Allantois During the embryonic stage of mammals, birds, and reptiles, the allantois, a small sac, is one of four extra-embryonic membranes (along with amnion, yolk sac, chorion) and serves several functions, such as a repository for the embryo’s nitrogenous waste (chiefly uric acid) in reptiles and birds (in the egg). The allan-tois provides oxygen to mammals, birds, and reptiles, as well as food in mammals (via the placenta).
The membrane of the allantois works with the chorion in respiratory functions, allowing the exchange of gases between the embryo and surrounding air. In humans, it is involved in the development of the urinary bladder.
Allele An allele is one of two or more alternative forms of a gene that can exist at a single locus. Each allele therefore has a unique nucleotide sequence and may lead to different phenotypes for a given trait. If the alleles for a gene are identical, the organism is called homozygous. If the alleles are different, the organism is heterozygous.
If two alleles are different, one becomes dominant and is fully expressed in appearance in the organism, while the other is recessive and has no noticeable effect on the appearance of the organism.
This is shown in the color of your eyes, determined by the genes inherited from your parents. The gene for brown eyes is dominant and overrides genes for other eye colors. The gene for blue eyes is recessive and will appear when there are no genes for other eye colors. A person with brown eyes may have a recessive, or "hidden," gene for blue eyes. Therefore, two brown-eyed parents may each give a recessive gene for blue eyes to their child, who would then have blue eyes. Gray, green, and other eye colors result from a complex mixing of different eye color genes.
Allen’s Rule In warm-blooded animals, the warmer the climate, the longer the appendages (ears, legs, wings) as compared with closely related taxa from colder areas.
An allele is one of the variant forms of a gene at a particular locus, or location, on a chromosome. Different alleles produce variation in inherited characteristics such as hair color or blood type. In an individual, one form of the allele (the dominant one) may be expressed more than another form (the recessive one).
Allochronic speciation Speciation that takes place related to time rather than space; populations that are reproductively isolated due to mating at different times.
Allometric growth The variation in the relative rates of growth of various parts of the body, which helps shape the organism. In other words, it is the pattern of growth whereby different parts of the body grow at different rates with respect to each other. Allometry is the study of relative growth and of changes in proportion with increase in size. For example, human arms and legs grow at a faster rate than the body and head, making adult proportions strikingly different from those of infants. Another striking example is the male fiddler crab Uca pugnax. In small males, the two claws are of equal weight, each weighing about 8 percent of the total crab weight. However, as the crab enlarges, its large crushing claw grows more rapidly, eventually constituting about 38 percent of the crab’s weight.
In 1932, Sir Julian Huxley described a simple mathematical method for the detection and measurement of the allometric growth. In order to compare the relative growth of two components (one of which may be the whole body), they are plotted logarithmically on x- and y-axes:
The slope of the resulting regression is called the allometric growth ratio, often designated as k.
k = 1, both components are growing at the same rate.
k <1, the component represented on the y-axis is growing more slowly than the component on the x-axis.
k >1, the y-axis component is growing faster than the x-axis component.
Another formula for measuring allometric growth is Y=bxa, where Y is equal to the mass of the organ, x = mass of the organism, a = growth coefficient of the organ, and b = a constant.
Yet another formula for measuring allometric growth is Y = bxa/c, where a and c are the growth rates for two body parts.
Allometric growth studies have also been applied to animal husbandry, archaeology, and urban systems studies.
Allometry The study of relative growth and of changes in proportion with increase in size.
Allopatric speciation one of two methods of speciation (the other is sympatric), allopatric speciation happens when the ancestral population becomes segregated by a geographical barrier. The Karner blue butterfly (Lycaeides melissa samualis) became allopatric from its parent the Melissa blue butterfly (Lycaeides melissa melissa) when the climate changed and restricted various populations along its range to northeastern pine barrens environments several thousand years ago. As populations become isolated, the isolated gene pools accumulate different genetic traits by microevolution. Small populations are more likely to evolve into separate species than larger isolated populations. Several populations of the Karner blue butterfly are now separated from each other by human-made development and may be evolving into separate subspecies or species, even though geographically they are isolated by only a few miles in some cases.
Conditions that favor allopatric speciation are when one population becomes isolated at the fringe of the parent population’s range. This splinter population, called a peripheral isolate, is likely to become allopatric because the gene pool of the isolate may already be different, since living on the border of the range encourages the expression of the extremes of any genotypic cllnes that existed in the original population. Furthermore, if the population is small enough, a founder effect will occur, giving rise to a gene pool that is not that of the parent population.
Genetic drift will also occur until the peripheral isolate becomes a larger population and will continue to change the gene pool at random until the population grows. New mutations or combinations of existing alleles that are neutral in adaptive value now may become fixed in the population by chance, causing genotypic or phenotypic divergence from the parent population. For example, the Karner blue butterfly has a row of orange spots on the top of the hindwing, whereas, the ancestral parent, the Melissa blue butterfly, has orange spots on the top of both front and hind-wing, a phenotypic variation.
Another factor in causing allopatric speciation is that evolution via natural selection may take a different road in the peripheral population. The isolate will encounter selection factors that are different from and perhaps more severe than that experienced by the parent because the isolate is living in an environment slightly, or completely, different from that of the parent. These small isolated populations are not guaranteed to become new species, as they are more often likely to become extinct, yet it is clear in evolutionary history that allopatric speciation does occur.
Allopolyploid A type of polyploid (having a nucleus that contains more than two sets of chromosomes) species, often a plant, resulting from two different species interbreeding and combining their chromosomes. Hybrids are often sterile because they do not have sets of homologous chromosomes, making pairing nonexistent unless two diploid hybrids double the chromosome numbers, resulting in a fertile allote-traploid that now contains two sets of homologous chromosomes. Plant breeders find that this is beneficial, since it is possible to breed the advantages of different species into one. Triticale (a "new" grain created by crossing rye and durum wheat) is an allopolyploid that was developed from wheat and rye. Some crops are naturally allopolyploid, such as cotton, oats, tall fescue, potatoes, wheat, and tobacco. It is estimated that half of all angiosperms (flowering plants) are polyploid.
Allosteric binding sites A type of binding site contained in many enzymes and receptors. As a consequence of the binding to allosteric binding sites, the interaction with the normal ligand (ligands are molecules that bind to proteins) may be either enhanced or reduced. Ligand binding can change the shape of a protein.
Allosteric effector Specific small molecules that bind to a protein at a site other than a catalytic site and that modulate (by activation or inhibition) the biological activity.
Allosteric enzyme An enzyme that contains a region, separate from the region that binds the substrate for catalysis, where a small regulatory molecule binds and affects that catalytic activity. This effector molecule may be structurally unrelated to the substrate, or it may be a second molecule of substrate. If the catalytic activity is enhanced by binding, the effector is called an activator; if it is diminished, the effector is called an inhibitor.
Allosteric regulation The regulation of the activity of allosteric enzymes.
Allosteric site A specific receptor site on an enzyme molecule not on the active site (the site on the surface of an enzyme molecule that binds the substrate molecule). Molecules bind to the allosteric site and change the shape of the active site, either enabling the substrate to bind to the active site or prevent the binding of the substrate.
The molecule that binds to the allosteric site is an inhibitor because it causes a change in the three-dimensional structure of the enzyme that prevents the substrate from binding to the active site.
Allozyme An enzyme form, a variant of the same enzyme (protein) that is coded for by different alleles at a single locus.
Alpha helix Most proteins contain one or more stretches of amino acids that take on a particular shape in three-dimensional space. The most common forms are alpha helix and beta sheet.
Alpha helix is spiral shaped, constituting one form of the secondary structure of proteins, arising from a specific hydrogen-bonding structure; the carbonyl group (-C=O) of each peptide bond extends parallel to the axis of the helix and points directly at the -N-H group of the peptide bond four amino acids below it in the helix.The alpha helix is right-handed and twists clockwise, like a corkscrew, and makes a complete turn every 3.6 amino acids. The distance between two turns is 0.54 nm. However, an alpha helix can also be left-handed. Most enzymes contain sections of alpha helix.
The alpha helix was discovered by Linus Pauling in 1948.
Alternation of generations A life cycle in plants where there is both a multicellular diploid form (the sporophyte generation) and a multicellular haploid form (the gametophyte generation).
Gametophytes produce haploid gametes that fuse zygotes that are forming. These zygotes then develop into diploid sporophytes. Meiosis in the sporophytes produces haploid spores, with division by meiosis giving rise to the next generation of gametophytes.
Alternation of generations occurs in plants and certain species of algae. Ferns and fern allies (such as the club moss) are common examples that display alternation of generations. The above ground parent fern plant (the diploid sporophyte, or spore-bearing plant) has two full sets of chromosomes (two of each kind of chromosome). It sheds its single-celled haploid spores, having one set of chromosomes (one of each kind), which fall to the ground, and these in turn grow into a different plant, the gametophyte or prothallus, also haploid. The gametophyte has special bodies within the plant called archegonia (female cells) and antheridia (male cells). Here sexual fertilization takes place, and a new diploid sporophyte then grows.
There are four main groups of plants considered to be "fern allies," a diverse group of vascular plants that are neither flowering plants nor ferns and that reproduce by shedding spore to initiate an alternation of generations. These are the Lycophyta (Lycopsida, the club mosses; Selaginellopsida, the spike mosses; and Isoetopsida, the quillworts); the Archeophyta (Sphenopsida, the horsetails and scouring-rushes; Psilopsida, the whiskbrooms; and ophioglossopsida, the adder’s-tongues and grape-ferns); the Pteridophyta (ferns); and Spermatophyta (flowering plants).
In some examples of alternation of generations—for example, in certain algae species such as in some green or brown forms—the alternation of generations takes on two different approaches. Where the sporophytes and gametophytes are structurally different, the two generations are heteromorphic. If the sporophytes and gametophytes look the same and have different chromosome pairs, the generations are said to be isomorphic.
Altruistic behavior The aiding of another individual at one’s own risk or expense. This can be in the form of one animal sending out a distress call to warn others of impending trouble, although putting itself in danger by giving out its location. Strangers coming to the rescue of other strangers, such as victims in an accident, hurricane, or earthquake, is another example of altruistic behavior.
Alveolus (plural, alveoli) Latin for "hollow cavity." There are several definitions for alveolus. It is a thin, multilobed air sac that exchanges gases in the lungs of mammals and reptiles at the end of each bronchiole, a very fine respiratory tube in the lungs. An alveolus is lined with many blood capillaries where the exchange of carbon dioxide and oxygen takes place.
It is also the name given to the socket in the jawbone in which a tooth is rooted by means of the peri-odontal membrane, the connective tissue that surrounds the root and anchors it.
Furthermore, it is the term used to describe a single hexagonal beehive cell found in a honeycomb. It is also the term that refers to the milk-secreting sacs of the mammary gland.
Ambidentate ligands, such as (NCS)-, that can bond to a central atom through either of two or more donor atoms.
Amicyanin An electron transfer protein containing a type 1 copper site, isolated from certain bacteria.
Amino acid An organic molecule possessing both acidic carboxylic acid (-COOH) and basic amino (-NH2) groups attached to the same tetrahedral carbon atom.
Amino acids are the principal building blocks of proteins and enzymes. They are incorporated into proteins by transfer RNA according to the genetic code while messenger RNA is being decoded by ribo-somes. The amino acid content dictates the spatial and biochemical properties of the protein or enzyme during and after the final assembly of a protein. Amino acids have an average molecular weight of about 135 daltons. While more than 50 have been discovered, 20 are essential for making proteins, long chains of bonded amino acids.
Amino acids comprise a group of 20 different kinds of small molecules that link together in long chains to form proteins. Often referred to as the "building blocks" of proteins.
Some naturally occurring amino acids are alanine, arginine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, thre-onine, tryptophan, tyrosine, and valine.
The two classes of amino acids that exist are based on whether the R-group is hydrophobic or hydrophilic. Hydrophobic or nonpolar amino acids tend to repel the aqueous environment and are located mostly in the interior of proteins. They do not ionize or participate in the formation of hydrogen bonds. on the other hand, the hydrophilic or polar amino acids tend to interact with the aqueous environment, are usually involved in the formation of hydrogen bonds, and are usually found on the exterior surfaces of proteins or in their reactive centers. It is for this reason that certain amino acid R-groups allow enzyme reactions to occur.
The hydrophilic amino acids can be further subdivided into polar with no charge, polar with negatively charged side chains (acidic), and polar with positively charged side chains (basic).
While all amino acids share some structural similarities, it is the side groups, or "R"-groups as they are called, that make the various amino acids chemically and physically different from each other so that they react differently with the environment. These groupings, found among the 20 naturally occurring amino acids, are ionic (aspartic acid, arginine, glutamic acid, lysine, and histidine), polar (asparagine, serine, threonine, cys-teine, tyrosine, and glutamine), and nonpolar amino acids (alanine, glycine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, and proline).
Amino acids are also referred to as amphoteric, meaning they can react with both acids and alkali, which makes them effective buffers in biological systems. A buffer is a solution where the pH usually stays constant when an acid or base is added.
In 1986 scientists found a 21st amino acid, seleno-cysteine. In 2002 two teams of researchers from ohio State University identified the 22nd genetically encoded amino acid, called pyrrolysine, a discovery that is the biological equivalent of physicists finding a new fundamental particle or chemists discovering a new element.
Amino acid supplements are widely used in exercise and dietary programs.