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Copy number variation is variation in the number of copies of a particular DNA segment among genomes. A deletion reduces the count; a duplication increases it. For an segment on a non-sex chromosome normally present once on each of two homologous chromosomes, a diploid cell usually has two copies, whereas one additional tandem copy produces a total of three.

The number must be interpreted against a specified baseline. In an ideal comparison with equal sample normalization, three copies versus two give a DNA abundance ratio of $3/2$, or ${\log }_2(3/2)\approx 0.585$ on a logarithmic readout. This arithmetic does not imply that gene expression must rise by the same proportion.

Genome-wide DNA hybridization experiments in 2004 found substantial large-scale copy number variation among otherwise healthy individuals. Such variation is part of ordinary human genetic diversity as well as a possible contributor to disease.^[ref]

DNA hybridization is association of nucleic-acid strands through complementary base pairing. It can form DNA-DNA or DNA-RNA duplexes, depending on the partners.

Sequence complementarity, temperature, ionic conditions, and strand concentration affect the equilibrium and rate of association. Mismatches can weaken binding, which makes hybridization useful for sequence-selective detection without making it an absolute all-or-nothing identity test.

A DNA microarray is a microarray whose spatially indexed sites carry known DNA sequences. A labelled sample binds through DNA hybridization, and signal at each site reports complementary material retained there. In a gene expression assay, material derived from messenger RNA is compared across many genes in parallel; the readout depends on transcript abundance and the chemistry of binding and labelling.^[ref]

The 1995 experiment used robotic printing of Complementary DNA onto glass, allowing many tests in a small hybridization volume.^[ref]

For a synthetic calculation, suppose the two sample signals are 1,000 and 600 units, with background 200 units in each. Background-corrected signals are 800 and 400, giving a ratio of $2$ and a base-two logarithm of $1$. The raw ratio $1000/600$ would give a different answer. Further correction for sample-wide effects may still be required. A transcript ratio supports an interpretation about gene expression; deriving a protein abundance ratio requires additional evidence about biological translation and turnover.

F. Clavel is the first-listed author of a1986 study isolating a new human retrovirus from two West African patients with AIDS. The investigators compared its biological and morphological properties, antigens and DNA hybridization behavior with previously characterized viruses. The work helped distinguish the agent subsequently called HIV-2; the original indexed initial-only author spelling is retained.^[ref][ref]

An illuminating feature was the separation of resemblance from identity. Similar viral morphology did not settle the classification: envelope glycoproteins differed antigenically, and DNA hybridization supplied a different comparison of genetic relatedness. Shared epitopes in one component could coexist with substantial differences in another component.^[ref]

A further1986 paper reported cloning the complete approximately9.5-kilobase HIV-2 genome and detecting restriction-site differences between isolates. This moved the characterization from an isolated agent toward a molecular comparison of genomes, without equating every difference between isolates with a distinct viral type.^[ref]

Crossing between genetically distinct lineages or biological populations. It can introduce combinations of alleles into descendants and contribute to gene flow or hybrid speciation. This process is distinct from DNA hybridization, the pairing of complementary nucleic-acid strands.

Julius Marmur was a biochemist whose work included DNA isolation, strand reassociation and DNA hybridization. His institutional biography at Albert Einstein College of Medicine identifies his collaboration with P. Doty at Harvard and his later research in yeast genetics.^[ref]

Marmur's contribution linked preparing intact DNA with testing its physical behavior. Purification makes it possible to compare changes in the nucleic acid itself rather than changes caused by a heterogeneous cell extract. DNA melting and subsequent pairing experiments then ask whether separated complementary strands can recover an associated structure. Isolation, strand separation and sequence-dependent reassociation are distinct experimental questions.

In the1962 thermal-denaturation study with Doty, the apparatus measured absorbance as a function of sample temperature. The authors used the midpoint of the hyperchromic effect and compared it with GC content determined independently. They corrected for thermal expansion of the solution, an example of separating a molecular absorption change from the changing concentration during heating.^[ref]

The resulting approach made bulk optical measurements informative about DNA composition and structural transitions. It did not directly read the order of bases along a strand: composition, pairing behavior and nucleic acid sequence remain different descriptions.

A microarray arranges many small test sites at known positions on a surface so their signals can be measured in parallel. In a DNA microarray, each site carries a known nucleic-acid sequence; DNA hybridization between that sequence and a labelled sample provides a readout of complementary material in the sample.

A classic expression experiment printed Complementary DNA at high density on glass and hybridized labelled material derived from cellular messenger RNA. Many genes could thus be compared in one small assay instead of one reaction at a time.^[ref]

A twofold difference in signal can suggest different transcript abundance after background correction and normalization. It is not automatically a twofold difference in protein abundance: probe binding, labelling and the steps between transcription and biological translation affect the correspondence.