what is DNA?

Answer 1

Whoa! Well, I couldn't even begin to explain as some of them already have....but....I can tell you that in our adoption of our son we had to have DNA tests done on him and his biological mother before anything could be completed. The test came back 99.99% a match. The company LabCorp does many DNA tests and if you need more info on them and DNA testing you can put "LabCorp" in the search engine and it will give you several sites.

Answer 2

1

Answer 3

Deoxyribonucleic acid (DNA) is a nucleic acid — usually in the form of a double helix — that contains the genetic instructions specifying the biological development of all cellular forms of life, and many viruses. DNA is a long polymer of nucleotides (a polynucleotide) and encodes the sequence of the amino acid residues in proteins using the genetic code, a triplet code of nucleotides. DNA is thought to date back to between approximately 3.5 to 4.6 billion years ago.

Answer 4

http://www.indigo.com/models/gph-dna-models/index.html

A lot has been identified since I studied it in early 70's.Always was my favorite thing to discuss in Biology.Thanks Mr Booth.

Answer 5

2

Answer 6

Don't Never Ask

Answer 7

http://dictionary.reference.com/browse/DNA
there you go

Answer 8

DNA
The general structure of a section of DNA
Deoxyribonucleic acid (DNA) is a nucleic acid — usually in the form of a double helix — that contains the genetic instructions specifying the biological development of all cellular forms of life, and many viruses. DNA is a long polymer of nucleotides (a polynucleotide) and encodes the sequence of the amino acid residues in proteins using the genetic code, a triplet code of nucleotides. DNA is thought to date back to between approximately 3.5 to 4.6 billion years ago.[1]
In complex eukaryotic cells such as those from plants, animals, fungi and protists, most of the DNA is located in the cell nucleus. By contrast, in simpler cells called prokaryotes, including the eubacteria and archaea, DNA is not separated from the cytoplasm by a nuclear envelope. The cellular organelles known as chloroplasts and mitochondria also carry DNA.
DNA is often referred to as the molecule of heredity as it is responsible for the genetic propagation of most inherited traits. In humans, these traits can range from hair color to disease susceptibility. During cell division, DNA is replicated and can be transmitted to offspring during reproduction. Lineage studies can be done based on the facts that the mitochondrial DNA only comes from the mother, and the male Y chromosome only comes from the father.
Every person's DNA, their genome, is inherited from both parents. The mother's mitochondrial DNA together with twenty-three chromosomes from each parent combine to form the genome of a zygote, the fertilized egg. As a result, with certain exceptions such as red blood cells, most human cells contain 23 pairs of chromosomes, together with mitochondrial DNA inherited from the mother.
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Space-filling model of a section of DNA molecule


DNA base pairing
Contrary to a common misconception, DNA is not a single molecule, but rather a pair of molecules joined by hydrogen bonds: it is organized as two complementary strands, head-to-toe, with the hydrogen bonds between them.[2] Each strand of DNA is a chain of chemical "building blocks", called nucleotides, of which there are four types: adenine (abbreviated A), cytosine (C), guanine (G) and thymine (T).[2] (Thymine should not be confused with thiamine, which is vitamin B1.) In some organisms, most notably the PBS1 phage, Uracil (U) replaces T in the organism's RNA.[3] These allowable base components of nucleic acids can be polymerized in any order giving the molecules a high degree of uniqueness.
Between the two strands, each base can only "pair up" with one single predetermined other base: A+T, T+A, C+G and G+C are the only possible combinations; that is, an "A" on one strand of double-stranded DNA will "mate" properly only with a "T" on the other, complementary strand; therefore, naming the bases on the conventionally chosen side of the strand is enough to describe the entire double-strand sequence.[2] Two nucleotides paired together are called a base pair. On rare occasions, wrong pairing can happen, when thymine goes into its enol form or cytosine goes into its imino form. The double-stranded structure of DNA provides a simple mechanism for DNA replication: the DNA double strand is first "unzipped" down the middle, and the "other half" of each new single strand is recreated by exposing each half to a mixture of the four bases. An enzyme makes a new strand by finding the correct base in the mixture and pairing it with the original strand. In this way, the base on the old strand dictates which base will be on the new strand, and the cell ends up with an extra copy of its DNA.
DNA contains the genetic information that is inherited by the offspring of an organism; this information is determined by the sequence of base pairs along its length. A strand of DNA contains genes, areas that regulate genes, and areas that either have no function, or a function yet unknown. Genes can be loosely viewed as the organism's "cookbook" or "blueprint".


DNA Under an electron microscope
Other interesting points:
•DNA is an acid because of the phosphate groups between each deoxyribose. This is the primary reason why DNA has a negative charge.
•The "polarity" of each pair is important: A+T is not the same as T+A, just as C+G is not the same as G+C (note that "polarity" as such is never used in this context -- it's just a suggestive way to get the idea across).
•Mutations are the results of the cells' attempts to repair chemical imperfections in this process, where a base is accidentally skipped, inserted, or incorrectly copied, or the chain is trimmed, or added to. Many mutations can be described as combinations of these accidental "operations". Mutations can also occur after chemical damage (through mutagens), light (UV damage), or through other more complicated gene swapping events.
•DNA molecules that act as enzymes are known in laboratories, but none have been known to be found in life so far.
•In addition to the traditionally viewed duplex form of DNA, DNA can also acquire triplex and quadruplex forms. Here instead of the Watson-Crick base pairing, Hoogsteen base pairing comes into the picture.
•DNA differs from ribonucleic acid (RNA) by having a sugar 2-deoxyribose instead of ribose in its backbone. This is the basic chemical distinction between RNA and DNA. In addition, in most[citation needed] RNA, the nucleotides thymine (T) are replaced by uracil (U).

Comparisons between DNA and single stranded RNA with the diagram of the bases showing.
Although sometimes called "the molecule of heredity", DNA macromolecules as people typically think of them are not single molecules. Rather, they are pairs of molecules, which entwine like vines to form a double helix (see the illustration at the right).
Each vine-like molecule is a strand of DNA: a chemically linked chain of nucleotides, each of which consists of a sugar (deoxyribose), a phosphate and one of five kinds of nucleobases ("bases"). Because DNA strands are composed of these nucleotide subunits, they are polymers.
The diversity of the bases means that there are five kinds of nucleotides, which are commonly referred to by the identity of their bases. These are adenine (A), thymine (T), uracil (U), cytosine (C), and guanine (G). U is rarely found in DNA except as a result of chemical degradation of C, but in some viruses, notably PBS1 phage DNA, U completely replaces the usual T in its DNA. Similarly, RNA usually contains U in place of T, but in certain RNAs such as transfer RNA, T is always found in some positions. Thus, the only true difference between DNA and RNA is the sugar, 2-deoxyribose in DNA and ribose in RNA.
In a DNA double helix, two polynucleotide strands can associate through the hydrophobic effect and pi stacking. Specificity of which strands stay associated is determined by complementary pairing. Each base forms hydrogen bonds readily to only one other, A to T forming two hydrogen bonds and C to G forming three hydrogen bonds. The GC content and length of each DNA molcule dictates the strength of the association; the more complementary bases exist, the stronger and longer-lasting the association characterised by the temperate required to break the hydrogen bonding, its Tm value.
The cell's machinery is capable of melting or disassociating a DNA double helix, and using each DNA strand as a template for synthesizing a new strand which is nearly identical to the previous strand. Errors that occur in the synthesis are known as mutations. The process known as PCR (polymerase chain reaction) mimics this process in vitro in a nonliving system.
Because pairing causes the nucleotide bases to face the helical axis, the sugar and phosphate groups of the nucleotides run along the outside; the two chains they form are sometimes called the "backbones" of the helix. In fact, it is chemical bonds between the phosphates and the sugars that link one nucleotide to the next in the DNA strand.
•Rotating DNA stick model (file info)
oAnimation of a section of DNA rotating. (1.00 MB, animated GIF format).
•Problems seeing the videos? See media help.
Nucleotide sequence
Within a gene, the sequence of nucleotides along a DNA strand defines a messenger RNA sequence which then defines a protein, that an organism is liable to manufacture or "express" at one or several points in its life using the information of the sequence. The relationship between the nucleotide sequence and the amino-acid sequence of the protein is determined by simple cellular rules of translation, known collectively as the genetic code. The genetic code consists of three-letter 'words' (termed a codon) formed from a sequence of three nucleotides (e.g. ACT, CAG, TTT). These codons can then be translated with messenger RNA and then transfer RNA, with a codon corresponding to a particular amino acid. There are 64 possible codons (4 bases in 3 places 43) that encode 20 amino acids. Most amino acids, therefore, have more than one possible codon. There are also three 'stop' or 'nonsense' codons signifying the end of the coding region, namely the UAA, UGA and UAG codons.
In many species, only a small fraction of the total sequence of the genome appears to encode protein. For example, only about 1.5% of the human genome consists of protein-coding exons. The function of the rest is a matter of speculation. It is known that certain nucleotide sequences specify affinity for DNA binding proteins, which play a wide variety of vital roles, in particular through control of replication and transcription. These sequences are frequently called regulatory sequences, and researchers assume that so far they have identified only a tiny fraction of the total that exist. "Junk DNA" represents sequences that do not yet appear to contain genes or to have a function. The reasons for the presence of so much non-coding DNA in eukaryotic genomes and the extraordinary differences in genome size ("C-value") among species represent a long-standing puzzle in DNA research known as the "C-value enigma".
Some DNA sequences play structural roles in chromosomes. Telomeres and centromeres typically contain few (if any) protein-coding genes, but are important for the function and stability of chromosomes. Some genes code for "RNA genes" (see tRNA and rRNA). Some RNA genes code for transcripts that function as regulatory RNAs (see siRNA) that influence the function of other RNA molecules. The intron-exon structure of some genes (such as immunoglobin and protocadeherin genes) is important for allowing alternative splicing of pre-mRNA which allows several different proteins to be made from the same gene. Some non-coding DNA represents pseudogenes, which have been hypothesized to serve as raw genetic material for the creation of new genes through the process of gene duplication and divergence. Some non-coding DNA provided hot-spots for duplication of short DNA regions; such sequence duplication has been the major form of genetic change in the human lineage (see evidence from the Chimpanzee Genome Project). Exons interspersed with introns allows for "exon shuffling" and the creation of modified genes that might have new adaptive functions. Large amounts of non-coding DNA is probably adaptive in that it provides chromosomal regions where recombination between homologous portions of chromosomes can take place without disrupting the function of genes. Some biologists such as Stuart Kauffman have speculated that non-coding DNA may modify the rate of evolution of a species.[citation needed]
Sequence also determines a DNA segment's susceptibility to cleavage by restriction enzymes, the quintessential tools of genetic engineering. The position of cleavage sites throughout an individual's genome determines one kind of an individual's "DNA fingerprint".
DNA replication
DNA replication or DNA synthesis is the process of copying the double-stranded DNA prior to cell division. The two resulting double strands are generally almost perfectly identical, but occasionally errors in replication or exposure to chemicals, or radiation can result in a less than perfect copy (see mutation), and each of them consists of one original and one newly synthesized strand. This is called semiconservative replication. The process of replication consists of three steps: initiation, elongation and termination.
Mechanical biological properties
Strands association and dissociation
The hydrogen bonds between the strands of the double helix are weak enough that they can be easily separated by enzymes. Enzymes known as helicases unwind the strands to facilitate the advance of sequence-reading enzymes such as DNA polymerase. The unwinding requires that helicases chemically cleave the phosphate backbone of one of the strands so that it can swivel around the other. The strands can also be separated by gentle heating, as used in PCR, provided they have fewer than about 10,000 base pairs (10 kilobase pairs, or 10 kbp). The intertwining of the DNA strands makes long segments difficult to separate.
Circular DNA
When the ends of a piece of double-helical DNA are joined so that it forms a circle, as in plasmid DNA, the strands are topologically knotted. This means they cannot be separated by gentle heating or by any process that does not involve breaking a strand. The task of unknotting topologically linked strands of DNA falls to enzymes known as topoisomerases. Some of these enzymes unknot circular DNA by cleaving two strands so that another double-stranded segment can pass through. Unknotting is required for the replication of circular DNA as well as for various types of recombination in linear DNA.
Great length versus tiny breadth
The narrow breadth of the double helix makes it impossible to detect by conventional electron microscopy, except by heavy staining. At the same time, the DNA found in many cells can be macroscopic in length -- approximately 2 meters long for strands in a human chromosome.[4] Consequently, cells must compact or "package" DNA to carry it within them. This is one of the functions of the chromosomes, which contain spool-like proteins known as histones, around which DNA winds.
Entropic stretching behavior
When DNA is in solution, it undergoes conformational fluctuations due to the energy available in the thermal bath. For entropic reasons, floppy states are more thermally accessible than stretched out states; for this reason, a single molecule of DNA stretches similarly to a rubber band. Using optical tweezers, the entropic stretching behavior of DNA has been studied and analyzed from a polymer physics perspective, and it has been found that DNA behaves like the Kratky-Porod worm-like chain model with a persistence length of about 53 nm.
Furthermore, DNA undergoes a stretching phase transition at a force of 65 pN; above this force, DNA is thought to take the form that Linus Pauling originally hypothesized, with the phosphates in the middle and bases splayed outward. This proposed structure for overstretched DNA has been called "P-form DNA," in honor of Pauling.
Different helix geometries
The DNA helix can assume one of three slightly different geometries, of which the "B" form described by James D. Watson and Francis Crick is believed to predominate in cells. It is 2 nanometres wide and extends 3.4 nanometres per 10 bp of sequence. This is also the approximate length of sequence in which the double helix makes one complete turn about its axis. This frequency of twist (known as the helical pitch) depends largely on stacking forces that each base exerts on its neighbors in the chain.
Supercoiled DNA
Main article: Supercoil
The B form of the DNA helix twists 360° per 10 bp in the absence of strain. But many molecular biological processes can induce strain. A DNA segment with excess or insufficient helical twisting is referred to, respectively, as positively or negatively "supercoiled". DNA in vivo is typically negatively supercoiled, which facilitates the unwinding of the double-helix required for RNA transcription.
Sugar pucker
There are four conformations that the ribofuranose rings in nucleotides can acquire:
1.C-2' endo
2.C-2' exo
3.C-3' endo
4.C-3' exo
Ribose is usually in C-3'endo, while deoxyribose is usually in the C-2' endo sugar pucker conformation. The A and B forms differ mainly in their sugar pucker. In the A form, the C3' configuration is above the sugar ring, whilst the C2' configuration is below it. Thus, the A form is described as "C3'-endo." Likewise, in the B form, the C2' configuration is above the sugar ring, whilst C3' is below; this is called "C2'-endo." Altered sugar puckering in A-DNA results in shortening the distance between adjacent phosphates by around one angstrom. This gives 11 to 12 base pairs to each helix in the DNA strand, instead of 10.5 in B-DNA. Sugar pucker gives uniform ribbon shape to DNA, a cylindrical open core, and also a deep major groove more narrow and pronounced that grooves found in B-DNA.
A and Z helices formation
The two other known double-helical forms of DNA, called A and Z, differ modestly in their geometry and dimensions. The A form appears likely to occur only in dehydrated samples of DNA, such as those used in crystallographic experiments, and possibly in hybrid pairings of DNA and RNA strands. Segments of DNA that cells have methylated for regulatory purposes may adopt the Z geometry, in which the strands turn about the helical axis like a mirror image of the B form.
Properties of different helical forms
Geometry attributeA-formB-formZ-form
Helix senseright-handedright-handedleft-handed
Repeating unit1 bp1 bp2 bp
Rotation/bp33.6°35.9°60°/2
Mean bp/turn10.710.412
Inclination of bp to axis+19°-1.2°-9°
Rise/bp along axis0.23 nm0.332 nm0.38 nm
Pitch/turn of helix2.46 nm3.32 nm4.56 nm
Mean propeller twist+18°+16°0°
Glycosyl angleantiantiC: anti,
G: syn
Sugar puckerC3'-endoC2'-endoC: C2'-endo,
G: C2'-exo
Diameter2.55 nm2.37 nm1.84 nm
Non-helical forms
There is an argument to be made that the native, intracellular form of DNA is not the B-form double helix, as commonly supposed. Rather, this argument proposes, the strands of DNA remain almost entirely topologically unlinked in their normal states. Watson-Crick base pairing is retained to link the two anti-parallel helical strands together but the two helices do not wind around each other with a net topological linkage when they are in a relaxed topological state.
The double helix, of course, in the B form, has one topological link every 10 base pairs, which is why a credible mode of unwinding the double helix held in a tightly packed three-dimensional ball inside a eukaryotic cell nucleus has remained a topic of continuing interest since the model was proposed in 1953.
Information on this alternative theory is available from this online book, presented in PDF format:
http://www.notahelix.com/delmonte/new_struct_mol_biol.pdf
and a recent research paper summarises some key experimental data which are better explained by SBS models than by the double helix:
http://www.ias.ac.in/currsci/dec102003/1564.pdf
with subsequent correspondence:
http://www.ias.ac.in/currsci/may252004/1352.pdf
However, these theories have been seen by some as having problems of their own, such as explaining the near-perfect symmetry of DNA in cells and the activity of DNA repair in the absence of a base-paired strand for comparison.
It is important to note that the Watson-Crick base pairing scheme is retained in side-by-side models of DNA structure, nevertheless, so the high symmetry of complementary base pairs in replication, for example, is retained. The key feature of the paranemic side-by-side model of duplex DNA is that it demonstrates how DNA operator sequences, antibody sensitivities, and target DNA sequences in general, are made rapidly and easily accessible, whereas the double helix model requires great lengths of duplex DNA to be partially unwound before such target sequences can be found since they are largely hidden and buried with the double helix model.
Additionally, the activity of topoisomerases would be entirely redundant, and not nearly as important to cellular function as it patently is, if not for the fact that base-paired double-strands are at least the primary form of cellular DNA.
The counterview, based upon the side-by-side models, however, is not that the functions of topoisomerases would be redundant. Even with side-by-side models there is still the phenomenon of supercoiling which is seen to be a valid topological constraint in cellular biochemical processes and enzymes such as the topoisomerases are still needed to separate DNA strands held together by supercoils.
The most demanding phenomenon for the double helix model to explain was reported inside solid fibers as early as 1973. Leslie et al. (1) reported that a fibre of poly(dI).poly(dC), in the absence of unwinding proteins, which gave sharp X-ray diffraction spots shortly after being drawn, was unstable and transformed irreversibly after a few days into poly(dC).poly(dI).poly(dC+). This new, three-stranded molecule also gave sharp spots in its own, different X-ray diffraction pattern which did not have any of the original spots.
Therefore the conversion was complete or substantially so and the product was both highly ordered and crystalline. Very similar results had been reported for fibres of poly(dA).poly(dT) converting to poly(dT).poly(dA).poly(dT) (2) and for the formation of poly(U).poly(A).poly(U) from poly(U).poly(A) (3).
The problem is evident: How and where would the torque, a vector, arise, and be sustained, inside a solid fibre that would permit one double helix to rotate against the friction of its neighbours so as to unwind one strand of an adjacent duplex and rewind it onto a triple helix having a different diameter and rotating at a different angular velocity, the conversion clearly being complete or largely so ?
Inside the fibre, individual molecules would have a random axial translation so only very rarely would two adjacent double helices have the same axial starting point down the fibre. They would be unlikely to be perfectly straight inside the fibre but would probably be at least partly entwined around neighbours, or at least twisted away from a true linear disposition. Moreover, any particular duplex would be equidistant from several neighbours, and, along its length, it could start to form a triplex by unwinding different neighbours.
A true, paranemic, antiparallel, side-by-side duplex DNA structure could form a side-by-side triplex, without any unwinding, by the linear transfer of a strand across to a neighbouring duplex.
1 Leslie, A. G. W., Arnott, S., Chandrasekaran, R. & Ratliff, R. L., J. Mol. Biol. 143, 49 - 72 (1980)
2 Arnott, S. & E. Selsing, E., J. Mol. Biol. 88, 509-521 (1974)
3 Arnott, S. & Bond, P. J., Nature New Biology 244, 99 -101 (1973)
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Strand direction
The asymmetric shape and linkage of nucleotides means that a DNA strand always has a discernible orientation or directionality. Because of this directionality, close inspection of a double helix reveals that nucleotides are heading one way along one strand (the "ascending strand"), and the other way along the other strand (the "descending strand"). This arrangement of the strands is called antiparallel.
Chemical nomenclature (5' and 3')
For reasons of chemical nomenclature, people who work with DNA refer to the asymmetric ends of ("five prime" and "three prime"). Within a cell, the enzymes that perform replication and transcription read DNA in the "3' to 5' direction", while the enzymes that perform translation read in the opposite directions (on RNA). However, because chemically produced DNA is synthesized and manipulated in the opposite or in non-directional manners, the orientation should not be assumed. In a vertically oriented double helix, the 3' strand is said to be ascending while the 5' strand is said to be descending.
Sense and antisense
Main article: Sense (molecular biology)
As a result of their antiparallel arrangement and the sequence-reading preferences of enzymes, even if both strands carried identical instead of complementary sequences, cells could properly translate only one of them. The other strand a cell can only read backwards. Molecular biologists call a sequence "sense" if it is translated or translatable, and they call its complement "antisense". It follows then, somewhat paradoxically, that the template for transcription is the antisense strand. The resulting transcript is an RNA replica of the sense strand and is itself sense.
A small proportion of genes in prokaryotes, and more in plasmids and viruses, blur the distinction made above between sense and antisense strands. Certain sequences of their genomes do double duty, encoding one protein when read 5' to 3' along one strand, and a second protein when read in the opposite direction (still 5' to 3') along the other strand. As a result, the genomes of these viruses are unusually compact for the number of genes they contain, which biologists view as an adaptation. This merely confirms that there is no biological distinction between the two strands of the double helix. Typically each strand of a DNA double helix will act as sense and antisense in different regions.
As viewed by topologists
Topologists like to note that the juxtaposition of the 3′ end of one DNA strand beside the 5′ end of the other at both ends of a double-helical segment makes the arrangement a "crab canon".
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Single-stranded DNA (ssDNA) and repair of mutations
In some viruses DNA appears in a non-helical, single-stranded form. Because many of the DNA repair mechanisms of cells work only on paired bases, viruses that carry single-stranded DNA genomes mutate more frequently than they would otherwise. As a result, such species may adapt more rapidly to avoid extinction. The result would not be so favorable in more complicated and more slowly replicating organisms, however, which may explain why only viruses carry single-stranded DNA. These viruses presumably also benefit from the lower cost of replicating one strand versus two.
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History of DNA research


James Watson in the Cavendish Laboratory at the University of Cambridge
The discovery that DNA was the carrier of genetic information was a process that required many earlier discoveries. The existence of DNA was discovered in the mid 19th century. However, it was only in the early 20th century that researchers began suggesting that it might store genetic information. This gained almost universal acceptance after the structure of DNA was elucidated by James D. Watson and Francis Crick in their 1953 Nature publication. Watson and Crick proposed the central dogma of molecular biology in 1957, describing the process whereby proteins are produced from nucleic DNA. In 1962 Watson, Crick, and Maurice Wilkins jointly received the Nobel Prize for their determination of the structure of DNA.
First isolation of DNA
Working in the 19th century, biochemists initially isolated DNA and RNA (mixed together) from cell nuclei. They were relatively quick to appreciate the polymeric nature of their "nucleic acid" isolates, but realized only later that nucleotides were of two types--one containing ribose and the other deoxyribose. It was this subsequent discovery that led to the identification and naming of DNA as a substance distinct from RNA.
Friedrich Miescher (1844-1895) discovered a substance he called "nuclein" in 1869. Somewhat later, he isolated a pure sample of the material now known as DNA from the sperm of salmon, and in 1889 his pupil, Richard Altmann, named it "nucleic acid". This substance was found to exist only in the chromosomes.
In 1929 Phoebus Levene at the Rockefeller Institute identified the components (the four bases, the sugar and the phosphate chain) and he showed that the components of DNA were linked in the order phosphate-sugar-base. He called each of these units a nucleotide and suggested the DNA molecule consisted of a string of nucleotide units linked together through the phosphate groups, which are the 'backbone' of the molecule. However Levene thought the chain was short and that the bases repeated in the same fixed order. Torbjorn Caspersson and Einar Hammersten showed that DNA was a polymer.
Chromosomes and inherited traits
Max Delbrück, Nikolai V. Timofeeff-Ressovsky, and Karl G. Zimmer published results in 1935 suggesting that chromosomes are very large molecules the structure of which can be changed by treatment with X-rays, and that by so changing their structure it was possible to change the heritable characteristics governed by those chromosomes. In 1937 William Astbury produced the first X-ray diffraction patterns from DNA. He was not able to propose the correct structure but the patterns showed that DNA had a regular structure and therefore it might be possible to deduce what this structure was.
In 1943, Oswald Theodore Avery and a team of scientists discovered that traits proper to the "smooth" form of the Pneumococcus could be transferred to the "rough" form of the same bacteria merely by making the killed "smooth" (S) form available to the live "rough" (R) form. Quite unexpectedly, the living R Pneumococcus bacteria were transformed into a new strain of the S form, and the transferred S characteristics turned out to be heritable. Avery called the medium of transfer of traits the transforming principle; he identified DNA as the transforming principle, and not protein as previously thought. He essentially redid Fredrick Griffith's experiment. In 1953, Alfred Hershey and Martha Chase did an experiment (Hershey-Chase experiment) that showed, in T2 phage, that DNA is the genetic material (Hershey shared the Nobel prize with Luria).


Francis Crick's first sketch of the deoxyribonucleic acid double-helix pattern
In 1944, the renowned physicist, Erwin Schrödinger, published a brief book entitled What is Life?, where he maintained that chromosomes contained what he called the "hereditary code-script" of life. He added: "But the term code-script is, of course, too narrow. The chromosome structures are at the same time instrumental in bringing about the development they foreshadow. They are law-code and executive power -- or, to use another simile, they are architect's plan and builder's craft -- in one." He conceived of these dual functional elements as being woven into the molecular structure of chromosomes. By understanding the exact molecular structure of the chromosomes one could hope to understand both the "architect's plan" and also how that plan was carried out through the "builder's craft." Three groups took up Schrödinger's challenge to work out the structure of the chromosomes and the question of how the segments of the chromosomes that were conceived to relate to specific traits could possibly do their jobs.
Just how the presence of specific features in the molecular structure of chromosomes could produce traits and behaviors in living organisms was unimaginable at the time. Because chemical dissection of DNA samples always yielded the same four nucleotides, the chemical composition of DNA appeared simple, perhaps even uniform. Organisms, on the other hand, are fantastically complex individually and widely diverse collectively. Geneticists did not speak of genes as conveyors of "information" in such words, but if they had, they would not have hesitated to quantify the amount of information that genes need to convey as vast. The idea that information might reside in a chemical in the same way that it exists in text--as a finite alphabet of letters arranged in a sequence of unlimited length--had not yet been conceived. It would emerge upon the discovery of DNA's structure, but few researchers imagined that DNA's structure had much to say about genetics.
Discovery of the structure of DNA
In the 1950s, three groups made it their goal to determine the structure of DNA. The first group to start was at King's College London and was led by Maurice Wilkins and was later joined by Rosalind Franklin. Another group consisting of Francis Crick and James D. Watson was at Cambridge. A third group was at Caltech and was led by Linus Pauling. Crick and Watson built physical models using metal rods and balls, in which they incorporated the known chemical structures of the nucleotides, as well as the known position of the linkages joining one nucleotide to the next along the polymer. At King's College Maurice Wilkins and Rosalind Franklin examined X-ray diffraction patterns of DNA fibers. Of the three groups, only the London group was able to produce good quality diffraction patterns and thus produce sufficient quantitative data about the structure.


Helix structure
In 1948 Pauling discovered that many proteins included helical (see alpha helix) shapes. Pauling had deduced this structure from X-ray patterns and from attempts to physically model the structures. (Pauling was also later to suggest an incorrect three chain helical structure based on Astbury's data.) Even in the initial diffraction data from DNA by Maurice Wilkins, it was evident that the structure involved helices. But this insight was only a beginning. There remained the questions of how many strands came together, whether this number was the same for every helix, whether the bases pointed toward the helical axis or away, and ultimately what were the explicit angles and coordinates of all the bonds and atoms. Such questions motivated the modeling efforts of Watson and Crick.
Complementary nucleotides
In their modeling, Watson and Crick restricted themselves to what they saw as chemically and biologically reasonable. Still, the breadth of possibilities was very wide. A breakthrough occurred in 1952, when Erwin Chargaff visited Cambridge and inspired Crick with a description of experiments Chargaff had published in 1947. Chargaff had observed that the proportions of the four nucleotides vary between one DNA sample and the next, but that for particular pairs of nucleotides — adenine and thymine, guanine and cytosine — the two nucleotides are always present in equal proportions.
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Watson and Crick's model


Crick and Watson DNA model built in 1953, was reconstructed largely from its original pieces in 1973 and donated to the National Science Museum in London.
Watson and Crick had begun to contemplate double helical arrangements, but they lacked information about the amount of twist (pitch) and the distance between the two strands. Rosalind Franklin had to disclose some of her findings for the Medical Research Council and Crick saw this material through Max Perutz's links to the MRC. Franklin's work confirmed that the phosphate "backbone" was on the outside of the molecule and also gave an insight into its symmetry, in particular that the two helical strands ran in opposite directions.
Watson and Crick were again greatly assisted by more of Franklin's data. This is controversial because Franklin's critical X-ray pattern was shown to Watson and Crick without Franklin's knowledge or permission. Wilkins showed the famous Photo 51 of the much simpler B type of DNA to Watson at his lab immediately after Watson had been unsuccessful in asking Franklin to collaborate to beat Pauling in finding the structure.
From the data in photograph 51 Watson and Crick were able to discern that not only was the distance between the two strands constant, but also to measure its exact value of 2 nanometres. The same photograph also gave them the 3.4 nanometre-per-10 bp "pitch" of the helix.
The final insight came when Crick and Watson saw that a complementary pairing of the bases could provide an explanation for Chargaff's puzzling finding. However the structure of the bases had been incorrectly guessed in the textbooks as the enol tautomer when they were more likely to be in the keto form. When Jerry Donohue pointed this fallacy out to Watson, Watson quickly realised that the pairs of adenine and thymine, and guanine and cytosine were almost identical in shape and so would provide equally sized 'rungs' between the two strands. Watson and Crick worked to develop a physical model of the double-helical structure out of wire which they used to confirm that the distances between the molecules were permissable. With the base-pairing, the Watson and Crick quickly converged upon a model, which they announced before Franklin herself had published any of her work.
Franklin was herself two steps away from the solution. She had not guessed the base-pairing and had not appreciated the implications of the symmetry that she had described. However she had been working almost alone and did not have regular contact with a partner like Crick and Watson, and with other experts such as Jerry Donohue. Her notebooks show that she was aware both of Jerry Donohue's work concerning tautomeric forms of bases (she had used the keto forms for three of the bases) and of Chargaff's work.
The disclosure of Franklin's data to Watson has angered some people who believe Franklin did not receive due credit at the time and that she might have discovered the structure on her own before Crick and Watson. In Crick and Watson's famous paper in Nature in 1953, they said that their work had been stimulated by the work of Wilkins and Franklin, whereas it had been the basis of their work. However they had agreed with Wilkins and Franklin that they all should publish papers in the same issue of Nature in support of the proposed structure. Additionally, in his autobiography, The Double Helix, Watson describes Franklin in very unflattering terms (commenting derisively on her lack of "feminine" traits) and all but implies that her work actually impaired that of Wilkins.
"Central Dogma"
Watson and Crick's model attracted great interest immediately upon its presentation. Arriving at their conclusion on February 21, 1953, Watson and Crick made their first announcement on February 28. Their paper, A Structure for Deoxyribose Nucleic Acid,[5] was published on April 25. In an influential presentation in 1957, Crick laid out the "Central Dogma", which foretold the relationship between DNA, RNA, and proteins, and articulated the "sequence hypothesis." A critical confirmation of the replication mechanism that was implied by the double-helical structure followed in 1958 in the form of the Meselson-Stahl experiment. Work by Crick and coworkers showed that the genetic code was based on non-overlapping triplets of bases, called codons, and Har Gobind Khorana and others deciphered the genetic code not long afterward. These findings represent the birth of molecular biology.
Watson, Crick, and Wilkins were awarded the 1962 Nobel Prize for Physiology or Medicine for discovering the molecular structure of DNA, by which time Franklin had died from cancer at 37. Nobel prizes are not awarded posthumously; had she lived, the difficult decision over whom to jointly award the prize would have been complicated as the prize can only be shared between a maximum of three; but because their work could be considered to be chemistry, it is conceivable that Wilkins and Franklin could have been awarded the Nobel Prize for Chemistry instead; see Graeme Hunter's biography of Sir Lawrence Bragg for more information on how scientists were nominated for Nobel Prizes.
DNA in practice
DNA in crime
Main article: Genetic fingerprinting
Forensic scientists can use DNA located in blood, semen, skin, saliva or hair left at the scene of a crime to identify a possible suspect, a process called genetic fingerprinting or DNA profiling. In DNA profiling the relative lengths of sections of repetitive DNA, such as short tandem repeats and minisatellites, are compared. DNA profiling was developed in 1984 by English geneticist Alec Jeffreys of the University of Leicester, and was first used to convict Colin Pitchfork in 1988 in the Enderby murders case in Leicestershire, England. Many jurisdictions require convicts of certain types of crimes to provide a sample of DNA for inclusion in a computerized database. This has helped investigators solve old cases where the perpetrator was unknown and only a DNA sample was obtained from the scene (particularly in rape cases between strangers). This method is one of the most reliable techniques for identifying a criminal, but is not always perfect, for example if no DNA can be retrieved, or if the scene is contaminated with the DNA of several possible suspects.
DNA in computation
Main article: DNA computing
DNA plays an important role in computer science, bioinformatics and computational biology, both as a motivating research problem and as a method of computation in itself.
Research on string searching algorithms, which find an occurrence of a sequence of letters inside a larger sequence of letters, was motivated in part by DNA research, where it is used to find specific sequences of nucleotides in a large sequence.[6] In other applications such as text editors, even simple algorithms for this problem usually suffice, but DNA sequences cause these algorithms to exhibit near-worst-case behavior due to their small number of distinct characters.
Database theory has been influenced by DNA research, which poses special problems for storing and manipulating DNA sequences. Databases specialized for DNA research are called genomic databases, and must address a number of unique technical challenges associated with the operations of approximate matching, sequence comparison, finding repeating patterns, and homology searching.
In 1994, Leonard Adleman of the University of Southern California made headlines when he discovered a way of solving the directed Hamiltonian path problem, an NP-complete problem, using tools from molecular biology, in particular DNA. The new approach, dubbed DNA computing, has practical advantages over traditional computers in power use, space use, and efficiency, due to its ability to highly parallelize the computation (see parallel computing), although there is labor worth mentioning involved in retrieving the answers. A number of other problems, including simulation of various abstract machines, the boolean satisfiability problem, and the bounded version of the Post correspondence problem, have since been analyzed using DNA computing.
Due to its compactness, DNA also has a theoretical role in cryptography, where in particular it allows unbreakable one-time pads to be efficiently constructed and used.[7]
DNA in historical and anthropological study
Because DNA collects mutations over time, which are then passed down from parent to offspring, it contains information about processes that have occurred in the past, becoming in time ancient DNA. By comparing different DNA sequences, geneticists can attempt to infer the history of organisms.
If DNA sequences from different species are compared, then the resulting family tree, or phylogeny can be used to study the evolution of these species. This field of phylogenetics is a powerful tool in evolutionary biology. If DNA sequences within a species are compared, population geneticists can glean information on the history of particular populations. This can be used in studies ranging from ecological genetics to anthropology (for example, DNA evidence is also being used to try to identify the Ten Lost Tribes of Israel[8][9]).
DNA has also been used to look at fairly recent issues of family relationships, such as establishing some manner of familial relationship between the descendants of Sally Hemings and the family of Thomas Jefferson. This usage is closely related to the use of DNA in criminal investigations detailed above. Indeed, some criminal investigations have been solved when DNA from crime scenes has fortuitously matched relatives of the guilty individual.[1][2]
References
Citations
1.^ http://proxy.arts.uci.edu/~nideffer/Hawking/early_proto/orgel.html
2.^ a b c Butler, John M. (2001) Forensic DNA Typing "Elsevier". pp. 14-15. ISBN 012147951X.
3.^ Takahashi I, Marmur J. (1963). " Replacement of thymidylic acid by deoxyuridylic acid in the deoxyribonucleic acid of a transducing phage for Bacillus subtilis". Nature 197: 794-5. PMID 13980287.
4.^ Length of a Human DNA Molecule. Retrieved on 2006-03-04.
5.^ Watson JD, Crick FH. (1953). " Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid". Nature 171 (4356): 737-8. PMID 13054692.
6.^ Gusfield, Dan. Algorithms on Strings, Trees, and Sequences: Computer Science and Computational Biology. Cambridge University Press, 15 January 1997. ISBN 0521585198.
7.^ Ashish Gehani, Thomas LaBean and John Reif. DNA-Based Cryptography. Proceedings of the 5th DIMACS Workshop on DNA Based Computers, Cambridge, MA, USA, 14 – 15 June 1999.
8.^ Lost Tribes of Israel, NOVA, PBS airdate: 22 February 2000. Transcript available from http://www.pbs.org/wgbh/nova/transcripts/2706israel.html (last accessed on 4 March 2006)
9.^ Kleiman, Yaakov. The Cohanim/DNA Connection. Retrieved on 2006-03-04.
General references
•Robert Olby; "The Path to The Double Helix: Discovery of DNA"; first published in 0ctober 1974 by MacMillan, with foreword by Francis Crick; ISBN 046681173; the definitive DNA textbook, revised in 1994, with a 9 page postscript.
•Ridley, Matt; Francis Crick: Discoverer of the Genetic Code (Eminent Lives) first published in June 2006 in the USA and then to be in the U.K. September 2006, by HarperCollins Publishers; 192 pp, ISBN 006082333X
•Watson, James D. and Francis H.C. Crick. A structure for Deoxyribose Nucleic Acid (PDF). Nature 171, 737 – 738, 25 April 1953.
•Watson, James D. DNA: The Secret of Life ISBN 0375415467.
•Watson, James D. The Double Helix: A Personal Account of the Discovery of the Structure of DNA (Norton Critical Editions). ISBN 0393950751
•Chomet, S. (Ed.), DNA Genesis of a Discovery, Newman-Hemisphere Press, London, 1994.
•Delmonte, C.S. and Mann, L.R.B. Variety in DNA tertiary structure. Current Science, 85 (11), 1564 – 1570, 10 December 2003.
•Miller, Kenneth R., and Levin, Joseph. Biology. Saddle River, New Jersey: Prentice Hall, 2002.
External links
•DNA from the beginning
•DNA hack: The website for Amateur Genetic Engineering
•on Francis Crick
•First press stories on DNA
•'Death' of DNA Helix (Crystaline) joke funeral card.
•Double helix: 50 years of DNA, Nature.
•DNA The Program (in Russian).
•U.S. National DNA Day Watch videos and participate in real-time chat with top scientists
•Genetic Education Modules for Teachers DNA from the Beginning Study Guide
•Talking Glossary of Genetic Terms In Spanish, too
•Linus Pauling and the Race for DNA
•Listen to Francis Crick and James Watson talking on the BBC in 1962, 1972, and 1974
•17 April, 2003, BBC News: Most ancient DNA ever?
•Latest Advances In Gene Research
•DNA Research News
•Using DNA in Genealogical Research
•DNA Interactive (requires Macromedia Flash)
•DNA: PDB molecule of the month
•DNA under electron microscope
•Left-handed DNA Hall of Fame
•My First Book About DNA Designed for children to learn more about DNA.
•Nucleic Acids at the Open Directory Project
•DNA Replication and Translation / Cell Biology
•DNA Articles DNA Articles and Information collected from various sources.
•Beyond Biology: Making Factories and Computers with DNA