The technician in a molecular biology laboratory is a resourceful scientist who specializes in the various experimental techniques critical to the mission of the laboratory. The work offers many rewards beyond the financial ones. For example, one of the rewards of working as a laboratory technician lies in being part of a team dedicated to scientific discovery. Another rewarding aspect of the labo…
The question of who owns tissues, DNA, and other biological materials raises numerous legal questions. One concern is that genetic information derived from someone's DNA sequences could be used to deny insurance coverage to people whose genes indicate that they have a disease or that they are at risk of contracting one. Another concern is that the profits made by hospitals and transplant ce…
Linkage refers to the association and co-inheritance of two DNA segments because they reside close together on the same chromosome. Recombination is the process by which they become separated during crossing over, Physical linkage of genes simply means they are on the same chromosome. To be genetically linked, a pair of genes must be close enough that they are unlikely to be separated by cross…
Maize (Zea mays L.), otherwise known as corn, is a highly unusual, economically important, and genetically well-characterized member of the grass family. It is believed to have originated some 8,000 to 10,000 years ago in the fields of the first agriculturalists of Mexico and Central America. These early farmers carefully selected traits that would ultimately transform the tiny, sparsely seeded sp…
Genetic mapping is the process of measuring the distance between two or more loci on a chromosome. In order to determine this distance, a number of things must be done. First, the loci (pronounced "low-sigh") have to be known, and alleles have to exist at each locus so that they can be observed. The specific pair of alleles that are present is usually referred to as a geno-type. Seco…
Marker systems are tools for studying the transfer of genes into an experimental organism. In gene transfer studies, a foreign gene, called a trans-gene, is placed into an organism, in a process called transformation. A common problem for researchers is to determine quickly and easily if the target cells of the organism have actually taken up the transgene. A marker allows the researcher to determ…
Mass spectrometry is a technique for separating and identifying molecules based on mass. It has become an important tool for proteomics, the analysis of the whole range of proteins expressed in a cell. Mass spectrometry is used to identify proteins and to determine their amino acid sequence. It can also be used to determine if a protein has been modified by the addition of phosphate groups or suga…
Barbara McClintock, having just received the prestigious $15,000 Lasker Award in 1981 for her many contributions to the field of genetics. Geneticist 1902-1992 Barbara McClintock was one of the most important geneticists of the twentieth century and among the most controversial women in the history of science. She made several fundamental contributions to our understanding of chromosome str…
Geneticist 1921- Victor Almon McKusick was born in the little town of Parkman in central Maine, on October 21, 1921. He went to Tufts College in Boston from Victor McKusick 1940 to 1943, and then received his medical degree from Johns Hopkins University in 1946. He has remained at Johns Hopkins ever since, rising from a medical student to physician-in-chief of the entire Johns Hopkins hospit…
Meiosis is a type of cell division that, in humans, occurs only in male testes and female ovary tissue, and, together with fertilization, it is the process that is characteristic of sexual reproduction. Meiosis serves two important purposes: it keeps the number of chromosomes from doubling each generation, and it provides genetic diversity in offspring. In this it differs from mitosis, which is th…
Gregor Mendel. Natural Scientist 1822-1884 Gregor Mendel laid the foundation for the modern understanding of inheritance with his experiments on transmission of traits in garden peas. The ideas he developed are still in use today, and his essential insights into the physical nature of inheritance led directly to the understanding of the gene as a physical entity within the cell. …
Gregor Mendel (1822-1884), an Austrian monk and botanist, was curious and loved nature. He grew plants with diverse flower colors, and he cross-pollinated plant varieties to create hybrids. Mendel's fascination with "the striking regularity with which the same hybrid forms always reappeared," broadened his quest into discovering laws for inheriting any trait, not just flower c…
Metabolism is the sum of the chemical processes and interconversions that take place in the cells and the fluids of the body. This includes the absorption of nutrients and minerals, the breakdown and buildup of large molecules, the interconversion of small molecules, and the production of energy from these chemical reactions. Virtually every chemical step of metabolism is catalyzed by an enzyme. D…
DNA methylation is a mechanism used to regulate genes and protect DNA from some types of cleavage. It is one of the regulatory processes that are referred to as epigenetic, in which an alteration in gene expression occurs without a change in the nucleotide sequence of DNA. Defects in this process cause several types of disease that afflict humans. …
A microbiologist studies living organisms that are invisible to the naked eye. Microbiologists study bacteria, fungi, and other one-celled organisms (microbes), as well as viruses. A microbiologist may study a single molecule isolated from a bacterium, or a complex ecosystem with many microbial species. In the course of their work, microbiologists do a variety of tasks. These include inoculating m…
Mitochondria are intracellular organelles that play a critical role in cellular metabolism. Mitochondria contain the electron transport chain, which transfers electrons to oxygen by means of a process called oxidative phosphorylation. This process releases energy for the production of adenosine triphosphate (ATP) by forming a pH and electrical gradient (called the chemiosmotic gradient) across the…
One of the defining features of eukaryotic cells is the presence of membrane-enclosed organelles. Two of these organelles, the mitochondria and chloroplast, are unique in that they contain their own genetic material necessary for proper functioning. These organelle genomes are evolutionary relics of free-living bacteria that entered into a symbiotic relationship with a host cell. Through the proce…
Mitosis is the process by which all cells divide. Many cells have a limited life span, and mitosis allows them to be renewed on a regular basis. Mitosis is also responsible for generating the many millions of cells that are needed for an embryo to develop into a fetus, an infant, and finally an adult. Most human cells continually undergo a cycle of different phases. The phases have distinct names …
A model organism is a species that biologists choose to study, not necessarily because it has any inherent medical, agricultural, or economic value, but because it has certain traits that make it easy and convenient to work with. Studying model organisms enables researchers to perform experiments that might be impossible to carry out, due to logistical, financial, or ethical constraints, on organi…
Anthropology is the study of the origin and development of the human species. Molecular anthropology uses the tools and techniques of molecular genetics to answer anthropological questions, especially those concerning the origins and spread of humans across the globe. These questions mainly fall under the heading of physical or biological anthropology, as opposed to cultural anthropology, which st…
Biologists study life on many different levels. For example, a cellular biologist is concerned with the most basic unit of life, the single cell, whereas an evolutionary biologist may investigate the origin and genealogical history of a particular species of plant or animal life. The molecular biologist is concerned with understanding the biological phenomena of life at the molecular level. Molecu…
Geneticist 1866-1945 Thomas Hunt Morgan proved the validity of the chromosomal theory of heredity and led a research group whose insights into the physical nature of inheritance propelled genetics into the center of biology in the twentieth century. …
In 1961 Mary Lyon, an English scientist, hypothesized that one of the two X chromosomes in females becomes genetically silent early in a female embryo's development. To understand how she arrived at this idea, which has come to be known as "the Lyon Hypothesis," we need to understand what was known about the sex chromosomes. Every time a cell divides, the genetic material asse…
Hermann Muller. Geneticist 1890-1967 Hermann Joseph Muller was one of the founding members of the "fly lab" that was initiated by Thomas Hunt Morgan. In the early part of the twentieth century, this lab was the center of important research into the role of chromosomes in inheritance, using the fruit fly Drosophila as a model organism in experiments. The major members included …
Alleles are alternative forms of a gene, and they are responsible for differences in phenotypic expression of a given trait (e.g., brown eyes versus green eyes). A gene for which at least two alleles exist is said to be polymorphic. Instances in which a particular gene may exist in three or more allelic forms are known as multiple allele conditions. It is important to note that while multiple alle…
Muscular dystrophies (MDs) are a group of disorders that share three characteristics: They are inherited, they cause progressive weakness and muscle wasting, and the primary defect is localized to skeletal muscle, sparing the nerves. Although selected limb muscles develop some degree of weakness in all dystrophies, to distinguish among the different types, it is critical to know the mode of inheri…
A mutagen is any substance or agent that can cause a mutation, or change in the sequence or structure of DNA. Mutagens are classified on the basis of their physical nature and the types of damage they do. A mutagen is not the same as a carcinogen. Carcinogens are agents that cause cancer. While many mutagens are carcinogens as well, many others are not. The Ames test is a widely used test to scree…
Mutagenesis is the process of inducing mutations. Mutations may occur due to exposure to natural mutagens such as ultraviolet (UV) light, to industrial or environmental mutagens such as benzene or asbestos, or by deliberate mutagenesis for purposes of genetic research. For geneticists, the study of mutagenesis is important because mutants reveal the genetic mechanisms underlying heredity and gene …
A mutation is any heritable change in the genome of an organism. For a population, heritable mutations provide the source of genetic variation, without which evolution could not occur: If all individuals of a species were genetically identical, every subsequent generation would be identical regardless of which members of the species reproduced successfully. For an individual organism, mutations ar…
Mutation rate refers to the frequency of new mutations per generation in an organism or a population. Mutation rates can be determined fairly precisely in experimental organisms with short generation times, such as bacteria or fruit flies. Human mutation rates are more difficult to determine accurately. Mutation rates can be used as a "molecular clock" to determine the time since two…
Although Wilhelm Johannsen coined the term "gene" in 1909, our understanding of the nature of the gene has changed significantly over the course of the twentieth century. Gregor Mendel's elements of inheritance were given a material basis in the chromosome theory of the early twentieth century. Attempts to understand the nature of gene action and mutation spurred interest in t…
Like any other field in science, genetics has its own language. However, genetics is also a multidisciplinary field that encompasses expertise, and hence terminology, from diverse areas of science, including molecular biology, statistics, clinical medicine, and, most recently, bioinformatics. Despite all of the new and changing language in the field, two of the most frequently used terms in geneti…
Nondisjunction is the failure of two members of a homologous pair of chromosomes to separate during meiosis. It gives rise to gametes with a chromosomal content that is different from the norm. The consequences of this are usually quite severe, and a number of clinical conditions are the result of this type of chromosome mutation. …
DNA and RNA are polymers made by linking together smaller units called nucleotides. Nucleases are enzymes that break the chemical bonds, called phosphodiester bonds, that hold the nucleotides of DNA or RNA polymers together. Enzymes that cleave the phosphodiester bonds of DNA are called deoxyribonucleases, and enzymes that cleave the phosphodiester bonds of RNA are called ribonucleases. Nucleases …
Nucleotides are the building blocks of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Individual nucleotide monomers (single units) are linked together to form polymers, or long chains. DNA chains store genetic information, while RNA chains perform a variety of roles integral to protein synthesis. Individual nucleotides also play important roles in cell metabolism. …
The largest of the membrane-bound organelles, the nucleus first was described in 1710 by Antoni van Leeuwenhoek using a simple microscope. In 1831 the Scottish botanist Robert Brown characterized the organelle in detail, calling it the "nucleus," from the Latin word for "little nut." The nucleus is the site of gene expression and gene regulation. By electron microscopy,…
An oncogene is a gene that causes cancer. Oncogenes arise from normal cellular genes, often ones that help regulate cell division. …
An operon is a genetic regulatory system found in prokaryotes and the bacterial viruses (bacteriophages) that attack bacteria. It is a cluster of genes that share regulatory elements and are usually functionally related. …
Overlapping genes are defined as a pair of adjacent genes whose coding regions are partially overlapping. In other words, a single stretch of DNA codes for portions of two separate proteins. Such an arrangement of genetic code is ubiquitous. Many overlapping genes have been identified in the genomes of prokaryotes, eukaryotes, mitochondria, and viruses. For two genes to overlap, the signal to begi…
A patent is a legal right granted by the government that gives the patent-holder the exclusive right to manufacture and profit from an invention. While naturally occurring substances in their natural form are not patentable, a very wide range of biological materials have been the subject of patents. In 1980 the U.S. Supreme Court decision in Diamond v. Chakrabarty indicated that "anything u…
One of the most important tools used by genetic professionals is the pedigree, a pictorial description of a family tree. A complete pedigree provides Pedigree symbols used to indicate familial relationships. information about the biological relationships of individuals in the family, their medical history, the pattern of inheritance of a genetic disorder in the family, variable expression of…
The role of the pharmaceutical scientist in drug discovery and development is highly varied. Duties range from the synthesis of novel compounds designed to alter disease processes, to the formulation of these compounds into a tablet or capsule, to the development of assays (tests) to measure the drug and its metabolites in the body, to the testing of compounds for their effects in animals and huma…
The complete sequencing of the human genome in 2000, along with new technologies, such as DNA microarrays, for analyzing human genes on a genome-wide scale, provides scientists with the tools to study the molecular basis of diseases on a level and scale that previously had not been possible. Pharmacogenomics is a biomedical science that aims to use this knowledge to tailor drug therapies based on …
According to Webster's II New College Dictionary, a physician is "a natural philosopher, a person skilled in physic, or the art of healing; one duly authorized to prescribe remedies for, and treat, diseases; a doctor of medicine." A scientist is one who is "learned in the observation, identification, description, experimental investigation, and theoretical explanation o…
"Plant genetic engineer" is a popular term that describes scientists working in any of several fields who manipulate DNA or organelles such as chloroplasts and mitochondria in plant cells. The specific titles of such a scientist can include plant physiologist, plant pathologist, weed scientist, cell biologist, botanist, molecular biologist, plant geneticist, and biochemist. The typic…
Plasmids are naturally occurring, stable genetic elements found in bacteria, fungi, and even in the mitochondria of some plants. They may be composed of DNA or RNA, double-stranded or single-stranded, linear or circular. The transfer gene (tra) makes a protein that binds to the origin of replication site (ori). The protein nicks the DNA, relaxing it and allowing it to be transferred to another…
Pleiotropy is the phenomenon whereby a single gene has multiple consequences in numerous tissues. Pleiotropic effects stem from both normal and mutated genes, but those caused by mutations are often more noticeable and easier to study. Pleiotropy is actually more common than its opposite, since in a complex organism, a protein from a single gene is likely to be expressed in more than one tissue, a…
The polymerase chain reaction (PCR) is a laboratory technique for "amplifying" a specific DNA sequence. PCR is extremely efficient and sensitive; it can make millions or billions of copies of any specific sequence of DNA, even when the sequence is in a complex mixture. Because of this power, researchers can use it to amplify sequences even if they only have a minute amount of DNA. A …
Genetic polymorphisms are different forms of a DNA sequence. "Poly" means many, and "morph" means form. Polymorphisms are a type of genetic diversity within a population's gene pool. They can be used to map (locate) genes such as those causing a disease, and they can help match two samples of DNA to determine if they come from the same source. Depending on its ex…
In eukaryotic organisms, chromosomes come in sets. The somatic cells, called soma, usually have a diploid chromosome number, which in scientific notation is abbreviated as 2N. The diploid state contains two sets of chromosomes, one set of which has been contributed by each parent. A single set of chromosomes composes the haploid chromosome number, which is abbreviated as N. The haploid set is foun…
A population bottleneck is a significant reduction in the size of a population that causes the extinction of many genetic lineages within that population, thus decreasing genetic diversity. Population bottlenecks have occurred in the evolutionary history of many species, including humans. Present-day bottlenecks are seen in endangered species such as the Yangtze River dolphin, whose numbers have d…
Population genetics is the study of the genetic structure of populations, the frequencies of alleles and genotypes. A population is a local group of organisms of the same species that normally interbreed. Defining the limits of a population can be somewhat arbitrary if neighboring populations regularly interbreed. All the humans in a small town in the rural United States could be defined as a popu…
As scientific research reveals more information about treating diseases and maintaining good health, it has become increasingly important to identify diseases in their early stages in order to treat them most effectively. Thus, researchers have developed tests for some diseases to identify people at high risk for the disease before the symptoms of the disease actually appear. These tests are routi…
Post-translational control can be defined as the mechanisms by which protein structure can be altered after translation. Proteins are polymers of amino acids, and there are twenty different amino acids. Both the order and identity of these amino acids are important for the role that the protein plays in the cell. In some cases, the chemical identity of these amino acids is changed after translatio…
The future health of a new individual can be predicted, to an extent, from clues that are apparent before birth. Prenatal diagnosis is the identification of a medical condition in a developing embryo or fetus. Prenatal testing can sample fetal cells to examine DNA sequences that correspond to specific disease-causing genes or chromosomes (the structures that carry the genes). Biochemicals obtained…
In 1997 Stanley Prusiner was awarded the Nobel Prize in physiology or medicine for a revolutionary theory about the mechanisms of infection. His theory, the "prion hypothesis," concerns an unusual protein, the prion, which occurs in the complete absence of DNA and RNA. According to Prusiner's theory, the prion differs from other well-known infections agents including bacteria …
As more diagnostic, screening, and monitoring tests based on genetic data become available, privacy issues are becoming increasingly important. There are concerns that the results of genetic tests showing a person to be pre-disposed to a particular disease will fall into the hands of commercial medical suppliers or financial, legal, insurance, or government agencies, all of which control important…
Probability measures the likelihood that something specific will occur. For example, a tossed coin has an equal chance, or probability, of landing with one side up ("heads") or the other ("tails"). If you drive without a seat belt, your probability of being injured in an accident is much higher than if you buckle up. Probability uses numbers to explain chance. If someth…
The molecules that give cells and entire organisms their shape as well as their ability to move, grow, and reproduce are the proteins. Although they come in an almost infinite variety of shapes and sizes, they have all been designed by the process of evolution to serve a defined and useful function in the processes of life. Some proteins, like actin and collagen, help to give a cell its physical s…
Proteins are polymers of amino acids that provide structure and control reactions in all cells. When humans think of expressing the meaning of life, they often resort to words. From poems to sonnets to short stories to novels, words tell the stories of life. But in biological terms, the words of life are proteins. While DNA holds the code of life, proteins are the language in which that code is ex…
Proteomics is the science of studying the multitude of proteomes found in living organisms. A proteome is the entire collection of proteins expressed by a genome or in a tissue. The contents of a proteome can differ in various tissue types, and it can change as a result of aging, disease, drug treatment, or environmental effects. This is contrary to the concept of a genome, which is an organism&#x…
Pseudogenes are defective copies of functional genes. These may be partial or complete duplicates derived from polypeptide-encoding genes or RNA genes. The DNA sequence of a pseudogene is characteristically very similar to its functional counterpart, but contains variant mutations that render the gene inactive. The functional polypeptide-encoding gene contains an open reading frame, a long stretch…
Genetic studies of psychiatric disorders have become an important specialty area within medical genetics. Much of the progress in the area is the result of advances in molecular genetics techniques, The Human Genome Project, developments in the neurosciences, and recent genetic findings in complex brain disorders such as Alzheimer's and Huntington's disease. …
As of 2002 more than ten thousand genes have been discovered, and it is estimated that 30,000 to 70,000 human genes will be identified as a result of the Human Genome Project over the following few years. Tests for more than 600 gene variants are already available in medical practice. Genetic variants, or polymorphisms, are a normal part of genetic viability that may or may not be associated with …
Many procedures in molecular biology require an initial pure sample of DNA. These procedures include the polymerase chain reaction, sequencing, gene cloning, blotting, and DNA profiling. Purification of DNA involves removing it and other constituents from the cell, separating it from the various other cell constituents, and protecting it from degradation by cellular enzymes. Isolation procedures m…
