Quantitative traits are those that vary continuously. This is in contrast to qualitative traits, in which the phenotype is discrete and can take on one of only a few different values. Examples of quantitative traits include height, weight, and blood pressure. There is no single gene for any of these traits, instead it is generally believed that continuous variation in a trait such as blood pressur…
Almost all organisms translate their genes into protein structures using an identical, universal codon dictionary in which each amino acid in the protein is represented by a combination of only three nucleotides. For example, the sequence AAA in a gene is transcribed into the sequence UUU in messenger RNA (mRNA) and is then translated as the amino acid phenylalanine. A group of several codons that…
Recombinant DNA refers to a collection of techniques for creating (and analyzing) DNA molecules that contain DNA from two unrelated organisms. One of the DNA molecules is typically a bacterial or viral DNA that is capable of accepting another DNA molecule; this is called a vector DNA. The other DNA molecule is from an organism of interest, which could be anything from a bacterium to a whale, or a …
The human genome contains approximately three billion base pairs of DNA. Within this there are between 30,000 and 70,000 genes, which together add up to less than 5 percent of the entire genome. Most of the rest is made up of several types of noncoding repeated elements. Most gene sequences are unique, found only once in the genome. In contrast, repetitive DNA elements are found in multiple copies…
DNA is the carrier of genetic information. Before a cell divides, DNA must be precisely copied, or "replicated," so that each of the two daughter cells can inherit a complete genome, the full set of genes present in the organism. In eukaryotes, the DNA molecules that make up the genome are packaged with proteins into chromosomes, each of which contains a single linear DNA molecule. E…
Successful pregnancy requires ovulation (when an ovary releases an egg into a fallopian tube), transport of the egg partway down the fallopian tube, movement of sperm from the vagina to the fallopian tube, penetration by the sperm of the egg's protective layer, and implantation of the fertilized egg in the uterus. In the United States, infertility is an issue of great concern to many couple…
Reproductive technology encompasses a range of techniques used to overcome infertility, increase fertility, influence or choose the genetic characteristics of offspring, or alter the characteristics of a population. Each type of reproductive technology brings with it a range of ethical issues. With the accelerated pace of progress in modern medical technology, these issues have been brought square…
Restriction enzymes are bacterial proteins that recognize specific DNA sequences and cut DNA at or near the recognition site. These enzymes are widely used in molecular genetics for analyzing DNA and creating recombinant DNA molecules. …
Retroviruses are RNA-containing viruses that use the enzyme reverse transcriptase to copy their RNA into the DNA of a host cell. Retroviruses have been isolated from a variety of vertebrate species, including humans, other mammals, reptiles, and fish. The family Retroviridae includes such important human pathogens as human immunodeficiency virus (HIV) and human Tlymphotropic virus (HTLV), the caus…
Reverse transcriptase is the replication enzyme of retroviruses. Because it polymerizes DNA precursors, reverse transcriptase is a DNA polymerase. However, whereas cellular DNA polymerases use DNA as a template for making new DNAs, reverse transcriptase uses the single-stranded RNA in retroviruses as the template for synthesizing viral DNA. This unusual process of making DNA from RNA is called &#x…
Ribosomes are the cellular organelles that carry out protein synthesis, through a process called translation. They are found in both prokaryotes and eukaryotes, these molecular machines are responsible for accurately translating the linear genetic code, via the messenger RNA, into a linear sequence of amino acids to produce a protein. All cells contain ribosomes because growth requires the continu…
Ribozymes are RNA molecules that catalyze chemical reactions. Most biological processes do not happen spontaneously. For example, the cleavage of a molecule into two parts or the linkage of two molecules into one larger molecule requires catalysts, that is, helper molecules that make a reaction go faster. The majority of biological catalysts are proteins called enzymes. For many years scientists a…
Ribonucleic acid (RNA) molecules, which are linear chains (or polymers) of ribonucleotides, perform a number of critical functions. Many of these functions are related to protein synthesis. Some RNA molecules bring genetic information from a cell's chromosomes to its ribosomes, where proteins are assembled. Others help ribosomes translate genetic information to assemble specific sequences o…
RNA interference is a process in which translation of some of a cell's messenger RNA (mRNA) sequences is prevented, because of the presence of (and consequent destruction of) matching double-stranded RNA sequences. RNA interference is believed to protect the cell against viruses and other threats. "Interference" refers to the interruption of the cell's translation of it…
RNA polymerases are enzyme complexes that synthesize RNA molecules using DNA as a template, in the process known as transcription. The RNAs created by transcription are either used as is (as ribosomal RNAs, transfer RNAs, or other types), or serve to guide the synthesis of a protein (as messenger RNAs). The word "polymerase" derives from "-ase," a suffix indicating an e…
RNA serves a multitude of functions within cells. These functions are primarily involved in converting the genetic information contained in a cell's DNA into the proteins that determine the cell's structure and function. All RNAs are originally transcribed from DNA by RNA polymerases, which are specialized enzyme complexes, but most RNAs must be further modified or processed before t…
Rodents play an important role in biology and medicine. Since the mouse and rat share many biological characteristics with humans, they are commonly used as model organisms for understanding disease processes and testing treatments. Moreover, it is relatively easy to experimentally manipulate the genetic composition of mice and rats and, thereby, to model human genetic disorders in these animals. …
Caenorhabditis elegans is a nonparasitic nematode that normally lives in the soil. Although studied since the 1800s, the modern use of C. elegans as a model system dates to the mid-1960s. By the start of that decade, scientists thought that the "classical" problems in molecular biology were about to be solved. They began to search for a model system that would support the new challen…
Molecular Biologist1918- Born August 13, 1918, in Rendcombe, Gloucestershire, United Kingdom, Fred Sanger has been breaking new ground in chemistry for decades. In fact, he is the only person to have won a Nobel Prize in chemistry twice, and is only one of four people ever to have won a Nobel Prize more than once. While at Cambridge University in England he developed a new method for sequencing am…
All good journalists are storytellers, and science writers are simply journalists who like to tell stories about science. Still, it has only been in the last few decades that science writing has become a profession in its own right, with journalists specifically trained to cover research ranging from genetics to particle physics. Beginning science writers may earn a starting salary of around $20,0…
Selection is a process in which members of a population reproduce at different rates, due to either natural or human-influenced factors. The result of selection is that some characteristic is found in increasing numbers of organisms within the population as time goes on. …
A gene is a segment of DNA that carries the information needed by the cell to construct a protein. Which protein that is, when it is made, and how damage to it can give rise to genetic disease all depend on the gene's sequence. In other words, they depend on how the building blocks of DNA, the nucleotides A, C, G, and T (adenine, cytosine, guanine, and thymine) are ordered along the DNA str…
The development of the immune system is a very complicated process. Stem cells in the bone marrow continually give rise to the white blood cells responsible for producing antibodies (B-lymphocytes), recognizing and destroying foreign cells (T-lymphocytes) and performing immune surveillance for cancer and foreign cells. Immunodeficiency results when the normal complex interactions of the immune sys…
Sex determination refers to the mechanisms employed by organisms to produce offspring that are of two different sexes. First we present an overview of the sex determination mechanisms used by mammals. Then we discuss the great variety of mechanisms used by animals other than mammals. A developing mammalian embryo's gender is determined by two sequential processes known as primary and second…
The biological basis of sexual orientation (heterosexuality, homosexuality, or bisexuality) has long been a topic of controversy in both science and society. A growing body of research supports the view that genetics and the environment work together to determine sexual orientation. Some issues remain unclear. First, how much of sexual orientation is genetic and how much is shaped by environmental…
To survive, an organism must constantly adjust its internal state to changes in the environment. To track environmental changes, the organism must receive signals. These may be in the form of chemicals, such as hormones or nutrients, or may take another form, such as light, heat, or sound. A signal itself rarely causes a simple, direct chemical change inside the cell. Instead, the signal sets off …
Speciation is the process by which new species of organisms arise. Earth is inhabited by millions of different organisms, all of which likely arose from one early life-form that came into existence about 3.5 billion years ago. It is the task of taxonomists to decide which out of the multitude of different types of organisms should be considered species. The wide range in the characteristics of ind…
Statistical geneticists are highly trained scientific investigators who are specialists in both statistics and genetics. Training in both statistics and genetics is necessary, as the nature of the work is highly interdisciplinary. Statistical geneticists must be able to understand molecular and clinical genetics, as well as mathematics and statistics, to effectively communicate with scientists fro…
Statistics is the set of mathematical tools and techniques that are used to analyze data. In genetics, statistical tests are crucial for determining if a particular chromosomal region is likely to contain a disease gene, for instance, or for expressing the certainty with which a treatment can be said to be effective. Statistics is a relatively new science, with most of the important developments o…
Tay-Sachs disease is a severe genetic disease of the nervous system that is nearly always fatal, usually by three to four years of age. It is caused by mutations in the HEXA gene, which codes for a component of the enzyme β-hexosaminidase A or "Hex A." The resulting accumulation of a brain lipid called GM2 ganglioside produces brain and spinal cord degeneration. It is a rare d…
A technical writer (sometimes called a technical communicator) designs, writes, edits, and produces documents for scientific, technical, industrial, and government organizations. These documents can include technical reports, specifications, reference manuals, operating instructions, policies and procedures, proposals, presentations, brochures, and Web pages. …
Telomeres are structures found at the ends of chromosomes in the cells of eukaryotes. Telomeres function by protecting chromosome ends from recombination, fusion to other chromosomes, or degradation by nucleases. They permit cells to distinguish between random DNA breaks and chromosome ends. They also play a significant role in determining the number of times that a normal cell can divide. Unicell…
Transcription is the process in which genetic information stored in a strand of DNA is copied into a strand of RNA. The sequence of the four bases in DNA, which are adenine (A), cytosine (C), guanine (G), and thymine (T), is preserved in the sequence of the four bases in RNA, which are A, C, G, and uracil (U). The first phase of RNA synthesis is initiation (Figure 1B). Initiation starts when the f…
Transcription factors are protein complexes that help RNA polymerase bind to DNA. RNA polymerase is the enzyme that transcribes genes to make messenger RNA, which is then used to make protein. By controlling RNA polymerase's access to the gene, transcription factors control the rate at which a gene is transcribed. Without transcription factors, cells would not be able to effectively regulat…
Transduction is one of three basic mechanisms for genetic exchange in bacteria. Like transformation and conjugation, transduction allows the movement of genetic information from a donor cell to a recipient. Unlike the other mechanisms, however, transduction requires the participation of a type of virus called a bacteriophage in order to accomplish this movement. While transduction has been studied…
Transformation is one of three basic mechanisms for genetic exchange in bacteria. Transformation may be either a natural process—that is, one that has evolved in certain bacteria—or it may be an artificial process whereby the recipient cells are forced to take up DNA by a physical, chemical, or enzymatic treatment. In both cases, exogenous DNA (DNA that is outside the host cell), is …
The term "transgenics" refers to the science of inserting a foreign gene into an organism's genome. Scientists do this, creating a "transgenic" organism, to study the function of the introduced gene and to identify genetic elements that determine which tissue and at what stage of an organism's development a gene is normally turned on. Transgenic animals ha…
A transgenic microorganism is a microbe, usually a bacterium, into which genetic information has been introduced from the outside and which possesses the ability to pass that information on to subsequent generations in a stable manner. This is not an entirely novel idea in microorganisms, since bacteria have been practicing and perfecting this art over billions of years of evolution. We, on the o…
A transgenic organism is a type of genetically modified organism (GMO) that has genetic material from another species that provides a useful trait. For instance, a plant may be given genetic material that increases its resistance to frost. Another example would be an animal that has been modified with genes that give it the ability to secrete a human protein. Bioethics addresses the impact of tec…
Transgenic plants are plants that have been genetically modified by inserting genes directly into a single plant cell. Transgenic crop plants modified for improved flavor, pest resistance, or some other useful property are being used increasingly. Transgenic plants are unique in that they develop from only one plant cell. In normal sexual reproduction, plant offspring are created when a pollen cel…
Translation is the cellular process in which the genetic information carried by the DNA is decoded, using an RNA intermediate, into proteins. This process is also known as protein synthesis. …
Modern medicine continues to offer many miracles that lengthen the life spans of humans, as well as greatly increase the quality of life that they enjoy. If one were to draw up a "top ten" list of technical feats, surely the ability to successfully transplant an entire organ from one human to another would be high on the list. Transplantation can be defined as the transfer of cells, …
Transposable genetic elements (TEs) are segments of DNA that can be integrated into new chromosomal (genomic) locations either through direct DNA transfer (transposons), or via an RNA intermediate (retrotransposons). Pseudonyms for TEs include mobile elements, jumping genes, genomic parasites, and selfish DNA. TEs are known to be responsible for several human genetic diseases and may play a role i…
Trinucleotide, or triplet repeats, consist of three consecutive nucleotides that are repeated within a region of DNA (for example, CCG CCG CCG CCG CCG). Expansion of a triplet repeat gene segment leads to increased disease severity and decreased age of onset in proportion to the degree of expansion. Repeats that are translated into protein alter the structure of the protein, conferring new fu…
Tumor suppressor genes regulate mitosis and cell division. When their function is impaired, the result is a high rate of uncontrolled cell growth or cancer. Damage to tumor suppressor genes contributes to a large number of different types of tumors. …
Twins are siblings carried together in the womb and born at the same time. Similarities and differences between twins can be used to answer questions about the role genes and the environment play in the development of traits such as personality, intelligence, and susceptibility to disease. While results from any single pair of twins cannot provide conclusive answers to such questions, the study of…
Viruses are infectious agents consisting of a nucleic acid genome made of DNA or RNA, a protein coat, and sometimes lipids. They are able to replicate only inside cells, and the viral genome contains genes coding for proteins. Viroids and virusoids are also infectious agents, but they differ from viruses in several ways. For instance, they have a single-stranded circular, RNA genome. Their genomes…
A virus is a parasite that must infect a living cell to reproduce. Although viruses share several features with living organisms, such as the presence of genetic material (DNA or RNA), they are not considered to be alive. Unlike cells, which contain all the structures needed for growth and reproduction, viruses are composed of only an outer coat (capsid), the genome, and, in some cases, a few enzy…
Geneticist 1928- James Dewey Watson was the codiscoverer of the structure of DNA. He has also made major contributions to research in genetics and molecular biology as an administrator, and has written widely read and influential books for both academic and nonscience audiences. …
The X chromosome occupies an exceptional place in the mammalian genome. Together with the Y chromosome, the X chromosome differentiates the sexes. Males have one X chromosome and a Y chromosome and females have two X chromosomes. Because of this fundamental genetic difference, diseases caused by genes located on the X chromosome affect males and females differently and thus present unusual inherit…
The diploid human genome is packaged within 46 chromosomes, as two pairs of 23 discrete elements, into all cells other than the haploid gametic egg and sperm cells. During the reproductive process, each parent's gametes contribute 22 nonsex chromosomes and either one X or one Y chromosome. While SRY is the most dramatic gene affiliated with the Y chromosome, about thirty other genes have be…
Yeast are single-celled eukaryotic organisms related to fungi. The baker's yeast Saccahromyces cerevisiae and the distantly related Schizosaccharomyces pombe are favored model organisms for genetic research. The interest in yeast research stems from the fact that, as eukaryotic organisms, the sub-cellular organization of yeast is similar to that of cells of more complex organisms. Thus, und…
The zebrafish (Brachydanio rerio) is a small tropical freshwater fish that began to be used as a genetic model system in the early 1980s. The zebrafish shares numerous anatomical and genetic similarities with higher vertebrates, The developing zebrafish is transparent, making it an ideal subject for the study of development. including humans, both in the general body plan and in specific org…
