a) Electronmicrograph of macrophage (pink) attacking Escherichia coli (green). The bacteria are phagocytized as described in part b and breakdown products secreted. The monocyte
  • (purple) has been recruited to the vicinity of the encounter by soluble factors secreted by the macrophage. The red sphere is an erythrocyte. (b) Schematic diagram of the steps in phagocytosis of a bacterium. [Part a, Dennis Kunkel Microscopy, Inc./Dennis Kunkel
  • systemic exposure of mammals to relatively small quantities of purified LPS leads to an acute inflammatory response (see below). The mechanism for this response is via a TLR on
  • macrophages that recognizes LPS and elicits a variety of molecules in the inflammatory response upon exposure.When the TLR is exposed to the LPS upon local invasion by a Gram-negative
  • bacterium, the contained response results in elimination of the bacterial challenge.

Thursday, 7 January 2016



  1. There are a number of different ways to determine whether an antibody has bound to its target antigen.
  2.  The enzyme-linked immuno sorbent assay (ELISA) is one method, and it is frequently used for diagnostic detection.
  3. The ELISA procedure may be either indirect or direct  
  4. A generalized indirect ELISA protocol  has the following steps.
  5. 1. Bind the sample being tested for the presence of a specific molecule or organism to a solid support, such as a plastic micro titer plate, which usually contains 96 sample wells. Wash the support to remove unbound molecules.
  6. 2. Add a marker-specific antibody (primary antibody directed against the target antigen) to the bound material, and then wash the support to remove unbound primary antibody.
  7. 3. Add a second antibody (secondary antibody) that binds specifically to the primary antibody and not to the target molecule. Bound (conjugated) to the secondary antibody is an enzyme, such as alkaline phosphatase, peroxidase, or urease, that can catalyze a reaction that converts a colorless substrate into a colored product.  Wash the mixture to remove any unbound secondary antibody–enzyme conjugate.
  8. 4. Add the colorless substrate.
  9. 5. Observe or measure the amount of colored product. If the primary antibody does not bind to a target site in the sample, the second washing step removes it. Consequently, the secondary antibody– enzyme conjugate has nothing to bind to and is removed during the third washing step, and the final mixture remains colorless. Conversely, if the target site is present in the sample, then the primary antibody binds to it,
  10. the secondary antibody binds to the primary antibody, and the attached enzyme catalyzes the reaction to form an easily detected colored product. Since secondary antibodies that are complexed with an enzyme are available commercially, each new diagnostic test requires only a unique primary antibody.
  11.  In addition, several secondary antibody molecules, each with
  12. several enzyme molecules attached, bind to one primary antibody molecule, thereby amplifying the intensity of the signal.
  13. With a direct ELISA protocol (Fig. 9.1B), a monoclonal antibody specific for the target antigen is first bound to the surface of the microtiter plate. 
  14. Generalized ELISA protocol for detecting a target antigen. The primary antibody is often obtained from rabbits that have been immunized with the target antigen, while the secondary antibody is from goats immunized with rabbit antibodies.
  15. The enzyme (E) is conjugated to the secondary antibody. (A) Indirect ELISA; (B) direct ELISA.
  16. assess the amount of a particular antigen in a sample, the sample is added to the well of the microtiter plate and allowed to interact with the bound antibody. 
  17. This is followed by a wash to remove any unbound molecules.
  18. Then, the primary antibody and the secondary antibody conjugated to an enzyme are added, as described above, before the presence of bound antigen is visualized.
  19. The principal feature of an ELISA system is the specific binding of the primary antibody to the target site. If the target molecule is, for example, a protein, then a purified preparation of this protein is generally used to generate the antibodies that will be used to detect the target. 
  20. The resulting antibody mixture, which is found in the serum (antiserum) of an inoculated animal, usually a rabbit, contains a number of different antibodies that would each bind to a different antigenic determinant (epitope) on the target molecule.
  21.  Such a mixture of antibodies is called a polyclonal preparation.
  22. For some diagnostic assays, the use of polyclonal antibodies has two drawbacks:
  23. (1) the amounts of the different antibodies within a polyclonal preparation may vary from one batch to the next, and (2) polyclonal antibodies cannot be used to distinguish between two similar targets, e.g., when the difference between the pathogenic form (target) and the nonpathogenic one (nontarget) is a single determinant. 
  24. However, these problems can be overcome,because it is now possible to generate an antibody preparation that is directed against a single antigenic determinant, namely, a monoclonal antibody.
  25. Also, despite these drawbacks, diagnostic assays employing polyclonal antibodies are widely used for a variety of purposes.

                                                DNA, RNA, and Protein Synthesis

                                                DNA, RNA, and Protein Synthesis

                                                         The information encoded in genetic material is responsible for establishing and maintaining the cellular and biochemical functions of an organism. 
                                                       In most organisms, the genetic material is a long double stranded DNA polymer. 
                                                       The sequence of units (deoxy ribo nucleotides) of
                                                one DNA strand is complementary to the deoxy ribo nucleotides of the other strand. 
                                                       This complementarity enables new DNA molecules to be synthesized with the same linear order of deoxyribonucleotides in each strand as an original DNA molecule.
                                                       The process of DNA synthesis is called replication.
                                                A specific order of deoxyribonucleotides determines the information content of an individual genetic element (gene). 
                                                       Some genes encode proteins, and others encode only ribonucleic acid (RNA) molecules. 
                                                        The protein- coding genes (structural genes) are decoded by two successive major cellular processes: RNA synthesis (transcription) and protein synthesis (translation).
                                                         First, a messenger RNA (mRNA) molecule is synthesized
                                                from a structural gene using one of the two DNA strands as a template.
                                                        Second, an individual mRNA molecule interacts with other components, including ribosomes, transfer RNAs (tRNAs), and enzymes, to produce a protein molecule. 
                                                       A protein consists of a precise sequence of amino acids,
                                                which is essential for its activity.
                                                       Although the deoxyribonucleotide sequences are different in genes  encoding different functions, and for genes encoding similar functions in different organisms, the chemical compositions are the same. 
                                                      This enables molecular biotechnologists to transfer genes among a variety of organisms to create beneficial products. 
                                                      To understand how this is accomplished, it is helpful to know about the structure of DNA, replication, transcription,                and translation.