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DNA

2007 Schools Wikipedia Selection. Related subjects: General Biology

   The general structure of a section of DNA
   Enlarge
   The general structure of a section of DNA

   Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic
   instructions for the biological development of a cellular form of life
   or a virus. All known cellular life and some viruses have DNA. DNA is a
   long polymer of nucleotides (a polynucleotide) that encodes the
   sequence of amino acid residues in proteins, using the genetic code.

Inheritance of DNA

   DNA is responsible for the genetic propagation of most inherited
   traits. In humans, these traits range from hair colour to disease
   susceptibility. The genetic information encoded by an organism's DNA is
   called its genome. During cell division, DNA is replicated, and during
   reproduction is transmitted to offspring.

   In eukaryotic cells, such as those of plants, animals, fungi and
   protists, most of the DNA is located in the cell nucleus, and each DNA
   molecule is usually packed into a chromosome that are passed to
   daughter cells during cell division. By contrast, in simpler cells
   called prokaryotes, including the eubacteria and archaea, DNA is found
   directly in the cytoplasm (not separated by a nuclear envelope) and is
   circular. The cellular organelles known as chloroplasts and
   mitochondria also carry DNA. DNA is thought to have originated
   approximately 3.5 to 4.6 billion years ago.

   In humans, the mother's mitochondrial DNA together with 23 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. Lineage studies can be
   done because mitochondrial DNA only comes from the mother, and the Y
   chromosome only comes from the father.
   Animation of a section of DNA rotating.
   Animation of a section of DNA rotating.
   DNA base pairing
   Enlarge
   DNA base pairing

Replication

   DNA replication
   DNA replication

   The double-stranded structure of DNA provides a mechanism for DNA
   replication: the two strands are separated, and then each strand's
   complement is recreated by exposing the strand to a mixture of the four
   bases. An enzyme makes the complement strand by finding the correct
   base in the mixture and bonding it with the original strand. In this
   way, the base on the old strand dictates which base appears on the new
   strand, and the cell ends up with an extra copy of its DNA.

Physical and chemical properties

   Comparisons between DNA and single stranded RNA with the diagram of the
   bases showing.
   Enlarge
   Comparisons between DNA and single stranded RNA with the diagram of the
   bases showing.

Molecular structure

   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, in the shape of a double helix (see the illustration at the
   right).

   DNA consists of a pair of molecules, organized as strands running
   start-to-end and joined by hydrogen bonds along their lengths. Each
   strand 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). (Thymine should not be confused with
   thiamine, which is vitamin B[1].) The DNA of some organisms, most
   notably of the PBS1 phage, have Uracil (U) instead of T.

   Each strand of DNA is a covalently linked chain of nucleotides, with
   alternating sugar ( deoxyribose)- phosphates forming the "backbone" for
   the nucleobases ("bases"). The negatively-charged phosphate groups
   between each deoxyribose make DNA an acid in solution and allow DNA
   molecules of different sizes to be separated by electrophoresis.
   Because DNA strands are composed of these nucleotide subunits, they are
   polymers. The major difference between DNA and RNA is the sugar,
   2-deoxyribose in DNA and ribose in RNA.

Base pairing

   DNA is composed of 4 bases: adenine (A), thymine (T), cytosine (C), and
   guanine (G). Uracil (U), is rarely found in DNA except as a result of
   chemical degradation of Cytosine, but the DNA of some viruses (notably
   PBS1 phage DNA) and RNA (Ribonucleic Acid), has Uracil instead of
   Thymine.

   Each base on one strand forms a bond with just one kind of base on
   another strand, called a "complementary" base: A bonds with T, and C
   bonds with G. Therefore, the whole double-strand sequence can be
   described by the sequence on one of the strands, chosen by convention.
   Two nucleotides paired together are called a base pair.

   In a DNA double helix, two polynucleotide strands can associate through
   the hydrophobic effect and pi stacking. Which strands associate depends
   on complementary pairing. Each base forms hydrogen bonds readily to
   only one other base, 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 temperature required to break the
   hydrogen bond, its melting temperature (also called T[m] value)).
   The chemical structure of DNA
   Enlarge
   The chemical structure of DNA

Strand direction

   The asymmetric shape and linkage of nucleotides means that a DNA strand
   always has a discernible orientation or directionality. Inspection of a
   double helix reveals that the direction of the nucleotides in one
   strand is opposite to their direction in the other strand. This
   arrangement of the strands is called antiparallel.

   Chemical nomenclature ( 5' and 3')

   The assymetric "ends" of the DNA bases are referred to as 5' (five
   prime) and 3' (three prime). Within the nucleus, the enzymes that
   perform replication and transcription read the DNA template in the "3'
   to 5' direction", although this directional reading should not be
   assumed in other cases. 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

   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.

Single-stranded DNA

   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.

   For further discussion of the physical structure of DNA see Mechanical
   properties of DNA.

DNA sequence

   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 are the units of heredity
   and can be loosely viewed as the organism's "cookbook" or "blueprint".
   DNA is often referred to as the molecule of heredity.

The genetic code

   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 4^3) 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.

Non-coding DNA

   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. Indeed, the 34,000
   human genes encode some 100,000 proteins. 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).

   Sequence also determines a DNA segment's susceptibility to cleavage by
   restriction enzymes, an important tool in genetic engineering. The
   position of cleavage sites throughout an individual's genome determines
   one kind of an individual's " DNA fingerprint".

Mutation

   A cell's machinery separates the DNA double helix, and uses 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 called mutations. 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. On rare occasions, wrong pairing can happen, when
   thymine goes into its enol form or cytosine goes into its imino form.
   Mutations can also occur after chemical damage (through mutagens),
   light (UV damage), or through other more complicated gene swapping
   events. This process of replication is mimiced in vitro by a process
   called Polymerase chain reaction (PCR).

The study 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 Frederick 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).

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.

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.
   Enlarge
   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.

   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.

   In spite of all this, the prize presented to Watson and Crick was
   indeed very controversial. In 1951, Rosalind Franklin, a physical
   chemist working in Paris, was researching DNA's structure at King's
   College and gave a department lecture on her work at the time on DNA.
   Watson attended this lecture and initially learned of Franklin's data,
   but he did not take notes. This led to an initial structure proposed by
   Watson and Crick, which Franklin refuted when she revealed information
   that Watson had neglected to write from attending her lecture.

   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. In the end,
   however, it turned out that much of Franklin's data from this MRC
   report had been presented in that open seminar where Watson had
   neglected to take notes.

   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 permissible. 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.

   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.

   Franklin died in 1958 and four years later, Watson, Crick and Wilkins
   won the Nobel Prize for their work on the structure of DNA. Because the
   Nobel Prize is not awarded posthumously, Franklin could not share in
   it.

"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, 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.

Forensics

   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 British geneticist Sir Alec Jeffreys of the
   University of Leicester, and was first used to convict Colin Pitchfork
   in 1988 in the Enderby murders case in Leicestershire, United Kingdom.
   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 and computation

   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. A Sequence profiling tool like
   Sequerome assists researchers working on sequence data by linking the
   entire Sequence alignment report ( BLAST) to many third party
   servers/sites that provide highly specific services in sequence
   manipulations such as restriction enzyme maps, open reading frame
   analyses for nucleotide sequences, and secondary structure prediction.

   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. 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
   behaviour 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.

History and anthropology

   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).

   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.

Global variation in copy number in the human genome

   In a report published in 2006 in Nature, researchers have found that
   the copy number variation (CNV) of DNA sequences in humans and other
   mammals, can be considerable. Deletions, insertions, duplications and
   complex multi-site variants, collectively termed copy number variations
   (CNVs) or copy number polymorphisms (CNPs), are found in all humans and
   other mammals examined.
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