Gene Pool and Genetic Structure

All of the alleles shared by all of the individuals in a population make up the population's gene pool. In diploid organisms such as humans, every gene is represented by two alleles. The pair of alleles may differ from one another, in which case it is said that the individual is heterozygous for that gene. If the two alleles are identical, it is said that the individual is homozygous for that gene. If every member of a population is homozygous for the same allele, the allele is said to be...

Non Fatal Human Aneuploid Conditions

The most common example of non-fatal trisomy in humans is that of Down syndrome, caused by the presence of an extra copy of chromosome 21. Affected individuals suffer from mental retardation, congenital heart disease, and increased suceptibility to infection. Physical characteristics include a short, stocky body, flattened facial features, and almond-shaped eyes. Down syndrome is an example of an autosomal trisomy as it involves one of the autosomes (autosome is the term that designates all the...

Chromosomal Aberrations

Meiosis is a very intricate process that requires, among other things, the precise alignment of homologous chromosome pairs and correct attachment of microtubules. During meiosis, errors in chromosome distribution may occur and lead to chromosomal aberrations in the offspring. One example is Down syndrome, where affected children carry three copies of chromosome 21 (trisomy 21). This may be explained by the failure of paired chromosomes or sister chromatids to separate in either sperm or egg,...

Polyploidy in Animals

Geneticist Hermann Muller argued that polyploidy is more rare in animals fthan plants because animals have a more complex development, with more organ systems that are fine-tuned to dosages of genes. Any given gene is represented three times in a triploid. If the amount (dosage) of gene product causes a heart, brain, or other vital organ not to form, the embryo will abort. When these developmental genes produce too much or too little of the products that induce organ formation, as they might if...

Fatal versus Nonfatal Conditions

In order to understand why some aneuploid conditions are fatal and others (such as those mentioned above) are not, one must understand the concept of gene dosage and its importance in development. A normal human possesses twenty-two pairs of autosomes and two sex chromosomes (XY in the case of males and XX in the case of females). Such an individual develops normally because there is a situation of genetic balance Each gene is present in the correct amount (or dose), such that its contribution...

Repairing the Damage

DNA is constantly being damaged, and it is constantly being repaired as well. It is only when the damage is not repaired that a mutation can lead to cancer or cell death. The DNA repair enzymes can recognize damaged nucleotides and remove and replace them. The human liver contains a large number of enzymes whose role is to detoxify toxic compounds, mutagenic or otherwise, by chemically reacting them. However, in some cases these enzymes (called cytochrome P450s) actually create mutagens during...

The Importance of the Electron Transport Chain

The origins of mitochondria are unknown, but the likely explanation, called the endosymbiont hypothesis, holds that they arose as free-living bacteria that colonized proto-eukaryotic cells, thereby establishing a symbiotic relationship. Primitive eukaryotic cells with intracellular mitochondria capable of metabolizing oxygen would have had an advantage in an oxygen-rich environment. The electron transport chain produces far more energy for each molecule of glucose consumed than is produced by...

Myotonic Muscular Dystrophy

Myotonic muscular dystrophy (DM, or dystrophia myotonica) is the most common adult-onset muscular dystrophy, having a frequency of one per twenty thousand persons in the general population. Myotonia, the delayed relaxation of a voluntary muscle after it is contracted, and muscle weakness are the hallmarks of the disorder. For example, a person with DM using a hammer will not immediately be able to release his grip on the handle when finished. It is an autosomal dominant disorder, but there is...

Table of Contents

Preface For Your Reference ix List of Contributors xvii Accelerated Aging Progeria 1 Addiction Aging and Life Span Agricultural Biotechnology 9 Alternative Splicing Alzheimer's Disease Ames Test Androgen Insensitivity Syndrome 21 Antibiotic Resistance Antisense Nucleotides 29 Apoptosis Arabidopsis thaliana Archaea Attention Deficit Hyperactivity Disorder 39 Attorney Automated Sequencer 43 Balanced Polymorphism 45 Behavior Bioinformatics Biopesticides Bioremediation Biotechnology Biotechnology...

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Tulane University Health Sciences Center Repetitive DNA Elements Joellen M. Schildkraut Duke University Medical Center Public Health, Genetic Techniques in Silke Schmidt Duke Center for Human Genetics Meiosis Mitosis David A. Scicchitano New York University Ames Test Carcinogens William K. Scott Duke Center for Human Genetics Aging and Life Span Epidemiologist Gene and Environment Gerry Shaw MacKnight Brain Institute of the University of Flordia Signal Transduction Alan R. Shuldiner University...

Other Typical Characteristics

Typical Operons

The clustered genes of the operon typically share a common promoter and a common regulatory region, called an operator. Gene expression requires Figure 2. Regulation of gene expression in the lac operon. RNA polymerase is represented as a five-subunit protein, with two a subunits, one p subunit, one p' subunit and one a subunit. The a subunit binds the promoter and dissociates from the polymerase once elongation of the RNA molecule begins. When lactose is present, some is converted to...

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Muscle biopsy of mitochondrial myopathy patient (modified Gomori trichrome stain) with ragged red fiber showing disorganization and mitochondrial proliferation. Mitochondria stain red. Other muscle fibers also have increased numbers of mitochondria. in coding for electron transport chain subunits is detailed in Figure 1. Human mtDNA contains 16,569 nucleotide bases and encodes thirteen polypeptides of the electron transport chain, twenty-two transfer RNAs (tRNAs) and two ribosomal...

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With the caveat that the intent is not to improve the gene pool, but to prevent suffering. This may mean terminating a pregnancy in which the fetus has a very bleak prognosis, which people opposed to abortion might find unethical. Opponents to this view point out that letting nature take its course can be painful for the fetus and may endanger the life of the woman. The ethics of prenatal diagnosis becomes more complicated when the goal is not to prevent suffering, but to choose a child of a...

The Causes of Nondisjunction and Its Frequency in Humans

Meiosis is a very tightly regulated process, and a whole series of control mechanisms (constituting a number of checkpoints) exist to ensure that everything proceeds in the correct manner. If an error should occur during the process, it is usually corrected. Nondisjunction is the result of a mistake at the level of chromosome segregation, which involves the spindle fibers. In normal meiosis, there is a mechanism that monitors the correct formation of the spindle fibers, the correct attachment...

Endosymbiosis and Genome Reduction

Given the bacterial-like features of mitochondria and chloroplasts (small size, circular genome, and ability to divide on their own), it is believed that each organelle traces its evolutionary history to a free-living bacterial ancestor that was engulfed by a larger cell and then entered into a symbiotic relationship with the host cell. This serial endosymbiotic theory proposes that the evolution of the modern eukaryotic cell was a step-wise association, with the acquisition of the mitochondria...

Enzyme Defects Cause Metabolic Disorders

The causes of enzyme defects are genetic mutations that affect the structure or regulation of the enzyme protein or create problems with the transport, processing, or binding of cofactors. In general, the consequences of an enzyme deficiency are due to perturbations of cellular chemistry, because of either a reduction in the amount of an essential product, the buildup of a toxic intermediate, or the production of a toxic side-product, as shown in Figure 1. Except as noted below, most metabolic...

Schizophrenia and Bipolar Disorder

Schizophrenia is a disorder characterized by psychotic symptoms such as hallucinations, delusions, and disordered thinking, as well as deficits in emotional and social behavior. Numerous family, twin, and adoption studies have provided substantial evidence for genetic factors in the etiology of this dis- etiology causation of disease, or the study of causation order. Nongenetic factors also appear to play an important role. The risk for relatives of an individual with schizophrenia is 6.5...

Tourettes Syndrome

Some psychiatric disorders, like Tourette's syndrome, have significant overlap (co-morbidity) with other psychiatric disorders (OCD and ADHD in the case of Tourette's). This may make the elucidation of genetic causes more difficult. Tourette's syndrome is characterized by multiple motor tics and one or more vocal tics. It has onset before eighteen years and is one and one-half to three times more common in males. Twin, adoption, and segregation analysis studies support a genetic etiology for...

Polymerase Chain Reaction

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, amplify produce many researchers can use it to amplify sequences even if they only have a minute amount of DNA. A single hair root, or a microscopic blood stain left at a crime scene, for example, contains ample...

Alterations of Amino Acids

Post-translational control of protein function or structure can be accomplished by chemical alteration of an amino acid side chain or by modification of the ends of the protein backbone. While there are many diverse chemical modifications of amino acids, three common examples are phosphorylation, glycosylation, and ubiquitination. Phosphorylation involves the addition of phosphate to an amino acid side chain, usually to the side chain hydroxyl -OH of serine, threonine, or tyrosine. This...

Viewing Chromosomes

Biologists first tried to visualize the chromosomes in a human cell in the late nineteenth century, with estimates of the total number ranging from 30 to 80. As methods to untangle and stain chromosomes improved, the count narrowed to 46 or 48, and by 1956 was confirmed as 46, or 23 pairs. By 1959, the first chromosomal abnormalities were identified using size and crude staining patterns to distinguish the chromosomes. In the 1970s, vastly cytogeneticists scien- improved staining techniques...

Duchenne Muscular Dystrophy

Duchenne muscular dystrophy DMD is an X-linked recessive disorder with a worldwide occurrence of one in four thousand newborn males, with approximately one-third of the cases arising from new mutations. DMD was named after the French neurologist Duchenne de Boulogne, who described the disorder in 1861. Becker muscular dystrophy BMD , named after Peter Becker, a German geneticist who first described it in the mid-1950s, is a disorder that is very similar to DMD but has a much milder course. In...

Chromosomal Aberrations and Transposons

Structural chromosomal aberrations, the second category of mutations, arise when DNA in chromosomes is broken. The broken ends may remain unrepaired or may be joined with those of another break, to form new combinations of genes, such as translocations. A translocation between chromosomes 8 and 21 in humans causes acute myeloid leukemia by increasing the activity of c-myc, a gene involved in cell replication. Translocations often cause human infertility, because they interfere with the normal...

Differences between Mitosis and Meiosis

Mitosis occurs in all eukaryotic cell tissues and produces genetically identical daughter cells with a complete set of chromosomes. In humans, mitosis produces somatic cells that are diploid, which means they contain two non-identical copies of each of the twenty-three chromosomes. One copy is derived from the person's mother and the other from the person's father. Meiosis, on the other hand, occurs only in testis and ovary tissues, producing sperm and ova eggs . The gametes that are produced...