Molecular biology

Molecular biology is the study of biological molecules and the molecular basis of structure and function in living organisms.

Molecular biology is an interdisciplinary approach to understanding biological functions and regulation at the level of molecules such as nucleic acids, proteins , and carbohydrates. Following the rapid advances in biological science brought about by the development and advancement of the Watson-Crick model of DNA (deoxyribonucleic acid ) during the 1950s and 1960s, molecular biologists studied gene structure and function in increasing detail. In addition to advances in understanding genetic machinery and its regulation, molecular biologists continue to make fundamental and powerful discoveries regarding the structure and function of cells and of the mechanisms of genetic transmission. The continued study of these processes by molecular biologists and the advancement of molecular biological techniques requires integration of knowledge derived from physics , chemistry , mathematics , genetics , biochemistry , cell biology and other scientific fields.

Molecular biology also involves organic chemistry, physics, and biophysical chemistry as it deals with the physicochemical structure of macromolecules (nucleic acids, proteins, lipids, and carbohydrates) and their interactions. Genetic materials including DNA in most of the living forms or RNA (ribonucleic acid ) in all plant viruses and in some animal viruses remain the subjects of intense study.

In 1945, William Astbury coined the term "molecular biology" referring to the study of the chemical and physical structure of biological macromolecules (large sized molecules). There was and still is a strong belief that all forms of life have uniformity in biological processes. The pioneer findings in prokaryotes (a simple or primitive cell type, e.g., bacteria and blue green alga) are extended to eukaryotes (a complex or well developed cell type, e.g., animal and plant cells).

The complete set of instructions for making an organism (i.e., the complete set of genes) is called its genome . It contains the master blueprint for all cellular structures and activities for the lifetime of the cell or organism. The human genome consists of tightly coiled threads of deoxyribonucleic acid (DNA) and associated protein molecules organized into structures called chromosomes. In humans, as in other higher organisms, a DNA molecule consists of two strands that wrap around each other to resemble a twisted ladder whose sides, made of sugar and phosphate molecules, are connected by rungs of nitrogen-containing chemicals called bases. Each strand is a linear arrangement of repeating similar units called nucleotides, which are each composed of one sugar, one phosphate, and a nitrogenous base. Four different bases are present in DNA adenine (A), thymine (T), cytosine (C), and guanine (G). The particular order of the bases arranged along the sugar-phosphate backbone is called the DNA sequence; the sequence specifies the exact genetic instructions required to create a particular organism with its own unique traits.

Each time a cell divides into two daughter cells, its full genome is duplicated; for humans and other complex organisms, this duplication occurs in the nucleus. During cell division the DNA molecule unwinds and the weak bonds between the base pairs break, allowing the strands to separate. Each strand directs the synthesis of a complementary new strand, with free nucleotides matching up with their complementary bases on each of the separated strands. Strict base-pairing rules are adhered to. Adenine will pair only with thymine (an A-T pair) and cytosine with guanine (a C-G pair). Each daughter cell receives one old and one new DNA strand. The cells adherence to these base-pairing rules ensures that the new strand is an exact copy of the old one. This minimizes the incidence of errors (mutations) that may greatly affect the resulting organism or its offspring.

Each DNA molecule contains many genes, the basic physical and functional units of heredity. A gene is a specific sequence of nucleotide bases, whose sequences carry the information required for constructing proteins, which provide the structural components of cells and tissues as well as enzymes for essential biochemical reactions.

The central dogma of molecular biology is that DNA is copied to make mRNA (messenger RNA) and mRNA is used as the template to make proteins. Formation of RNA is called transcription and formation of protein is called translation. Transcription and translation processes are regulated at various stages and the regulation steps are unique to prokaryotes and eukaryotes. DNA regulation determines what type and amount of mRNA should be transcribed, and this subsequently determines the type and amount of protein. This process is the bottom line for growth and morphogenesis.

All living organisms are composed largely of proteins, the product of genes. Humans can synthesize at least 100,000 different kinds. Proteins are large, complex molecules made up of long chains of subunits called amino acids. The protein-coding instructions from the genes are transmitted indirectly through messenger ribonucleic acid (mRNA), a transient intermediary molecule similar to a single strand of DNA. For the information within a gene to be expressed, a complementary RNA strand is produced (a process called transcription) from the DNA template in the nucleus. This mRNA is moved from the nucleus to the cellular cytoplasm, where it serves as the template for protein synthesis.

Twenty different kinds of amino acids are usually found in proteins. Within the gene, each specific sequence of three DNA bases (codons ) directs the cells protein-synthesizing machinery to add specific amino acids. For example, the base sequence ATG codes for the amino acid methionine. Since three bases code for one amino acid, the protein coded by an average-sized gene (3,000 bp) will contain 1,000 amino acids. The genetic code is thus a series of codons that specify which amino acids are required to make up specific proteins.

Molecular biology also deals with: (1) the processes of DNA replication (making an exact copy of DNA) and DNA repair; (2) mutations (sudden alterations in nitrogen containing bases of DNA), their effects, and the agents that cause mutations (e.g., ultra-violet rays and chemicals); and (3) mechanisms and rearrangement and exchange of genetic materials via small segments of DNA such as plasmids, transposable elements, insertion sequences, and transposons to obtain recombinant DNA (DNA with recombined or exchanged nitrogenous bases).

Genetic engineering is an offshoot of molecular biology. Several biochemical, microbial, and molecular biological techniques are combined to obtain desirable DNA sequences in larger quantities, which may be subsequently used to manufacture proteins in larger quantities (e.g. insulin production).

Advances in molecular biology have led to significant discoveries about such changes in cell function and behavior as the development of higher organisms, the immunologic response, cancer and cell evolution . It has contributed tremendously to applications in the field of medicine, forensic science , biotechnology and biomedical industries.

See also Archaeogenetics; Bioinformatics and computational biology; Biotechnology; Ethnoarchaeology; Evolutionary change, rate of; Gene chips and microarrays; Gene mutation; Gene splicing; Gene therapy; Genetic identification of microorganisms; Genetic testing; Genetically modified foods and organisms; Genetics; Genomics (comparative); Genotype and phenotype; Human cloning; Human Genome Project; Ligand; Microbial genetics; PCR; Proteomics; Rare genotype advantage; Restoration ecology; Ribonucleic acid (RNA); RNA splicing; Shotgun cloning.

Resources

books

Alberts, Bruce, et. al., Molecular Biology of the Cell. 4th ed. Garland Press, 2002.

Brown, Terence A., ed. Genomes. 2nd ed. New York: John Wiley & Sons, 2002.

periodicals

International Human Genome Sequencing Consortium. "Initial Sequencing and Analysis of the Human Genome." Nature 409 (2001): 860–921.

organizations

National Institutes of Health. "The National Human Genome Research Institute: Advancing Human Health Through Genetic Research." NHGRI. February 2003 [cited February 28, 2003]. <http://www.genome.gov>.

other

Johnson, Lianna, University of California, Los Angeles. "Tutorials in Molecular Biology" [cited March 10, 2003]. <http://www.lsic.ucla.edu/ls3/tutorials/contact.html>.

KEY TERMS

Lipids

—Any fatty substance which tends not to dissolve in water but instead dissolves in relatively nonpolar organic solvents.

MOLECULAR BIOLOGY

MOLECULAR BIOLOGY is the science, or cluster of scientific activities, that seeks to explain the phenomena of life through investigation of the molecules found in living things. The term was apparently invented in the late 1930s by Warren Weaver, a mathematician-turned official of the Rockefeller Foundation, who from 1933 through World War II (1939–1945) channeled much of this philanthropy's considerable resources into a program to promote medical advances by making the life sciences more like physics in intellectual rigor and technological sophistication. There is considerable debate about the extent to which Weaver successfully altered the intellectual direction of the wide range of life sciences with which he interacted. However, there can be little doubt that his program made important new instruments and methods available for biologists. For instance, Rockefeller support greatly furthered the development of X-ray crystallography, ultracentrifuge and electrophoresis instrumentation, and the electron microscope, all used for analyzing the structure and distribution in organisms of proteins, nucleic acids, and other large biomolecules. In the 1930s and 1940s, these biological macromolecules were studied not mainly by biochemists, since the traditional methods of biochemistry were adequate only for the study of compounds orders of magnitude smaller (with molecular weights in the hundreds), but rather by scientists from the ill-defined fields known as "biophysics" and "general physiology."

A general postwar enthusiasm for science made rich resources available to biologists from federal agencies such as the National Science Foundation and the National Institutes of Health. Thanks to this new funding, and also to a postatomic urge to make physics benefit mankind peacefully, the research topics and methods of biophysicists made great headway in the 1950s. New radioisotopes and accelerators spurred radiobiology. Electron microscopes were turned on cells and viruses. Protein structure was probed by crystallography, electrophoresis, and ultracentrifugation; furthermore, chemical methods were developed allowing determination of the sequence of the string of amino acids making up smaller proteins. This kind of macromolecule-focused research in the 1950s has been described as the "structural school of molecular biology" (or biophysics). In the immediate postwar era, another approach also developed around Max Delbrück, a physicist-turned-biologist fascinated since the 1930s with explaining the gene, who attracted many other physicists to biology. Now regarded as the beginning of molecular genetics, this style has been called the "informational school of molecular biology," since during the 1940s and 1950s the school probed the genetic behavior of viruses and bacteria without any attempt to purify and characterize genes chemically. To the surprise of many, largely through the combined efforts of James Watson and Francis Crick—a team representing both schools—in the mid-1950s, the gene was found to be a double-helical form of nucleic acid rather than a protein. From this point through the early 1960s, molecular geneticists concentrated much of their efforts on "cracking" the "code" by which sequences of nucleic acid specify the proteins that carry out the bulk of biological functions. After the "coding" problem was settled in the mid-1960s, they turned mainly to the mechanisms by which genes are activated under particular circumstances, at first in viruses and bacteria, and from the 1970s, in higher organisms. While the extent to which physics actually influenced the development of molecular biology is controversial, some impact can clearly be seen in the use of cybernetic concepts such as feedback in explaining genetic control, as well as in early thinking about genetics as a cryptographic problem.

Although many projects associated with biophysics flourished in the 1950s, the field as a whole did not. Rather, some areas pioneered by biophysicists, such as protein structure, were partly absorbed by biochemistry, while others split off in new disciplines. For example, electron microscopists studying cell structure split when they established cell biology, and radiobiologists largely left biophysics to join (with radiologists) in the newly emerging discipline of nuclear medicine. By the later 1960s, departments bearing the name "molecular biology" were becoming more common, typically including molecular genetics as well as certain types of "structural" biophysics. In the 1970s a new generation of convenient methods for identifying particular nucleic acids and proteins in biological samples (RNA and DNA hybridization techniques, monoclonal antibodies) brought the study of genes and their activation to virtually all the experimental life sciences, from population genetics to physiology to embryology. Also in the 1970s, methods to determine the sequence of nucleic acids making up genes began to be developed—culminating during the 1990s in the government-funded, international Human Genome Project—as well as methods for rearranging DNA sequences in an organism's chromosomes, and then reintroducing these altered sequences to living organisms, making it possible for molecular geneticists to embark upon "genetic engineering." In the early twenty-first century, there is virtually no branch of life science and medicine that is not "molecular," in that all explain biological phenomena partly in terms of nucleic acid sequences and protein structure. Thus, from its beginnings, molecular biology has resisted definition as a discipline. But however defined—as a style of investigation, a set of methods or questions, or a loosely knit and overlapping set of biological fields based in several disciplines—the enterprise of explaining life's properties through the behavior of its constituent molecules has, since its origins in the interwar era, become one of the most intellectually fruitful and medically useful movements ever to engage the life sciences.

BIBLIOGRAPHY

Abir-Am, Pnina. "The Discourse of Physical Power and Biological Knowledge in the 1930's: A Reappraisal of the Rockefeller Foundation's 'Policy' in Molecular Biology," Social Studies of Science 12: 341–382 (1982).

———. "Themes, Genres and Orders of Legitimation in the Consolidation of New Scientific Disciplines: Deconstructing the Historiography of Molecular Biology." History of Science 23 (1985): 73–117.

Chadarevian, Soraya de. Designs for Life: Molecular Biology After World War II. Cambridge, UK: Cambridge University Press, 2002.

Creager, Angela N. H. The Life of a Virus: Tobacco Mosaic Virus As an Experimental Model, 1930–1965. Chicago: University of Chicago Press, 2002.

Kay, Lily E. The Molecular Vision of Life: Caltech, the Rockefeller Vision, and the Rise of the New Biology. New York: Oxford University Press, 1993.

———. Who Wrote the Book of Life?: A History of the Genetic Code. Stanford, Calif.: Stanford University Press, 2000.

Keller, Evelyn Fox. Refiguring Life: Metaphors of Twentieth-Century Biology. New York: Columbia University Press, 1995.

Kohler Jr., Robert E. "The Management of Science: The Experience of Warren Weaver and the Rockefeller Foundation Program in Molecular Biology." Minerva 14 (1976): 279–306.

Olby, Robert C. The Path to the Double Helix. Seattle: University of Washington Press, 1974.

Pauly, Philip. "General Physiology and the Discipline of Physiology, 1890–1935," in G. L. Geison, ed., Physiology in the American Context, 1850–1940. Baltimore: American Physiological Society, 1987, 195–207.

Rasmussen, Nicolas. "The Midcentury Biophysics Bubble: Hiroshima and the Biological Revolution in America, Revisited." History of Science 35 (1997): 245–293.

———. Picture Control: The Electron Microscope and the Transformation of Biology in America, 1940–1960. Stanford, Calif.: Stanford University Press, 1997.

NicolasRasmussen

See alsoDNA ; Genetic Engineering ; Genetics ; Human Genome Project .

Molecular Biology

Resources

Molecular biology is an interdisciplinary approach to understanding biological functions and regulation at the level of molecules such as nucleic acids, proteins, and carbohydrates. Following the rapid advances in biological science brought about by the development and advancement of the Watson-Crick model of DNA (deoxyribonucleic acid) during the 1950s and 1960s, molecular biologists studied gene structure and function in increasing detail. In addition to advances in understanding genetic machinery and its regulation, molecular biologists continue to make fundamental and powerful discoveries regarding the structure and function of cells and of the mechanisms of genetic transmission. The continued study of these processes by molecular biologists and the advancement of molecular biological techniques requires integration of knowledge derived from physics, microbiology, mathematics, genetics, biochemistry, cell biology and other scientific fields.

Molecular biology also involves organic chemistry, physics, and biophysical chemistry as it deals with the physicochemical structure of macromolecules (nucleic acids, proteins, lipids, and carbohydrates) and their interactions. Genetic materials including DNA in most of the living forms or RNA (ribonucleic acid) in all plant viruses and in some animal viruses remain the subjects of intense study.

The complete set of genes containing the genetic instructions for making an organism is called its genome. It contains the master blueprint for all cellular structures and activities for the lifetime of the cell or organism. The human genome consists of tightly coiled threads of deoxyribonucleic acid (DNA) and associated protein molecules organized into structures called chromosomes. In humans, as in other higher organisms, a DNA molecule consists of two strands that wrap around each other to resemble a twisted ladder whose sides, made of sugar and phosphate molecules are connected by rungs of nitrogen-containing chemicals called bases (nitrogenous bases). Each strand is a linear arrangement of repeating similar units called nucleotides, which are each composed of one sugar, one phosphate, and a nitrogenous base. Four different bases are present in DNA adenine (A), thymine (T), cytosine (C), and guanine (G). The particular order of the bases arranged along the sugar-phosphate backbone is called the DNA sequence; the sequence specifies the exact genetic instructions required to create a particular organism with its own unique traits.

Each time a cell divides into two daughter cells, its full genome is duplicated; for humans and other complex organisms, this duplication occurs in the nucleus. During cell division the DNA molecule unwinds and the weak bonds between the base pairs break, allowing the strands to separate. Each strand directs the synthesis of a complementary new strand, with free nucleotides matching up with their complementary bases on each of the separated strands. Nucleotides match up according to strict base-pairing rules. Adenine will pair only with thymine (an A-T pair) and cytosine with guanine (a C-G pair). Each daughter cell receives one old and one new DNA strand. The cell’s adherence to these base-pairing rules ensures that the new strand is an exact copy of the old one. This minimizes the incidence of errors (mutations) that may greatly affect the resulting organism or its offspring.

Each DNA molecule contains many genes, the basic physical and functional units of heredity. A gene is a specific sequence of nucleotide bases, whose sequences carry the information required for constructing proteins, which provide the structural components of cells and as well as enzymes for essential biochemical reactions.

The chromosomes of prokaryotic microorganisms differ from eukaryotic microorganisms, in terms of shape and organization of genes. Prokaryotic genes are more closely packed and are usually is arranged along one circular chromosome.

The central dogma of molecular biology states that DNA is copied to make mRNA (messenger RNA), and mRNA is used as the template to make proteins. Formation of RNA is called transcription and formation of protein is called translation. Transcription and translation processes are regulated at various stages and the regulation steps are unique to prokaryotes and eukaryotes. DNA regulation determines what type and amount of mRNA should be transcribed, and this subsequently determines the type and amount of protein. This process is the fundamental control mechanism for growth and development (morphogenesis).

All living organisms are composed largely of proteins, the end product of genes. Proteins are large, complex molecules made up of long chains of subunits called amino acids. The protein-coding instructions from the genes are transmitted indirectly through messenger ribonucleic acid (mRNA), a transient intermediary molecule similar to a single strand of DNA. For the information within a gene to be expressed, a complementary RNA strand is produced (a process called transcription) from the DNA template. In eukaryotes, messenger RNA (mRNA) moves from the nucleus to the cellular cytoplasm, but in both eukaryotes and prokaryotes mRNA serves as the template for protein synthesis.

Twenty different kinds of amino acids are usually found in proteins. Within the gene, sequences of three DNA bases serve as the template for the construction of mRNA with sequence complimentary codons that serve as the language to direct the cell’s protein-synthesizing machinery. Cordons specify the insertion of specific amino acids during the synthesis of protein. For example, the base sequence ATG codes for the amino acid methionine. Because more than one codon sequence can specify the same amino acid, the genetic code is termed a degenerate code (i.e., there is not a unique codon sequence for every amino acid).

Areas of intense study by molecular biology include the processes of DNA replication, repair, and mutation (alterations in base sequence of DNA). Other areas of study include the identification of agents that cause mutations (e.g., ultra-violet rays, chemicals) and the mechanisms of rearrangement and exchange of genetic materials (e.g. the function and control of small segments of DNA such as plasmids, transposable elements, insertion sequences, and transposons to obtain recombinant DNA).

Recombinant DNA technologies and genetic engineering are an increasingly important part of molecular biology. Advances in biotechnology and molecular medicine also carry profound clinical and social significance. Advances in molecular biology have led to significant discoveries concerning the mechanisms of the embryonic development, disease, immunologic response, and evolution.

Resources

BOOKS

Alberts, Bruce, et. al., Molecular Biology of the Cell. 4th ed. Garland Press, 2002.

Brown, Terence A., ed. Genomes. 2nd ed. New York: John Wiley & Sons, 2002.

Clark, David P. Molecular Biology made Simple and Fun. 3rd ed. St. Louis: Cache River Press, 2005.

Weaver, Robert F. Molecular Biology. New York: McGraw-Hill, 2004.

PERIODICALS

International Human Genome Sequencing Consortium. “Initial sequencing and analysis of the human genome.” Nature 409 (2001): 860–921.

Molecular Biology

Deoxyribonucleic acid, better known as DNA, is located in the nucleus of all living cells. DNA dictates which creatures walk, fly, or bore through the soil. Each DNA strand is made up of four nucleotides or "building blocks": adenine, cytosine, guanine, and thymine. These nucleotides, in turn, are made up of a variety of proteins, called amino acids. The strands of DNA, made up of bonded nucleotide pairs, are very long. Molecular biologists have worked toward breaking the genetic code by identifying the nucleotides in order and searching for patterns.

Nature gave scientists one big hint: adenine always bonds with thymine, and guanine always bonds with cytosine. In the past, researchers found fluorescent molecules that mimic the natural nucleotides. When the known fluorescent molecule bonded with an unknown nucleotide, scientists could identify that particular bonded pair of the DNA strand. The process took a decade or more. Researchers hope to speed up this painstaking work with help from computers, so they can accomplish such tasks in hours instead of years.

The challenges are formidable. Scientists must find a way to isolate and copy genes. Sensitive equipment must be developed to allow DNA sequences to be "read" as they are drawn through some kind of microscopic portal. The monitoring equipment must be fast enough, or the process slowed enough, to allow for an accurate identification of these bonded molecules. Finally, researchers must have algorithms to help them process the multitude of data they would receive from even one single strand of DNA, which could have 70,000 nucleotides. Computers are then needed to transmit and compare the sequence of the genes being studied with known gene sequence databases, a monumental task for which computers are particularly well suited.

The benefits of such research are profound. The study of human genetics is the first most obvious benefit, since these findings will aid research in fields as diverse as inherited diseases and anthropology. Since DNA holds the key to every aspect of the human body, genetic studies could potentially be used to mimic the way different cells work. This would allow researchers to develop and experiment with the effect of medications on the body without using living subjects. Disease processes, such as various forms of cancer, could potentially be duplicated in an electronic model and studied. This kind of understanding would aid in the development of successful treatments. Scientists also hope to use DNA sequences to identify and classify organisms, from the discovery of new bacteria to tracing the evolution of animals. Models of bacterial DNA could help in the study of the spread of diseases.

Gene therapy is a field in which the genes of living things are manipulated and even exchanged with one another to provide a beneficial result. For instance, spider silk, one of the strongest materials on Earth, has been produced by potatoes. Frogs and earthworms have been made to glow in the dark. A type of corn has been modified to fight tooth decay effectively. Vitamin A has been added to rice, making the grain more nutritious. Many of these applications of gene therapy have met controversy, since the long-term effects of gene manipulation in food are unknown.

Bioinformatics is the field in which software is developed to aid in the study of molecular biology. Many of the studies currently being done in molecular biology would be impossible without the help of computers and computer software. This software comes from various places. One researcher developed a basic bioinformatics software application and posted it on the web for others to download and improve upon, as did the creators of the Linux operating system , resulting in a versatile software application. Other firms have hired professionals to develop and copyright applications, which are then sold to researchers and private firms.

The U.S. government has played a pivotal role in the quest for information by establishing the National Center for Biotechnology Information, or NCBI. The NCBI is a division of the National Library of Medicine (NLM), which stores biomedical (such as gene sequence) databases. The NLM itself is a division of the National Institutes of Health (NIH). With all of the resources of the NIH, it is considered the largest biomedical research facility in the world. The NCBI is the result of a cooperative effort. Researchers, academic institutions, and similar agencies from other countries around the world access and contribute to the databases available at the NCBI.

Molecular biology and computers are also finding uses within the medical field. For example, scientists have developed a device that electronically smells the presence of bacterial infections in the lungs. Breath samples are taken from patients and put into an aroma-detection device. The machine measures the electrical resistance of the molecules in the sample of air. The results are then displayed in a two-dimensional "map." Different bacteria produce distinct characteristics on this map. This is a potentially life-saving tool because it allows physicians to treat patients immediately with the correct antibiotic for pneumonia instead of waiting two or three days for sample cultures of the bacteria to be grown and identified.

Although the use of computers is rapidly advancing the field of molecular biology, there is growing evidence that molecular biology is also important to computer science. Researchers at Syracuse University in New York are working with a purple protein called bacteriorhodopsin, produced by a type of bacteria native to salt marshes. This protein is quite stable, readily produced, and easily processed. Many believe it will eventually replace silicon microchips. Bacteriorhodopsin changes shape upon exposure to light. One shape is designated as binary 0, and the other shape is designated as binary 1. Bacteriorhodopsin is suspended in organized layers within a polymer gel. Because an individual protein changes shape upon reacting to different colors of lasers, the shape of an individual protein within the cube can be manipulated.

Floppy drives, CD-ROMs (compact disc-read only memory), and hard drives are different forms of memory that operate on a two-dimensional basis. The bacteriorhodopsin gel would be a type of three-dimensional memory. Researchers believe that one cubic centimeter of this bacteriorhodopsin protein/gel will be able to store between eight and ten gigabytes of information.

Experts agree that future molecular biology studies would be unthinkable without computers. Perhaps in the future, computers will be equally dependent upon molecular biology.

see also Binary Number System; Biology; Image Analysis; Molecular Computing; Pattern Recognition.

Mary McIver Puthawala

Bibliography

Cimino, Daniela. "Modeling a Drug." Software Magazine 18, no.1 (1998): 12(1).

Cooke, Robert. "Brave New Bacterial World." MIT Technology Review 100, no. 3 (1997): 14(2).

Levin, Carol. "High Protein Computers." PC Magazine 14, no. 10 (1995): 29(1).

Molecular biology

Molecular biology is the study of life at the level of atoms and molecules. Suppose, for example, that one wishes to understand as much as possible about an earthworm. At one level, it is possible to describe the obvious characteristics of the worm, including its size, shape, color, weight, the foods it eats, and the way it reproduces.

Long ago, however, biologists discovered that a more basic understanding of any organism could be obtained by studying the cells of which that organism is made. They could identify the structures of which cells are made, the way cells change, the substances needed by the cell to survive, products made by the cell, and other cellular characteristics.

Molecular biology takes this analysis of life one step further. It attempts to study the molecules of which living organisms are made in much the same way that chemists study any other kind of molecule. For example, they try to find out the chemical structure of these molecules and the way this structure changes during various life processes, such as reproduction and growth. In their research, molecular biologists make use of ideas and tools from many different sciences, including chemistry, biology, and physics.

The Central Dogma

The key principle that dominates molecular biology is known as the Central Dogma. (A dogma is an established belief.) The Central Dogma is based on two facts. The first fact is that the key players in the way any cell operates are proteins. Proteins are very large, complex molecules made of smaller units known as amino acids. A typical protein might consist, as an example, of a few thousand amino acid molecules joined to each other end-to-end. Proteins play a host of roles in cells. They are the building blocks from which cell structures are made; they act as hormones (chemical messengers) that deliver messages from one part of a cell to another or from one cell to another cell; and they act as enzymes, compounds that speed up the rate at which chemical reactions take place in cells.

The second basic fact is that proteins are constructed in cells based on master plans stored in molecules known as deoxyribonucleic acids (DNA) present in the nuclei of cells. DNA molecules consist of very long chains of units known as nucleotides joined to each other end-to-end. The sequence in which nucleotides are arranged act as a kind of code that tells a cell what proteins to make and how to make them.

Words to Know

Amino acid: An organic compound from which proteins are made.

Cell: The basic unit of a living organism; cells are structured to perform highly specialized functions.

Cytoplasm: The semifluid substance of a cell containing organelles and enclosed by the cell membrane.

DNA (deoxyribonucleic acid): The genetic material in the nucleus of cells that contains information for an organism's development.

Enzyme: Any of numerous complex proteins that are produced by living cells and spark specific biochemical reactions.

Hormone: A chemical produced in living cells that is carried by the blood to organs and tissues in distant parts of the body, where it regulates cellular activity.

Nucleotide: A unit from which DNA molecules are made.

Protein: A complex chemical compound that consists of many amino acids attached to each other that are essential to the structure and functioning of all living cells.

Ribosome: Small structures in cells where proteins are produced.

The Central Dogma, then, is very simple and can be expressed as follows:

DNA → mRNA → proteins

What this equation says in words is that the code stored in DNA molecules in the nucleus of a cell is first written in another kind of molecule known as messenger ribonucleic acid (mRNA). Once they are constructed, mRNA molecules leave the nucleus and travel out of the nucleus into the cytoplasm of the cell. They attach themselves to ribosomes, structures inside the cytoplasm where protein production takes place. Amino acids that exist abundantly in the cytoplasm are then brought to the ribosomes by another kind of RNA, transfer RNA (tRNA), where they are used to construct new protein molecules. These molecules have their structure dictated by mRNA molecules which, in turn, have structures originally dictated by DNA molecules.

Significance of molecular biology

The development of molecular biology has provided a new and completely different way of understanding living organisms. We now know, for example, that the functions a cell performs can be described in chemical terms. Suppose that we know that a cell makes red hair. What we have learned is that the reason the cell makes red hair is that DNA molecules in its nucleus carry a coded message for red-hair-making. That coded message passes from the cell's DNA to its mRNA. The mRNA then directs the production of red-hair proteins.

The same can be said for any cell function. Perhaps a cell is responsible for producing antibodies against infection, or for making the hormone insulin, or assembling a sex hormone. All of these cell functions can be specified as a set of chemical reactions.

But once that fact has been realized, then humans have exciting new ways of dealing with living organisms. If the master architect of cell functions is a chemical molecule (DNA), then that molecule can be changed, like any other chemical molecule. If and when that happens, the functions performed by the cell are also changed. For these reasons, the development of molecular biology is regarded by many people as one of the greatest revolutions in all of scientific history.