| For other uses, see Gene (disambiguation).Introns are regions often found in eukaryote genes which are removed in the splicing process (after the DNA is transcribed into RNA): only the exons encode the protein.In reality most genes are hundreds of times larger, and the relationships between Introns and exons can be highly complex.Base pair
A pair of nucleotide bases on complementary DNA or RNA strands organized in a double helix.DNA molecules that encode many genes.Cytosine
One of the four nucleotide bases in DNA or RNA; pairs with guanine.DNA
A polymeric molecule made of deoxyribonucleotides, hence then name deoxyribonucleic acid.Gene
The unit of heredity in living organisms, typically encoded in a sequence of nucleotide monomers that make up a long strand of DNA, or deoxyribonucleic acid.Gene expression
The process in which the infomation encoded in a gene is converted into a form useful for the cell.The first step is transcription, which produces a messenger RNA molecule complementary to the DNA molecule on which a gene is encoded.RNA is read by the ribosome to produce a protein.Gene pool
The sum of all the alleles shared by members of a single population.Genetics
The field of biology that studies genes and their inheritance.Guanine
One of the four nucleotide bases in DNA or RNA; pairs with cytosine.Locus
A location on a chromosome where a particular gene resides.Protein
A linear polymeric molecule made of amino acids linked by peptide bonds.RNA
A polymeric molecule made of ribonucleotides, hence the name ribonucleic acid, similar to but less stable than DNA.One type, messenger RNA, plays an important role in gene expression.Ribosomes are also made largely of RNA.Thymine
One of the four nucleotide bases in DNA; pairs with adenine.Transcription
The first step in gene expression, in which a messenger RNA molecule complementary to particular gene encoded in DNA is synthesized by enzymes called RNA polymerases.To produce a functional protein, transcription is followed by translation.Translation is always preceded by transcription.Uracil
One of the four nucleotide bases in RNA; pairs with adenine.In DNA, uracil is replaced with thymine.RNA genes, has been proposed by Gerstein et al.RNA copy of the gene's information.The genes of eukaryotic organisms can contain regions called introns that are removed from the messenger RNA in a process called splicing.The regions encoding gene products are called exons.In prokaryotes (bacteria and archaea), introns are less common and genes often contain a single uninterrupted stretch of DNA, called a cistron, that codes for a product.Prokaryotic genes are often arranged in groups called operons with promoter and operator sequences that regulate transcription of a single long RNA.Amoeba dubia, with over 670 billion base pairs, some 200 times larger than the human genome.Composition of the genome
6.Mendel was also the first to hypothesize independent assortment, the distinction between dominant and recessive traits, the distinction between a heterozygote and homozygote, and the difference between what would later be described as genotype and phenotype.Wilhelm Johannsen abbreviated this term to "gene" ("gen" in Danish and German) two decades later.In the early 1900s, Mendel's work received renewed attention from scientists.In 1910, Thomas Hunt Morgan showed that genes reside on specific chromosomes.With this knowledge, Morgan and his students began the first chromosomal map of the fruit fly Drosophila.In 1941, George Wells Beadle and Edward Lawrie Tatum showed that mutations in genes caused errors in certain steps in metabolic pathways.Watson and Francis Crick demonstrated the molecular structure of DNA.In 1972, Walter Fiers and his team at the Laboratory of Molecular Biology of the University of Ghent (Ghent, Belgium) were the first to determine the sequence of a gene: the gene for Bacteriophage MS2 coat protein.In particular, genes do not seem to sit side by side on DNA like discrete beads.Darwin used the term Gemmule to describe a microscopic unit of inheritance, and what would later become known as Chromosomes had been observed separating out during cell division by Wilhelm Hofmeister as early as 1848.The idea that chromosomes were the carriers of inheritance was expressed in 1883 by Wilhelm Roux.William Bateson in 1905.The word pangenesis is made from the Greek words pan (a prefix meaning "whole", "encompassing") and genesis ("birth") or genos ("origin").Different genes for the same trait, which give rise to different phenotypes, are known as alleles.Alleles may be dominant or recessive; dominant alleles give rise to their corresponding phenotypes when paired with any other allele for the same trait, while recessive alleles give rise to their corresponding phenotype only when paired with another copy of the same allele.For example, if the allele specifying tall stems in pea plants is dominant over the allele specifying short stems, then pea plants that inherit one tall allele from one parent and one short allele from the other parent will also have tall stems.Although Mendel's work was largely unrecognized after its first publication in 1866, it was rediscovered in 1900 by three European scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak, who had reached similar conclusions from their own research.However, these scientists were not yet aware of the identity of the 'discrete units' on which genetic material resides.DNA (deoxyribonucleic acid), a polymeric molecule found in all cells on which the 'discrete units' of Mendelian inheritance are encoded.The modern study of genetics at the level of DNA is known as molecular genetics and the synthesis of molecular genetics with traditional Darwinian evolution is known as the modern evolutionary synthesis.The vast majority of living organisms encode their genes in long strands of DNA.Thus, nucleotides in DNA or RNA are typically called 'bases'; consequently they are commonly referred to simply by their purine or pyrimidine original base components adenine, cytosine, guanine, thymine.The other end contains an exposed phosphate group, this is the 5' end.DNA replication occur in only one direction.The expression of genes encoded in DNA begins by transcribing the gene into RNA, a second type of nucleic acid that is very similar to DNA, but whose monomers contain the sugar ribose rather than deoxyribose.RNA also contains the base uracil in place of thymine.The genetic code specifies the correspondence during protein translation between codons and amino acids.The genetic code is nearly the same for all known organisms.Because they use RNA to store genes, their cellular hosts may synthesize their proteins as soon as they are infected and without the delay in waiting for transcription.While RNA is common as genetic storage material in viruses, in mammals in particular RNA inheritance has been observed very rarely.These weak promoters usually permit a lower rate of transcription than the strong promoters, because the transcription machinery binds to them and initiates transcription less frequently.Other possible regulatory regions include enhancers, which can compensate for a weak promoter.Eukaryotic promoter regions are much more complex and difficult to identify than prokaryotic promoters.By contrast, eukaryotic genes are transcribed only one at a time, but may include long stretches of DNA called introns which are transcribed but never translated into protein (they are spliced out before translation).Splicing can also occur in prokaryotic genes, but is less common than in eukaryotes.Chromosomes
The total complement of genes in an organism or cell is known as its genome, which may be stored on one or more chromosomes; the region of the chromosome at which a particular gene is located is called its locus.DNA helix on which thousands of genes are encoded.For example, the genes for antibiotic resistance are usually encoded on bacterial plasmids and can be passed between individual cells, even those of different species, via horizontal gene transfer.The manner in which DNA is stored on the histone, as well as chemical modifications of the histone itself, are regulatory mechanisms governing whether a particular region of DNA is accessible for gene expression.The ends of eukaryotic chromosomes are capped by long stretches of repetitive sequences called telomeres, which do not code for any gene product but are present to prevent degradation of coding and regulatory regions during DNA replication.The length of the telomeres tends to decrease each time the genome is replicated in preparation for cell division; the loss of telomeres has been proposed as an explanation for cellular senescence, or the loss of the ability to divide, and by extension for the aging process in organisms.DNA", or regions of DNA that serve no obvious function.DNA, while the genomes of complex multicellular organisms, including humans, contain an absolute majority of DNA without an identified function.The genetic code is the set of rules by which a gene is translated into a functional protein.Each gene consists of a specific sequence of nucleotides encoded in a DNA (or sometimes RNA) strand; a correspondence between nucleotides, the basic building blocks of genetic material, and amino acids, the basic building blocks of proteins, must be established for genes to be successfully translated into functional proteins.There are 64 possible codons (four possible nucleotides at each of three positions, hence 43 possible codons) and only 20 standard amino acids; hence the code is redundant and multiple codons can specify the same amino acid.The correspondence between codons and amino acids is nearly universal among all known living organisms.RNA molecule known as messenger RNA, whose nucleotide sequence is complementary to the DNA from which it was transcribed.In prokaryotes, transcription occurs in the cytoplasm; for very long transcripts, translation may begin at the 5' end of the RNA while the 3' end is still being transcribed.The splicing of introns present within the transcribed region is a modification unique to eukaryotes; alternative splicing mechanisms can result in mature transcripts from the same gene having different sequences and thus coding for different proteins.Translation
Translation is the process by which a mature mRNA molecule is used as a template for synthesizing a new protein.The genetic code is read three nucleotides at a time, in units called codons, via interactions with specialized RNA molecules called transfer RNA (tRNA).DNA, known as the template strand, and synthesize a new complementary strand.The process of DNA replication is semiconservative; that is, the copy of the genome inherited by each daughter cell contains one original and one newly synthesized strand of DNA.Binary fission is extremely fast compared to the rates of cell division in eukaryotes.Molecular inheritance
The duplication and transmission of genetic material from one generation of cells to the next is the basis for molecular inheritance, and the link between the classical and molecular pictures of genes.In asexually reproducing organisms, the offspring will be a genetic copy or clone of the parent organism.In sexually reproducing organisms, a specialized form of cell division called meiosis produces cells called gametes or germ cells that are haploid, or contain only one copy of each gene.DNA on one chromatid is swapped with a length of DNA on the corresponding sister chromatid.The Mendelian principle of independent assortment asserts that each of a parent's two genes for each trait will sort independently into gametes; which allele an organism inherits for one trait is unrelated to which allele it inherits for another trait.This is known as genetic linkage.Rare, spontaneous alterations in the base sequence of a particular gene arise from a number of sources, such as errors in DNA replication and the aftermath of DNA damage.Variants of a single gene are known as alleles, and differences in alleles may give rise to differences in traits.Extrachromosomal DNA is present in many prokaryotes and some simple eukaryotes as small, circular pieces of DNA called plasmids, which usually contain only a few genes each.Generally, regulatory regions and junk DNA are considered to be part of an organism's genome, but structural regions such as telomeres are not.The location (or locus) of a gene and the chromosome on which it is situated is in a sense arbitrary.Two genes positioned near one another on a chromosome may encode proteins that figure in the same cellular process or in completely unrelated processes.Many species carry more than one copy of their genome within each of their somatic cells.Cells or organisms with only one copy of each gene are called haploid; those with two copies are called diploid; and those with more than two copies are called polyploid.When more than one copy is present, the two copies are not necessarily identical; in sexually reproducing organisms, one copy is normally inherited from each parent.Although it was believed before the completion of the Human Genome Project that the human genome would contain many more genes than simpler animals such as mice or fruit flies, the completion of the project has revealed that the human genome has an unexpectedly low gene density.Estimates of the number of genes in a genome are difficult to compile because they depend on gene finding algorithms that search for patterns resembling those present in known genes, such as open reading frames, promoter regions with sequences resembling the consensus promoter sequence, and related regulatory regions such as TATA boxes in eukaryotes.DNA that resembles expressed genes but usually lacks appropriate promoters and other control sequences; such regions are hypothesized to be the results of gene duplication events in a lineage's evolutionary past.All approved symbols are stored in the HGNC Database.Each symbol is unique and each gene is only given one approved gene symbol.It is necessary to provide a unique symbol for each gene so that people can talk about them.In preference each symbol maintains parallel construction in different members of a gene family and can be used in other species, especially the mouse.Evolutionary concept of a gene
George C.He proposed an evolutionary concept of gene to be used when we are talking about natural selection favoring some genes.The definition is: "that which segregates and recombines with appreciable frequency."According to this definition, even an asexual genome could be considered a gene, insofar it have an appreciable permanency through many generations.The difference is: the molecular gene transcribes as a unit, and the evolutionary gene inherits as a unit.In The Selfish Gene Dawkins attempts to redefine the word 'gene' to mean "an inheritable unit" instead of the generally accepted definition of "a section of DNA coding for a particular protein".The gene concept is still changing
The concept of the gene has changed considerably (see history section).And the definition of gene is still changing.T(H)2 cytokine locus on chromosome 11 come into close proximity in the nucleus possibly to be jointly regulated.The concept that genes are clearly delimited is also being eroded.There is evidence for fused proteins stemming from two adjacent genes that can produce two separate protein products.This new data has led to an updated, and probably tentative, definition of a gene as "a union of genomic sequences encoding a coherent set of potentially overlapping functional products.""Genetics: what is a gene?".Gerstein MB, Bruce C, Rozowsky JS, Zheng D, Du J, Korbel JO, Emanuelsson O, Zhang ZD, Weissman S, Snyder M (2007).History and updated definition".The Evolution of Genome Size Chichester: John Wiley.International Human Genome Sequencing Consortium (2004).Watson JD, Baker TA, Bell SP, Gann A, Levine M, Losick R (2004).Peason Benjamin Cummings (Cold Spring Harbor Laboratory Press).Cold Spring Harbor Laboratory Press: Cold Spring Harbor, New York.References and databases
HUGO Gene Nomenclature Committee, HGNC
OMIM NIH's National Library of Medicine NCBI website link to Online Mendelian Inheritance in Man.This page was last modified 19:19, 3 January 2008.All text is available under the terms of the GNU Free Documentation License.See Copyrights for details.View Related Articles
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