Succinate dehydrogenase or succinate-coenzyme Q reductase or respiratory Complex II is an enzyme complex, found in many bacterial cells and in the inner mitochondrial membrane of eukaryotes. It is the only enzyme that participates in both the citric acid cycle and the electron transport chain. Histochemical analysis showing high succinate dehydrogenase in muscle demonstrates high mitochondrial content and high oxidative potential. In step 6 of the citric acid cycle, SQR catalyzes the oxidation of succinate to fumarate with the reduction of ubiquinone to ubiquinol; this occurs in the inner mitochondrial membrane by coupling the two reactions together. Mitochondrial and many bacterial monomer SQRs are composed of four subunits: two hydrophilic and two hydrophobic; the first two subunits, a flavoprotein and an iron-sulfur protein, are hydrophilic. SdhA contains a covalently attached flavin adenine dinucleotide cofactor and the succinate binding site and SdhB contains three iron-sulfur clusters:, and.
The second two subunits are hydrophobic membrane anchor subunits, SdhC and SdhD. Human mitochondria contain two distinct isoforms of SdhA, these isoforms are found in Ascaris suum and Caenorhabditis elegans; the subunits form a membrane-bound cytochrome b complex with six transmembrane helices containing one heme b group and a ubiquinone-binding site. Two phospholipid molecules, one cardiolipin and one phosphatidylethanolamine, are found in the SdhC and SdhD subunits, they serve to occupy the hydrophobic space below the heme b. These subunits are displayed in the attached image. SdhA is green, SdhB is teal, SdhC is fuchsia, SdhD is yellow. Around SdhC and SdhD is a phospholipid membrane with the intermembrane space at the top of the image. Ubiquinone's binding site, image 4, is located in a gap composed of SdhB, SdhC, SdhD. Ubiquinone is stabilized by the side chains of His207 of subunit B, Ser27 and Arg31 of subunit C, Tyr83 of subunit D; the quinone ring is surrounded by Ile28 of subunit C and Pro160 of subunit B.
These residues, along with Il209, Trp163, Trp164 of subunit B, Ser27 of subunit C, form the hydrophobic environment of the quinone-binding pocket. SdhA provides the binding site for the oxidation of succinate; the side chains Thr254, His354, Arg399 of subunit A stabilize the molecule while FAD oxidizes and carries the electrons to the first of the iron-sulfur clusters. This can be seen in image 5; the succinate-binding site and ubiquinone-binding site are connected by a chain of redox centers including FAD and the iron-sulfur clusters. This chain extends over 40 Å through the enzyme monomer. All edge-to-edge distances between the centers are less than the suggested 14 Å limit for physiological electron transfer; this electron transfer is demonstrated in image 8. Little is known about the exact succinate oxidation mechanism. However, the crystal structure shows that FAD, Glu255, Arg286, His242 of subunit A are good candidates for the initial deprotonation step. Thereafter, there are two possible elimination mechanisms: E1cb.
In the E2 elimination, the mechanism is concerted. The basic residue or cofactor deprotonates the alpha carbon, FAD accepts the hydride from the beta carbon, oxidizing the bound succinate to fumarate—refer to image 6. In E1cb, an enolate intermediate is shown in image 7, before FAD accepts the hydride. Further research is required to determine which elimination mechanism succinate undergoes in Succinate Dehydrogenase. Oxidized fumarate, now loosely bound to the active site, is free to exit the protein. After the electrons are derived from succinate oxidation via FAD, they tunnel along the relay until they reach the cluster; these electrons are subsequently transferred to an awaiting ubiquinone molecule within the active site. The Iron-Sulfur electron tunneling system is shown in image 9; the O1 carbonyl oxygen of ubiquinone is oriented at the active site by hydrogen bond interactions with Tyr83 of subunit D. The presence of electrons in the iron sulphur cluster induces the movement of ubiquinone into a second orientation.
This facilitates a second hydrogen bond interaction between the O4 carbonyl group of ubiquinone and Ser27 of subunit C. Following the first single electron reduction step, a semiquinone radical species is formed; the second electron arrives from the cluster to provide full reduction of the ubiquinone to ubiquinol. This mechanism of the ubiquinone reduction is shown in image 8. Although the functionality of the heme in succinate dehydrogenase is still being researched, some studies have asserted that the first electron delivered to ubiquinone via may tunnel back and forth between the heme and the ubiquinone intermediate. In this way, the heme cofactor acts as an electron sink, its role is to prevent the interaction of the intermediate with molecular oxygen to produce reactive oxygen species. The heme group, relative to ubiquinone, is shown in image 4, it has been proposed that a gating mechanism may be in place to prevent the electrons from tunneling directly to the heme from the cluster. A potential candidate is residue His207, which lies directly between the heme.
His207 of subunit B is in direct proximity to the cluster, the bound ubiquinone, the heme. To reduce the quinone in SQR, two electrons as well as two protons are needed, it has been argued that a water molecule arrives at the active site and is coordinated by His207 of subunit B, Arg31 of subunit C, Asp82 of subunit D. The semiquinone species is protonated by protons deliv
Gömöri trichrome stain
Gömöri trichrome stain is a histological stain used on muscle tissue. It can be used to test for certain forms of mitochondrial myopathy, it is named for George Gömöri, who developed it in 1950. Media related to Gömöri trichrome stain at Wikimedia Commons
A micrograph or photomicrograph is a photograph or digital image taken through a microscope or similar device to show a magnified image of an object. This is opposed to a macrograph or photomacrograph, an image, taken on a microscope but is only magnified less than 10 times. Micrography is the art of using microscopes to make photographs. A micrograph contains extensive details of microstructure. A wealth of information can be obtained from a simple micrograph like behavior of the material under different conditions, the phases found in the system, failure analysis, grain size estimation, elemental analysis and so on. Micrographs are used in all fields of microscopy. A light micrograph or photomicrograph is a micrograph prepared using an optical microscope, a process referred to as photomicroscopy. At a basic level, photomicroscopy may be performed by connecting a camera to a microscope, thereby enabling the user to take photographs at reasonably high magnification. Scientific use began in England in 1850 by Prof Richard Hill Norris FRSE for his studies of blood cells.
Roman Vishniac was a pioneer in the field of photomicroscopy, specializing in the photography of living creatures in full motion. He made major developments in light-interruption photography and color photomicroscopy. Photomicrographs may be obtained using a USB microscope attached directly to a home computer or laptop. An electron micrograph is a micrograph prepared using an electron microscope. Micrographs have micron bars, or magnification ratios, or both. Magnification is a ratio between the size of an object on its real size. Magnification can be a misleading parameter as it depends on the final size of a printed picture and therefore varies with picture size. A scale bar, or micron bar, is a line of known length displayed on a picture; the bar can be used for measurements on a picture. When the picture is resized the bar is resized making it possible to recalculate the magnification. Ideally, all pictures destined for publication/presentation should be supplied with a scale bar. All but one of the micrographs presented on this page do not have a micron bar.
The microscope has been used for scientific discovery. It has been linked to the arts since its invention in the 17th century. Early adopters of the microscope, such as Robert Hooke and Antonie van Leeuwenhoek, were excellent illustrators. After the invention of photography in the 1820s the microscope was combined with the camera to take pictures instead of relying on an artistic rendering. Since the early 1970s individuals have been using the microscope as an artistic instrument. Websites and traveling art exhibits such as the Nikon Small World and Olympus Bioscapes have featured a range of images for the sole purpose of artistic enjoyment; some collaborative groups, such as the Paper Project have incorporated microscopic imagery into tactile art pieces as well as 3D immersive rooms and dance performances. Close-up Digital microscope Macro photography Microphotograph Microscopy USB microscope Make a Micrograph – This presentation by the research department of Children's Hospital Boston shows how researchers create a three-color micrograph.
Shots with a Microscope – a basic, comprehensive guide to photomicrography Scientific photomicrographs – free scientific quality photomicrographs by Doc. RNDr. Josef Reischig, CSc. Micrographs of 18 natural fibres by the International Year of Natural Fibres 2009 Seeing Beyond the Human Eye Video produced by Off Book - Solomon C. Fuller bio Charles Krebs Microscopic Images Dennis Kunkel Microscopy Andrew Paul Leonard, APL Microscopic Cell Centered Database - Montage Nikon Small World Olympus Bioscapes Other examples
Embryonic is the twelfth studio album by experimental rock band The Flaming Lips released on October 13, 2009 on Warner Bros. It is the first double album to be released by the band, announced during an interview with the band's frontman Wayne Coyne. Somewhere along the way it occurred to me that we should do a double album... Just this idea that you can weave a couple of themes into there and you can sprawl a little bit. Several other artists made contributions to various tracks on the album. German mathematician Dr. Thorsten Wörmann contributed to the track "Gemini Syringes", psychedelic rock band MGMT contributed to the song "Worm Mountain", Karen O contributed to the songs "I Can Be a Frog" and "Watching the Planets". Karen O's contributions were recorded by Wayne Coyne over the phone. On August 13, 2009, the song "See the Leaves" was reviewed and streamed on Pitchfork.com On September 3, 2009, the album was previewed in its entirety on The Fly website, using Wayne Coyne's own track-by-track guide.
On September 17, 2009, the band appeared on The Colbert Report and announced that the album would stream in its entirety on Colbertnation.com until September 21, 2009. Embryonic was streamed in full on the UK music site clashmusic.com on October 5, just over a week ahead of its release. Embryonic was selected as fourth best album of 2009 by Pitchfork Media Embryonic received general acclaim from critics upon release, garnering an critic score of 81 out of 100 on Metacritic. NME wrote that "ten years after their last masterpiece, The Flaming Lips have produced another one," while Paste described the record as "a wonderfully weird parade of sonic delights: an arresting consummation of the Lips' two-and-a-half decade career." Other critics praised the album but were quick to note its different sound in comparison to previous releases. Mojo remarked that " themes may be familiar, but its fine, dazzlingly outlandish music is fresh and utterly fearless." As of 2011, the album has sold 103,000 copies in the United States.
The style of the tracks on Embryonic differs from the styles of previous albums, Yoshimi Battles the Pink Robots and At War with the Mystics, has been reported to be similar to the style of Joy Division, Miles Davis, John Lennon. Wayne Coyne says the new record solves their perpetual "dilemma" of what to include on each album, by dumping all their ideas on the follow-up to 2006's At War with the Mystics. Coyne had this to say about the double-LP decision to Billboard: "Some of my favorite records – thinking Beatles' White Album, Zeppelin's Physical Graffiti and some of the longer things that The Clash have done – part of the reason I like them is that they're not focused, they go everywhere. It's not because we're prolific, I think we always stay in a sort of perpetual panic of like we never have more songs than we need and we always wonder if any of them are any good to begin with." Coyne notes that Embryonic is less polished than Mystics or 2002's Yoshimi Battles the Pink Robots and has a "freak-out vibe".
The frontman notes the influence of Miles Davis's group and slow-burn songs like John Lennon's "Instant Karma!". A deluxe version of the album was released on October 13, 2009; the deluxe edition includes the original 18 tracks as well as a bonus DVD-Audio which features the album in full dynamic range at 24bit/96 kHz audio. A further variant sold through the band's website is packaged in a "fur pack" with an extended booklet which features additional art and band photos; this web-only deluxe edition comes with a 14 inch by 28 inch lithograph featuring the full album cover. A limited number of pre-orders received an additional lithograph autographed by the band, shipped 2–3 weeks after the release date. On one retail edition of the release, all tracks are included on one disc, though both the "deluxe" and "fur-pack" variations of the album spread the songs over two discs, containing 9 songs each. Disc one "Convinced of the Hex" – 3:56 "The Sparrow Looks Up at the Machine" – 4:14 "Evil" – 5:38 "Aquarius Sabotage" – 2:11 "See the Leaves" – 4:24 "If" – 2:05 "Gemini Syringes" – 3:41 "Your Bats" – 2:35 "Powerless" – 6:57Disc two "The Ego's Last Stand" – 5:40 "I Can Be a Frog" – 2:14 "Sagittarius Silver Announcement" – 2:59 "Worm Mountain" – 5:21 "Scorpio Sword" – 2:02 "The Impulse" – 3:30 "Silver Trembling Hands" – 3:58 "Virgo Self-Esteem Broadcast" – 3:45 "Watching the Planets" – 5:16iTunes exclusive bonus tracks "UFOs Over Baghdad" – 5:18 "What Does It Mean?"
– 5:10 "Just Above Love" – 4:49 "Anything You Say Now, I Believe You" – 6:40 The Flaming Lips Wayne Coyne – lead vocals, theremin, vocoder, production Steven Drozd – guitars, drums, backing vocals, lead vocals on "If", production Michael Ivins – bass, backing vocals, production, additional engineering Kliph Scurlock – drums, percussionAdditional personnel Scott Booker – production Dave Fridmann – production, programming, mastering MGMT – additional singing and playing on "Worm Mountain" Karen O – additional singing, animal sounds and noises on "Gemini Syringes", "I Can Be a Frog" and "Watching the Planets" Thorsten Wörmann – spoken announcements on "Gemini Syringes" and "Virgo Self-Esteem Broadcast" Embryonic moved 32,000 copies in its first week on US charts
Dominance in genetics is a relationship between alleles of one gene, in which the effect on phenotype of one allele masks the contribution of a second allele at the same locus. The first allele is dominant and the second allele is recessive. For genes on an autosome, the alleles and their associated traits are autosomal dominant or autosomal recessive. Dominance is a key concept in Mendelian inheritance and classical genetics; the dominant allele codes for a functional protein whereas the recessive allele does not. A classic example of dominance is the inheritance of seed shape in peas. Peas associated with allele r. In this case, three combinations of alleles are possible: RR, Rr, rr; the RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals have round peas. Thus, allele R is dominant to allele r, allele r is recessive to allele R; this use of upper case letters for dominant alleles and lower case ones for recessive alleles is a followed convention.
More where a gene exists in two allelic versions, three combinations of alleles are possible: AA, Aa, aa. If AA and aa individuals show different forms of some trait, Aa individuals show the same phenotype as AA individuals allele A is said to dominate, be dominant to or show dominance to allele a, a is said to be recessive to A. Dominance is not inherent to either its phenotype, it is a relationship between two alleles of their associated phenotypes. An allele may be dominant for a particular aspect of phenotype but not for other aspects influenced by the same gene. Dominance differs from epistasis, a relationship in which an allele of one gene affects the expression of another allele at a different gene; the concept of dominance was introduced by Gregor Johann Mendel. Though Mendel, "The Father of Genetics", first used the term in the 1860s, it was not known until the early twentieth century. Mendel observed that, for a variety of traits of garden peas having to do with the appearance of seeds, seed pods, plants, there were two discrete phenotypes, such as round versus wrinkled seeds, yellow versus green seeds, red versus white flowers or tall versus short plants.
When bred separately, the plants always produced generation after generation. However, when lines with different phenotypes were crossed and only one of the parental phenotypes showed up in the offspring. However, when these hybrid plants were crossed, the offspring plants showed the two original phenotypes, in a characteristic 3:1 ratio, the more common phenotype being that of the parental hybrid plants. Mendel reasoned that each parent in the first cross was a homozygote for different alleles, that each contributed one allele to the offspring, with the result that all of these hybrids were heterozygotes, that one of the two alleles in the hybrid cross dominated expression of the other: A masked a; the final cross between two heterozygotes would produce AA, Aa, aa offspring in a 1:2:1 genotype ratio with the first two classes showing the phenotype, the last showing the phenotype, thereby producing the 3:1 phenotype ratio. Mendel did not use the terms gene, phenotype, genotype and heterozygote, all of which were introduced later.
He did introduce the notation of capital and lowercase letters for dominant and recessive alleles still in use today. Most animals and some plants have paired chromosomes, are described as diploid, they have two versions of each chromosome, one contributed by the mother's ovum, the other by the father's sperm, known as gametes, described as haploid, created through meiosis. These gametes fuse during fertilization during sexual reproduction, into a new single cell zygote, which divides multiple times, resulting in a new organism with the same number of pairs of chromosomes in each cell as its parents; each chromosome of a matching pair is structurally similar to the other, has a similar DNA sequence. The DNA in each chromosome functions as a series of discrete genes that influence various traits. Thus, each gene has a corresponding homologue, which may exist in different versions called alleles; the alleles at the same locus on the two homologous chromosomes may be different. The blood type of a human is determined by a gene that creates an A, B, AB or O blood type and is located in the long arm of chromosome nine.
There are three different alleles that could be present at this locus, but only two can be present in any individual, one inherited from their mother and one from their father. If two alleles of a given gene are identical, the organism is called a homozygote and is said to be homozygous with respect to that gene; the genetic makeup of an organism, either at a single locus or over all its genes collectively, is called its genotype. The genotype of an organism directly and indirectly affects its molecular and other traits, which individually or collectively are called its phenotype. At heterozygous gene loci, the two alleles interact to produce the phenotype. In complete dominance, the effect of one allele in a heterozygous genotype masks the effect of the other; the allele that mas
A neuromuscular junction is a chemical synapse formed by the contact between a motor neuron and a muscle fiber. It is at the neuromuscular junction that a motor neuron is able to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to function—and just to maintain muscle tone, avoiding atrophy. Synaptic transmission at the neuromuscular junction begins when an action potential reaches the presynaptic terminal of a motor neuron, which activates voltage-dependent calcium channels to allow calcium ions to enter the neuron. Calcium ions bind to sensor proteins on synaptic vesicles, triggering vesicle fusion with the cell membrane and subsequent neurotransmitter release from the motor neuron into the synaptic cleft. In vertebrates, motor neurons release acetylcholine, a small molecule neurotransmitter, which diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors on the cell membrane of the muscle fiber known as the sarcolemma. NAChRs are ionotropic receptors.
The binding of ACh to the receptor can depolarize the muscle fiber, causing a cascade that results in muscle contraction. Neuromuscular junction diseases can be of autoimmune origin. Genetic disorders, such as Duchenne muscular dystrophy, can arise from mutated structural proteins that comprise the neuromuscular junction, whereas autoimmune diseases, such as myasthenia gravis, occur when antibodies are produced against nicotinic acetylcholine receptors on the sarcolemma. At the neuromuscular junction presynaptic motor axons terminate 30 nanometers from the cell membrane or sarcolemma of a muscle fiber; the sarcolemma at the junction has invaginations called postjunctional folds, which increase its surface area facing the synaptic cleft. These postjunctional folds form the motor endplate, studded with nicotinic acetylcholine receptors at a density of 10,000 receptors/micrometer2; the presynaptic axons terminate in bulges called terminal boutons that project toward the postjunctional folds of the sarcolemma.
In the frog each motor nerve terminal contains about 300,000 vesicles, with an average diameter of 0.05 micrometers. The vesicles contain acetylcholine; some of these vesicles are gathered into groups of fifty, positioned at active zones close to the nerve membrane. Active zones are about 1 micrometer apart; the 30 nanometer cleft between nerve ending and endplate contains a meshwork of acetylcholinesterase at a density of 2,600 enzyme molecules/micrometer2, held in place by the structural proteins dystrophin and rapsyn. Present is the receptor tyrosine kinase protein MuSK, a signaling protein involved in the development of the neuromuscular junction, held in place by rapsyn. About once every second in a resting junction randomly one of the synaptic vesicles fuses with the presynaptic neuron's cell membrane in a process mediated by SNARE proteins. Fusion results in the emptying of the vesicle's contents of 7000-10,000 acetylcholine molecules into the synaptic cleft, a process known as exocytosis.
Exocytosis releases acetylcholine in packets that are called quanta. The acetylcholine quantum diffuses through the acetylcholinesterase meshwork, where the high local transmitter concentration occupies all of the binding sites on the enzyme in its path; the acetylcholine that reaches the endplate activates ~2,000 acetylcholine receptors, opening their ion channels which permits sodium ions to move into the endplate producing a depolarization of ~0.5 mV known as a miniature endplate potential. By the time the acetylcholine is released from the receptors the acetylcholinesterase has destroyed its bound ACh, which takes about ~0.16 ms, hence is available to destroy the ACh released from the receptors. When the motor nerve is stimulated there is a delay of only 0.5 to 0.8 msec between the arrival of the nerve impulse in the motor nerve terminals and the first response of the endplate The arrival of the motor nerve action potential at the presynaptic neuron terminal opens voltage-dependent calcium channels and Ca2+ ions flow from the extracellular fluid into the presynaptic neuron's cytosol.
This influx of Ca2+ causes several hundred neurotransmitter-containing vesicles to fuse with the presynaptic neuron's cell membrane through SNARE proteins to release their acetylcholine quanta by exocytosis. The endplate depolarization by the released acetylcholine is called an endplate potential; the EPP is accomplished when ACh binds the nicotinic acetylcholine receptors at the motor end plate, causes an influx of sodium ions. This influx of sodium ions generates the EPP, triggers an action potential which travels along the sarcolemma and into the muscle fiber via the transverse tubules by means of voltage-gated sodium channels; the conduction of action potentials along the transverse tubules stimulates the opening of voltage-gated Ca2+ channels which are mechanically coupled to Ca2+ release channels in the sarcoplasmic reticulum. The Ca2+ diffuses out of the sarcoplasmic reticulum to the myofibrils so it can stimulate contraction; the endplate potential is thus responsible for setting up an action potential in the muscle fiber which triggers muscle contraction.
The transmission from nerve to muscle is so rapid because each quantum of acetylcholine reaches the endplate in millimolar concentrations, high enough to combine with a receptor with a low affinity, which swiftly releases the bound transmitter. Acetylcholine is a neurotransmitter synthesized from dietary choline and acetyl-CoA, is involved in the stimulation of muscle tissue in vertebrates as well as i
Non-Mendelian inheritance is any pattern of inheritance in which traits do not segregate in accordance with Mendel's laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. In Mendelian inheritance, each parent contributes one of two possible alleles for a trait. If the genotypes of both parents in a genetic cross are known, Mendel’s laws can be used to determine the distribution of phenotypes expected for the population of offspring. There are several situations in which the proportions of phenotypes observed in the progeny do not match the predicted values. Non-Mendelian Inheritance is applicable in incomplete dominance. Non-Mendeian inheritance plays a role in several disease processes. Extranuclear inheritance is a form of non-Mendelian inheritance first discovered by Carl Correns in 1908. While working with Mirabilis jalapa Correns observed that leaf color was dependent only on the genotype of the maternal parent. Based on these data, he determined that the trait was transmitted through a character present in the cytoplasm of the ovule.
Research by Ruth Sager and others identified DNA present in chloroplasts as being responsible for the unusual inheritance pattern observed. Work on the poky strain of the mold Neurospora crassa begun by Mary and Hershel Mitchell led to the discovery of genetic material in mitochondria as well. According to the endosymbiont theory and chloroplasts were once free living organisms that were each taken up by a eukaryotic cell. Over time and chloroplasts formed a symbiotic relationship with their eukaryotic hosts. Although the transfer of a number of genes from these organelles to the nucleus prevents them from living independently, each still possesses genetic material in the form of double stranded DNA, it is the transmission of this organellar DNA, responsible for the phenomenon of extranuclear inheritance. Both chloroplasts and mitochondria are present in the cytoplasm of maternal gametes only. Paternal gametes do not have cytoplasmic mitochondria. Thus, the phenotype of traits linked to genes found in either chloroplasts or mitochondria are determined by the maternal parent.
In humans, mitochondrial diseases are a class of diseases, many of which affect the muscles and the eye. Gene conversion can be one of the major forms of non-Mendelian inheritance. Gene conversion arises during DNA repair via DNA recombination, by which a piece of DNA sequence information is transferred from one DNA helix to another DNA helix, whose sequence is altered; this may occur as a mismatch repair between the strands of DNA which are derived from different parents. Thus the mismatch repair can convert one allele into the other; this phenomenon can be detected through the offspring non-Mendelian ratios, is observed, e.g. in fungal crosses. Another form of non-Mendelian inheritance is known as infectious heredity. Infectious particles such as viruses may infect host cells and continue to reside in the cytoplasm of these cells. If the presence of these particles results in an altered phenotype this phenotype may be subsequently transmitted to progeny; because this phenotype is dependent only on the presence of the invader in the host cell’s cytoplasm, inheritance will be determined only by the infected status of the maternal parent.
This will result in a uniparental transmission of the trait, just as in extranuclear inheritance. One of the most well studied examples of infectious heredity is the killer phenomenon exhibited in yeast. Two double-stranded RNA viruses, designated L and M, are responsible for this phenotype; the L virus codes for the capsid proteins of both viruses, as well as an RNA polymerase. Thus the M virus can only infect cells harboring L virus particles; the M viral RNA encodes a toxin, secreted from the host cell. It kills susceptible cells growing in close proximity to the host; the M viral RNA renders the host cell immune to the lethal effects of the toxin. For a cell to be susceptible it harbor only the L virus; the L and M viruses are not capable of exiting their host cell through conventional means. They can only transfer from cell to cell. All progeny of a mating involving a doubly infected yeast cell will be infected with the L and M viruses. Therefore, the killer phenotype will be passed down to all progeny.
Heritable traits that result from infection with foreign particles have been identified in Drosophila. Wild type flies full recover after being anesthetized with carbon dioxide. Certain lines of flies have been identified that die off after exposure to the compound; this carbon dioxide sensitivity is passed down from mothers to their progeny. This sensitivity is due to infection with σ virus, a rhabdovirus only capable of infecting Drosophila. Although this process is associated with viruses, recent research has shown that the Wolbachia bacterium is capable of inserting its genome into that of its host. Genomic imprinting represents yet another example of non-Mendelian inheritance. Just as in conventional inheritance, genes for a given trait are passed down to progeny from both parents. However, these genes are epigenetically marked before transmission, altering their levels of expression; these imprints are created before gamete formation and are erased during the creation of germ line cells. Therefore, a new pattern of imprinting can be made with each generation.
Genes are imprinted differently depending on the parental origin of the chromosome that contains them. In mice, the insulin-like growth factor 2 gene undergoes imprinting; the protein e