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Genome

Science EncyclopediaScience & Philosophy: Gastrula to Glow discharge

The genome (sometimes spelled geneome) is, in the broadest use of the term, the full set of genes or genetic material carried by a particular organism representing a particular species or population. The size of a genome is usually measured in numbers of genes or base pairs.

With the success of the Human Genome Project and other international genome projects and programs, by 2003, scientists have, to a great extent, constructed genetic maps delineating individual base sequences that constitute the basis of human genome.

A genomic sequence is the actual order of the nitrogenous bases in the DNA nucleotide sequence that, with subtle alterations that create differing gene forms (alleles), comprise an organism's genetic material.

In humans, the genome comprises one representative of each of the chromosome pairs of the adult diploid parent. In this sense, a genome is a single set of genetic instructions.

Not all of alleles within a genome are expressed, some are masked by the presence of dominant forms. The genomic formula is a mathematical expression of the number of subsets of genomes present in an individual cell or organism. One of the commonly encountered genomic formulae designations is the haploid number (n) that represents a set with a single copy of each gene. This is sometimes called the basic number. The diploid form contains two sets of genes and is designated 2n; the triploid is 3n, and the tetraploid is 4n. Genetic abnormalities, where one chromosome is missing from the genome, can be represented in the same manner. For example, a diploid organism with one chromosome missing is a monosomic cell and is represented by 2n-1. A diploid with two chromosomes missing is termed a nullisomic and is represented by 2n-2. Additions of chromosomes can also occur and are represented in the same form; for example, 2n+1 is trisomic.

In a report published in the February 2001 issue of the international scientific journal nature (usually spelled in the lowercase) researchers reported findings that indicated that the human genome consisted of far fewer genes than previous projected by estimation of phylogenic relationships between humans and other species. Since then additional estimates fix the size of the human genome at about 30,000 genes. By contrast, some worms carry about 22,000 genes in their genome.

As of 2003, while work still continues on the Human Genome Project scientists are also beginning a "Genomes to Life" research program designed to identify and characterize the protein complexes important in animal, especially human, and microbial cell reactions and to further identify the specific genes that regulate these processes.

Genomic libraries have been used in human genetics as part of the Human Genome Project.

A genomic library is a comprehensive collection of cloned DNA fragments derived from a genome. Each part of the genome is represented in the library several times, and the number of times it is represented on average is called the coverage of a library. The library can be screened for the presence of the sequence of interest by radioactively labelling the DNA (usually between 100 and 500 nucleotides long) and using this as a probe to identify the clone that contains the selected sequence. The clone selected can then be grown in bacteria to produce large amounts of clone DNA, which can be studied. If for example, the sequence of interest was part of a gene, by using this sequence as a probe, the clone containing, hopefully, the whole gene could be isolated.

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