Name: Norman Anthony Aguero Currently a student at FIU. My major is chemistry and my minor is physics. My goal is to hopefully earn a Ph.D. in physical organic chemistry.
According to Wikipedia, Cro-Magnon man lived about 35,000 to 10,000 years ago in the Upper Paleothici period of the Pleistocene epoch. (Apparently, having difficult to pronounce names gives the illusion of significance in the scientific world.)
But anywho, I beg to differ. And don't think I'm on some creationist trip either. Granted, I might possibly believe the Earth to be 4.5 billion years old. But that shouldn't discredit the information I am about to share.
You see, I'm not on a rampage to prove evolutionists wrong. I just like to prove myself correct. And at last I have proof.
Cro-Magnon man didn't live 10,000+ years ago. That fucker still walks the Earth today. I seen him with my own eyes. So in a sense, I'm about to prove how both creationists and evolutionists are correct.
Johnny "Neanderthal" Damon is living proof that Cro-Magnon man exists. I mean, good God look at the size of that dude's head. If Johnny Damon died today, and I had some scientific nitwit undig his bones 10 years from now he would swear up and down he found the missing link.
One of South America's few remaining uncontacted indigenous tribes has been spotted and photographed on the border between Brazil and Peru.
The Brazilian government says it took the images to prove the tribe exists and help protect its land.
The pictures, taken from an aeroplane, show red-painted tribe members brandishing bows and arrows.
More than half the world's 100 uncontacted tribes live in Brazil or Peru, Survival International says.
Stephen Corry, the director of the group - which supports tribal people around the world - said such tribes would "soon be made extinct" if their land was not protected.
'Monumental crime'
Survival International said that although this particular group is increasing in number, others in the area are at risk from illegal logging.
The photos were taken during several flights over one of the most remote parts of the Amazon rainforest in Brazil's Acre region.
They show tribe members outside thatched huts, surrounded by the dense jungle, pointing bows and arrows up at the camera.
"We did the overflight to show their houses, to show they are there, to show they exist," the group quoted Jose Carlos dos Reis Meirelles Junior, an official in the Brazilian government's Indian affairs department, as saying.
"This is very important because there are some who doubt their existence."
He described the threats to such tribes and their land as "a monumental crime against the natural world" and "further testimony to the complete irrationality with which we, the 'civilised' ones, treat the world".
Disease is also a risk, as members of tribal groups that have been contacted in the past have died of illnesses that they have no defence against, ranging from chicken pox to the common cold.
A third red spot has appeared alongside the Great Red Spot and Red Spot Jr. in the turbulent Jovian atmosphere.
A potentially historic change is occurring on Jupiter. An upstart storm now rivals the gas giant's Big Red Spot as king of storms, astronomers announced last week.
The Little Red Spot, as it was named upon discovery in 2006, shows both size and speed in threatening to knock the former champion off its perch, with Junior's maximum winds reaching 384 mph (172 meters per second).
"In terms of maximum wind speed, the Little Red Spot as measured in 2007 and the Great Red Spot when last measured in 2000 are just about the same," said Andrew Cheng, physicist and lead study author at Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
Those winds far outstrip the 156 mph threshold that defines a Category 5 hurricane on Earth, and the Little Red Spot itself appears nearly as big as our whole planet.
Seeing spots
A third red spot on Jupiter was also announced last week by a different team, joining its larger super-storm cousins. The Great Red Spot has raged on for at least two centuries and perhaps as much as 350 years, ancient observations suggest.
Cheng's team used image maps made by the New Horizons spacecraft to gauge wind speed and direction.
The Hubble Space Telescope provided visible-light images of the storms, while the Very Large Telescope in Chile used mid-infrared to glimpse the thermal structure of the storms below the visible cloud tops.
The thermal heat images showed that the Little Red Spot may already match the Great Red Spot for size, although the latter still appears almost twice as large on the surface of Jupiter's atmosphere when examined in visible light.
"In the infrared, which sees deeper beneath those clouds, the Little Red Spot appears to be part of an interacting system that is actually larger than the Great Red Spot," Cheng told SPACE.com.
The Little Red Spot has steadily gained strength even as the Big Red Spot shrinks.
Both storms have winds that circulate in the opposite direction to that of a cyclone, or counterclockwise, and appear "strikingly similar," Cheng said.
Seeing red
Astronomers remain mystified by the angry red color of the storms. The Little Red Spot only changed color in late 2005 after it formed from earlier mergers of three smaller storms.
Similarly, the newest third red spot began as an oval white storm.
These latest findings support the theory that the most powerful storms dredge up material from below Jupiter's clouds and lift it into the upper atmosphere. That exposes the material to solar ultraviolet radiation and causes the color change to red.
The newcomer storm may end up merging with the Great Red Spot or getting pushed away when the two encounter each other in August, assuming their paths remain the same. The Little Red Spot lies at a lower latitude and will pass the Great Red Spot in June.
The idea that Jupiter is undergoing global climate change was proposed in 2004 by Phil Marcus, a mechanical engineer at the University of California, Berkeley.
He predicted large changes in the southern hemisphere starting around 2006 that would destabilize jet streams and spawn new storms.
Much of the activity in the gas giant's South Equatorial Belt has disappeared and left the Great Red Spot isolated, foreshadowing even greater changes to come.
"The Great Red Spot may not always be the largest and strongest storm on Jupiter," said Glenn Orton, planetary scientist and study coauthor at NASA's Jet Propulsion Laboratory in Pasadena
Use and abuse of substances such as cigarettes, alcohol, and illegal drugs may begin in childhood or the teen years. Certain risk factors may increase someone's likelihood to abuse substances.
Factors within a family that influence a child's early development have been shown to be related to increased risk of drug abuse.
Chaotic home environment
Ineffective parenting
Lack of nurturing and parental attachment
Factors related to a child's socialization outside the family may also increase risk of drug abuse.
Inappropriately aggressive or shy behavior in the classroom
In geometry, a torus (pl. tori) is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle, which does not touch the circle. Examples of tori include the surfaces of doughnuts and inner tubes. A circle rotated about a chord of the circle is called a torus in some contexts, but this is not a common usage in mathematics. The shape produced when a circle is rotated about a chord resembles a round cushion. Torus was the Latin word for a cushion of this shape.
Geometry
A torus can be defined parametrically by:
where
u, v are in the interval [0, 2π],
R is the distance from the center of the tube to the center of the torus,
These formulas are the same as for a cylinder of length 2πR and radius r, created by cutting the tube and unrolling it by straightening out the line running around the centre of the tube. The losses in surface area and volume on the inner side of the tube happen to exactly cancel out the gains on the outer side.
According to a broader definition, the generator of a torus need not be a circle but could also be an ellipse or any other conic section.
I learned this crap in (u,v) space, thanks to Dr. R.
Enzymes are proteinmolecules that manipulate other molecules — the enzymes' substrates. These target molecules bind to an enzyme's active site and are transformed into products through a series of steps known as the enzymatic mechanism. These mechanisms can be divided into single-substrate and multiple-substrate mechanisms. Kinetic studies on enzymes that only bind one substrate, such as triosephosphate isomerase, aim to measure the affinity with which the enzyme binds this substrate and the turnover rate.
When enzymes bind multiple substrates, such as dihydrofolate reductase (shown right), enzyme kinetics can also show the sequence in which these substrates bind and the sequence in which products are released. An example of enzymes that bind a single substrate and release multiple products are proteases, which cleave one protein substrate into two polypeptide products. Others join two substrates together, such as DNA polymerase linking a nucleotide to DNA. Although these mechanisms are often a complex series of steps, there is typically one rate-determining step that determines the overall kinetics. This rate-determining step may be a chemical reaction or a conformational change of the enzyme or substrates, such as those involved in the release of product(s) from the enzyme.
Knowledge of the enzyme's structure is helpful in interpreting the kinetic data. For example, the structure can suggest how substrates and products bind during catalysis; what changes occur during the reaction; and even the role of particular amino acid residues in the mechanism. Some enzymes change shape significantly during the mechanism; in such cases, it is helpful to determine the enzyme structure with and without bound substrate analogs that do not undergo the enzymatic reaction.
Not all biological catalysts are protein enzymes; RNA-based catalysts such as ribozymes and ribosomes are essential to many cellular functions, such as RNA splicing and translation. The main difference between ribozymes and enzymes is that the RNA catalysts perform a more limited set of reactions, although their reaction mechanisms and kinetics can be analysed and classified by the same methods.
General principles
The rate of reaction will increase as substrate concentration increases, eventually becoming saturated at very high concentrations of substrate.
The reaction catalysed by an enzyme uses exactly the same reactants and produces exactly the same products as the uncatalysed reaction. Like other catalysts, enzymes do not alter the position of equilibrium between substrates and products.[1] However, unlike normal chemical reactions, enzymes are saturable. This means as more substrate is added, the reaction rate will increase, because more active sites become occupied. This can continue until all the enzyme becomes saturated with substrate and the rate reaches a maximum. The two most important kinetic properties of an enzyme are how quickly the enzyme becomes saturated with a particular substrate, and the maximum rate it can achieve. Knowing these properties suggests what an enzyme might do in the environment of the cell and can show how the enzyme will respond to changes in these conditions.
Enzyme assays
Progress curve for an enzyme reaction. The slope in the initial rate period is the initial rate of reactionv. The Michaelis-Menten equation describe how this slope varies with the concentration of substrate.
Enzyme assays are laboratory procedures that measure the rate of enzyme reactions. Because enzymes are not consumed by the reactions they catalyse, enzyme assays usually follow changes in the concentration of either substrates or products to measure the rate of reaction. There are many methods of measurement. Spectrophotometric assays observe change in the absorbance of light between products and reactants; radiometric assays involve the incorporation or release of radioactivity to measure the amount of product made over time. Spectrophotometric assays are most convenient since they allow the rate of the reaction to be measured continuously. Although radiometric assays require the removal and counting of samples (i.e., they are discontinuous assays) they are usually extremely sensitive and can measure very low levels of enzyme activity.[2] An analogous approach is to use mass spectrometry to monitor the incorporation or release of stable isotopes as substrate is converted into product.
The most sensitive enzyme assays use lasers focused through a microscope to observe changes in single enzyme molecules as they catalyse their reactions. These measurements either use changes in the fluorescence of cofactors during an enzyme's reaction mechanism, or of fluorescent dyes added onto specific sites of the protein to report movements that occur during catalysis.[3] These studies are providing a new view of the kinetics and dynamics of single enzymes, as opposed to traditional enzyme kinetics, which observes the average behaviour of populations of millions of enzyme molecules.[4][5]
On the left is shown a typical progress curve for an enzyme assay. The enzyme produces product at a linear initial rate at the start of the reaction. Later in this progress curve, the rate slows down as substrate is used up or products accumulate. The length of the initial rate period depends on the assay conditions and can range from milliseconds to hours. Enzyme assays are usually set up to produce an initial rate lasting over a minute, to make measurements easier. However, equipment for rapidly mixing liquids allows fast kinetic measurements on initial rates of less than one second.[6] These very rapid assays are essential for measuring pre-steady-state kinetics, which are discussed below.
Most enzyme kinetics studies concentrate on this initial, linear part of enzyme reactions. However, it is also possible to measure the complete reaction curve and fit this data to a non-linear rate equation. This way of measuring enzyme reactions is called progress-curve analysis.[7] This approach is useful as an alternative to rapid kinetics when the initial rate is too fast to measure accurately.
Single-substrate reactions
Enzymes with single-substrate mechanisms include isomerases such as triosephosphateisomerase or bisphosphoglycerate mutase, intramolecular lyases such as adenylate cyclase and the hammerhead ribozyme, a RNA lyase.[8] However, some enzymes that only have a single substrate do not fall into this category of mechanisms. Catalase is an example of this, as the enzyme reacts with a first molecule of hydrogen peroxide substrate, becomes oxidised and is then reduced by a second molecule of substrate. Although a single substrate is involved, the existence of a modified enzyme intermediate means that the mechanism of catalase is actually a ping–pong mechanism, a type of mechanism that is discussed in the Multi-substrate reactions section below.
Michaelis–Menten kinetics
Saturation curve for an enzyme showing the relation between the concentration of substrate and rate.
Single-substrate mechanism for an enzyme reaction. k1, k-1 and k2 are the rate constants for the individual steps.
As enzyme-catalysed reactions are saturable, their rate of catalysis does not show a linear response to increasing substrate. If the initial rate of the reaction is measured over a range of substrate concentrations (denoted as [S]), the reaction rate (v) increases as [S] increases, as shown on the right. However, as [S] gets higher, the enzyme becomes saturated with substrate and the rate reaches Vmax, the enzyme's maximum rate.
The Michaelis-Menten kinetic model of a single-substrate reaction is shown on the right. There is an initial bimolecular reaction between the enzyme E and substrate S to form the enzyme–substrate complex ES. Although the enzymatic mechanism for the unimolecular reactionreaction can be quite complex, there is typically one rate-determining enzymatic step that allows the mechanism to be modelled as a single catalytic step of rate constant k2.
(Equation 1).
k2 is also called kcat or the turnover number, the maximum number of enzymatic reactions catalyzed per second.
At low concentrations of substrate [S], the enzyme exists in an equilibrium between both the free form E and the enzyme–substrate complex ES; increasing [S] likewise increases [ES] at the expense of [E], shifting the binding equilibrium to the right. Since the rate of the reaction depends on the concentration [ES], the rate is sensitive to small changes in [S]. However, at very high [S], the enzyme is entirely saturated with substrate, and exists only in the ES form. Under these conditions, the rate (v≈k2[E]tot=Vmax) is insensitive to small changes in [S]; here, [E]tot is the total enzyme concentration
which is approximately equal to the concentration [ES] under saturating conditions.
The Michaelis–Menten equation[9] describes how the reaction rate v depends on the position of the substrate-binding equilibrium and the rate constant k2. Michaelis and Menten showed when k2 is much less than k-1 (called the equilibrium assumption)[10] they could derive the following equation:
(Equation 2)
This Michaelis-Menten equation is the basis for most single-substrate enzyme kinetics.
The Michaelis constant Km is defined as the concentration at which the rate of the enzyme reaction is half Vmax. This may be verified by substituting [S] = Km into the Michaelis-Menten equation. If the rate-determining enzymatic step is slow compared to substrate dissociation (k2 << k-1), the Michaelis constant Km is roughly the dissociation constant of the ES complex, although this situation is relatively rare.
The more normal situation where k2 > k-1 is sometimes called Briggs-Haldane kinetics.[11] The Michaelis–Menten equation still holds under these more general conditions, as may be derived from the steady-state approximation.[10] During the initial-rate period, the reaction rate v is roughly constant, indicating that [ES] is similarly constant (cf. equation 1):
Therefore, the concentration [ES] is given by the formula
where the Michaelis constant Km is defined
([E] is the concentration of free enzyme). Taken together, the general formula for the reaction rate v is again the Michaelis-Menten equation:
The specificity constant kcat / Km is a measure of how efficiently an enzyme converts a substrate into product. Using the definition of the Michaelis constant Km, the Michaelis-Menten equation may be written in the form
where [E] is the concentration of free enzyme. Thus, the specificity constant is an effective second-order rate constant for free enzyme to react with free substrate to form product. The specificity constant is limited by the frequency with which the substrate and enzyme encounter each other in solution, as high as 1010M−1s−1.[12] Remarkably, this maximum rate is largely unaffected by the size of either the substrate or the enzyme.[13] The ratio of the specificity constants for two substrates is a quantitative comparison of how efficient the enzyme is in converting those substrates. The slope of the Michaelis-Menten graph at low substrate concentration [S] (when [S] << Km) also yields the specificity constant.
Lineweaver–Burk or double-reciprocal plot of kinetic data, showing the significance of the axis intercepts and gradient.
Using an interactive Michaelis–Menten kinetics tutorial at the University of Virginia,[α] the effects on the behaviour of an enzyme of varying kinetic constants can be explored.
The plot of v versus [S] above is not linear; although initially linear at low [S], it bends over to saturate at high [S]. Before the modern era of nonlinear curve-fitting on computers, this nonlinearity could make it difficult to estimate Km and Vmax accurately. Therefore, several researchers developed linearizations of the Michaelis-Menten equation, such as the Lineweaver-Burk plot, the Eadie-Hofstee diagram and the Hanes-Woolf plot. All of these linear representations can be useful for visualizing data, but none should be used to determine kinetic parameters, as computer software is readily available that allows for more accurate determination by nonlinear regression methods.[14]
The Lineweaver-Burk plot or double reciprocal plot is common way of illustrating kinetic data. This is produced by taking the reciprocal of both sides of the Michaelis–Menten equation. As shown on the right, this is a linear form of the Michaelis–Menten equation and produces a straight line with the equation y = mx + c with a y-intercept equivalent to 1/Vmax and an x-intercept of the graph representing -1/Km.
Naturally, no experimental values can be taken at negative 1/[S]; the lower limiting value 1/[S] = 0 (the y-intercept) corresponds to an infinite substrate concentration, where 1/v=1/Vmax as shown at the right; thus, the x-intercept is an extrapolation of the experimental data taken at positive concentrations. More generally, the Lineweaver-Burk plot skews the importance of measurements taken at low substrate concentrations and, thus, can yield inaccurate estimates of Vmax and Km.[15] A more accurate linear plotting method is the Eadie-Hofstee plot. In this case, v is plotted against v/[S]. In the third common linear representation, the Hanes-Woolf plot, [S]/v is plotted against [S]. In general, data normalisation can help diminish the amount of experimental work and can increase the reliability of the output, and is suitable for both graphical and numerical analysis.[16]
Practical significance of kinetic constants
The study of enzyme kinetics is important for two basic reasons. Firstly, it helps explain how enzymes work, and secondly, it helps predict how enzymes behave in living organisms. The kinetic constants defined above, Km and Vmax, are critical to attempts to understand how enzymes work together to control metabolism.
Making these predictions is not trivial, even for simple systems. For example, oxaloacetate is formed by malate dehydrogenase within the mitochondrion. Oxaloacetate can then be consumed by citrate synthase, phosphoenolpyruvate carboxykinase or aspartate aminotransferase, feeding into the citric acid cycle, gluconeogenesis or aspartic acid biosynthesis, respectively. Being able to predict how much oxaloacetate goes into which pathway requires knowledge of the concentration of oxaloacetate as well as the concentration and kinetics of each of these enzymes. This aim of predicting the behaviour of metabolic pathways reaches its most complex expression in the synthesis of huge amounts of kinetic and gene expression data into mathematical models of entire organisms. Although this goal is far in the future for any eukaryote, attempts are now being made to achieve this in bacteria, with models of Escherichia coli metabolism now being produced and tested.[17][18]
Mmm... Universe. Calculations show it really might be shaped like the snack favourite.
The doughnut is making a comeback – at least as a possible shape for our Universe.
The idea that the universe is finite and relatively small, rather than infinitely large, first became popular in 2003, when cosmologists noticed unexpected patterns in the cosmic microwave background (CMB) – the relic radiation left behind by the Big Bang.
The CMB is made up of hot and cold spots that represent ripples in the density of the infant Universe, like waves in the sea. An infinite Universe should contain waves of all sizes, but cosmologists were surprised to find that longer wavelengths were missing from measurements of the CMB made by NASA’s Wilkinson Microwave Anisotropy Probe.
“You can think of the Universe as a musical instrument - it cannot sustain vibrations that have a wavelength that is bigger than the length of the instrument itself,” explains Frank Steiner, a physicist at Ulm University in Germany.
Cosmologists have suggested various 'wrap-around' shapes for the Universe: it might be shaped like a football or even a weird 'doughnut'. In each case, the Universe would appear to be infinite, because you would never physically reach its edge - if you travelled far enough in any direction you would end up back where you started, just as if you were circumnavigating the globe.
But the notion soon suffered a setback. Cosmologists predicted that a wrap-around Universe would act like a hall of mirrors, with images from distant objects being repeated multiple times across the sky. Glenn Starkman at Case Western Reserve University in Cleveland, Ohio, and his colleagues searched for the predicted patterns, but found nothing.
Undeterred, Steiner and his colleagues have re-analysed the 2003 data from NASA's Wilkinson Microwave Anisotropy Probe, looking for different shapes, including the so-called '3-torus', also dubbed the 'doughnut universe'.
Despite its catchy nickname, this shape is tough to visualize, says Steiner. The 3-torus is an extension of the familiar doughnut shape and can be formed from a rectangular piece of paper. You can imagine gluing together first one set of opposite edges to make a cylinder, and then the second set of opposing edges to make a doughnut shape, explains Steiner.
The 3-torus is formed in a similar way, but you begin with a cube and glue together each of the opposite faces. So if you were to attempt to exit one of the cube's faces, you would immediately find yourself entering again through the opposite one.
Other shapes are possible
Steiner’s team used three separate techniques to compare predictions of how the temperature fluctuations in different areas of the sky should match up in both an infinite Universe and a doughnut one. In each case, the doughnut gave the best match to the Wilkinson Microwave Anisotropy Probe data. The team has even been able to pin point the probable size of the Universe, which would take around 56 billion light years to cross.
Jean-Pierre Luminet at the Paris Observatory in France, who proposed the football-shaped universe in 2003, likes Steiner's work. He agrees that the analysis shows that the doughnut is still a likely candidate, but adds that other shapes are also possible. “One must remember that the (football universe) is still alive and well,” says Luminet.
Starkman, however, is not convinced that Steiner’s team has done enough to win people over. “It could be true that the Universe is small,” he says, “but this doesn’t provide an answer one way or the other.”
Steiner believes that new and more precise measurements of the cosmic microwave background to be made by Europe's Planck satellite, which is due to be launched later this year, will help answer the question.
“Philosophically, I like the idea that the Universe is finite and one day we could fully explore it and find out everything about it,” Starkman says. “But since physics cannot be decided by philosophy, I hope it will be answered by Planck.”
Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of blueprints or a recipe, since it contains the instructions needed to construct other components of cells, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.
Chemically, DNA is a long polymer of simple units called nucleotides, with a backbone made of sugars and phosphate groups joined by ester bonds. Attached to each sugar is one of four types of molecules called bases. It is the sequence of these four bases along the backbone that encodes information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called transcription.
Within cells, DNA is organized into structures called chromosomes. These chromosomes are duplicated before cells divide, in a process called DNA replication. Eukaryotic organisms (animals, plants, and fungi) store their DNA inside the cell nucleus, while in prokaryotes (bacteria and archae) it is found in the cell's cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.
Physical and chemical properties
The chemical structure of DNA. Hydrogen bonds are shown as dotted lines.
DNA is a long polymer made from repeating units called nucleotides. The DNA chain is 22 to 26 Ångströms wide (2.2 to 2.6 nanometres), and one nucleotide unit is 3.3 Å (0.33 nm) long. Although each individual repeating unit is very small, DNA polymers can be enormous molecules containing millions of nucleotides. For instance, the largest human chromosome, chromosome number 1, is approximately 220 million base pairs long.
In living organisms, DNA does not usually exist as a single molecule, but instead as a tightly-associated pair of molecules. These two long strands entwine like vines, in the shape of a double helix. The nucleotide repeats contain both the segment of the backbone of the molecule, which holds the chain together, and a base, which interacts with the other DNA strand in the helix. In general, a base linked to a sugar is called a nucleoside and a base linked to a sugar and one or more phosphate groups is called a nucleotide. If multiple nucleotides are linked together, as in DNA, this polymer is called a polynucleotide.
The backbone of the DNA strand is made from alternating phosphate and sugar residues. The sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. These asymmetric bonds mean a strand of DNA has a direction. In a double helix the direction of the nucleotides in one strand is opposite to their direction in the other strand. This arrangement of DNA strands is called antiparallel. The asymmetric ends of DNA strands are referred to as the 5′ (five prime) and 3′ (three prime) ends, with the 5' end being that with a terminal phosphate group and the 3' end that with a terminal hydroxyl group. One of the major differences between DNA and RNA is the sugar, with 2-deoxyribose being replaced by the alternative pentose sugar ribose in RNA.
The DNA double helix is stabilized by hydrogen bonds between the bases attached to the two strands. The four bases found in DNA are adenine (abbreviated A), cytosine (C), guanine (G) and thymine (T). These four bases are attached to the sugar/phosphate to form the complete nucleotide, as shown for adenosine monophosphate.
These bases are classified into two types; adenine and guanine are fused five- and six-membered heterocyclic compounds called purines, while cytosine and thymine are six-membered rings called pyrimidines. A fifth pyrimidine base, called uracil (U), usually takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring. Uracil is not usually found in DNA, occurring only as a breakdown product of cytosine.
Major and minor grooves
Animation of the structure of a section of DNA. The bases lie horizontally between the two spiraling strands. Large version[9]
The double helix is a right-handed spiral. As the DNA strands wind around each other, they leave gaps between each set of phosphate backbones, revealing the sides of the bases inside (see animation). There are two of these grooves twisting around the surface of the double helix: one groove, the major groove, is 22 Å wide and the other, the minor groove, is 12 Å wide.The narrowness of the minor groove means that the edges of th
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