Genes are the units of heredity in living organisms. They are encoded in the organism's genome, composed of DNA or RNA, and direct the physical development and behavior of the organism. Most genes encode proteins, or biological macromolecules composed of linear chains of amino acids that effect most of the chemical reactions carried out by the cell. Some genes do not encode proteins; these genes produce non-coding RNA molecules that play key roles in protein biosynthesis and gene regulation. Molecules that result from gene expression, whether RNA or protein, are collectively known as gene products.
Most genes contain regions, called non-coding regions, than do not code for the gene products. Much of this non-coding region dictates gene regulation. A critical non-coding region of a gene is the promoter, a short DNA sequence that is required for initiation of gene expression. The genes of eukaryotic organisms often contain non-coding regions called introns which are removed from the messenger RNA in a process known as alternative splicing. The regions that actually encode the gene product, which can be much smaller than the introns, are known as exons.
The word "gene" was coined in 1909 by Danish botanist Wilhelm Johannsen for the fundamental physical and functional unit of heredity. The word was derived from Hugo De Vries' term pangen, itself a derivative of the word pangenesis coined by Darwin (1868).[citationneeded] The word pangenesis is made from the Greek words pan (a prefix meaning "whole", "encompassing") and genesis ("birth") or genos ("origin").
Although classical genetics and evolutionary biology use the term "gene" to refer to a conceptual entity or "unit of inheritance", modern molecular genetics typically uses the term to refer to a physical molecule. The Sequence Ontology Project, an effort directed by the larger Gene Ontology system, defines a gene as "a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions".
The word "gene" is also used in common speech to refer to the inheritance of a trait, as in "a cancer gene" or "the gene for obesity"; however, biologists rarely use the term in this sense because it is highly unlikely that such complex and large-scale phenomena would be attributable to the influence of a single molecular gene.
In molecular biology, a gene is a region of DNA (or RNA, in the case of some viruses) that determines the amino acid sequence of a protein (the coding sequence) and the surrounding sequence that controls when and where the protein will be produced (the regulatory sequence). The genetic code determines how the coding sequence is converted into a protein sequence. The protein-coding regions of genes are composed of a series of three-nucleotide sequences called codons. Each codon specifies a particular amino acid to be added to the protein chain; thus genes determine the protein's primary structure. Most genes are expressed in a two-stage process: first, the DNA is transcribed by enzymes known as RNA polymerases to produce an RNA molecule known as messenger RNA (mRNA), and second, the mRNA is translated by specialized cellular machinery known as the ribosome into a polypeptide chain that then folds into a functional protein. The genetic code is essentially the same for all known life, from bacteria to humans.
Through the proteins they encode, genes govern the cells in which they reside. In multicellular organisms, they control the development of the individual from the fertilized egg and the day-to-day functions of the cells that make up tissues and organs. The roles of their protein products range from mechanical support of the cell structure to the transportation and manufacture of other molecules and to the regulation of other proteins' activities.
Due to rare, spontaneous errors (e.g. in DNA replication), mutations in the sequence of a gene may arise. Once propagated to the next generation, this mutation may lead to variations within a species' population. Variants of a single gene are known as alleles, and differences in alleles may give rise to differences in traits, for example eye colour. A gene's most common allele is called the wild type allele, and rare alleles are called mutants. (However, this does not imply that the wild-type allele is the ancestor from which the mutants are descended.)
In most cases, RNA is an intermediate product in the process of manufacturing proteins from genes. However, for some gene sequences, the RNA molecules are the actual functional products. For example, RNAs known as ribozymes are capable of enzymatic function, and small interfering RNAs have a regulatory role. The DNA sequences from which such RNAs are transcribed are known as non-coding RNA, or RNA genes.
Most living organisms carry their genes and transmit them to offspring as DNA, but some viruses carry only RNA. Because they use RNA, their cellular hosts may synthesize their proteins as soon as they are infected and without the delay in waiting for transcription. On the other hand, RNA retroviruses, such as HIV, require the reverse transcription of their genome from RNA into DNA before their proteins can be synthesized.
For each known human gene the HUGO Gene Nomenclature Committee (HGNC) approve a gene name and symbol (short-form abbreviation). 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. This also facilitates electronic data retrieval from publications. In preference each symbol maintains parallel construction in different members of a gene family and can be used in other species, especially the mouse.
The table below gives typical numbers of genes and genome size for some organisms. Estimates of the number of genes in an organism are somewhat controversial because they depend on the discovery of genes, and no techniques currently exist to prove that a DNA sequence contains no gene. (In early genetics, genes could be identified only if there were mutations, or alleles.) Nonetheless, estimates are made based on current knowledge:
Human, mouse or rat: 25,000
Fugu fish: 40,000
Fruit Fly: 13,767
Mycoplasma genitalium: 500
DNA virus: 10–900
RNA virus: 1–25
Four kinds of sequentially linked nucleotides compose a DNA molecule or strand (more at DNA). These four nucleotides constitute the genetic alphabet. A sequence of three consecutive nucleotides, called a codon, is the protein-coding vocabulary. The sequence of codons in a gene specifies the amino-acid sequence of the protein it encodes.
In most eukaryotic species, very little of the DNA in the genome encodes proteins, and the genes may be separated by vast sequences of so-called junk DNA. Moreover, the genes are often fragmented internally by non-coding sequences called introns, which can be many times longer than the coding sequence. Introns are removed on the heels of transcription by splicing. In the primary molecular sense, they represent parts of a gene, however.
All the genes and intervening DNA together make up the genome of an organism, which in many species is divided among several chromosomes and typically present in two or more copies. The location (or locus) of a gene and the chromosome on which it is situated is in a sense arbitrary. Genes that appear together on the chromosomes of one species, such as humans, may appear on separate chromosomes in another species, such as mice. Two genes positioned near one another on a chromosome may encode proteins that figure in the same cellular process or in completely unrelated processes. As an example of the former, many of the genes involved in spermatogenesis reside together on the Y chromosome.
Many species carry more than one copy of their genome within each of their somatic cells. These organisms are called diploid if they have two copies or polyploid if they have more than two copies. In such organisms, the copies are practically never identical. With respect to each gene, the copies that an individual possesses are liable to be distinct alleles, which may act synergistically or antagonistically to generate a trait or phenotype. The ways that gene copies interact are explained by chemical dominance relationships (more at genetics, allele).
For various reasons, the relationship between DNA strand and a phenotype trait is not direct. The same DNA strand in two different individuals may result in different traits because of the effect of other DNA strands or the environment.- The DNA strand is expressed into a trait only if it is transcribed to RNA. Because the transcription starts from a specific base-pair sequence (a promoter) and stops at another (a terminator), our DNA strand needs to be correctly placed between the two. If not, it is considered as junk DNA, and is not expressed.
This complex process helps explain the different meanings of "gene":- a nucleotide sequence in a DNA strand;
The latter meaning of gene is the result of more "material entity" than the first one.
Just as there are many factors influencing the expression of a particular DNA strand, there are many ways to have genetic mutations.
For example, natural variations within regulatory sequences appear to underlie many of the heritable characteristics seen in organisms. The influence of such variations on the trajectory of evolution through natural selection may be as large as or larger than variation in sequences that encode proteins. Thus, though regulatory elements are often distinguished from genes in molecular biology, in effect they satisfy the shared and historical sense of the word. Indeed, a breeder or geneticist, in following the inheritance pattern of a trait, has no immediate way to know whether this pattern arises from coding sequences or regulatory sequences. Typically, he or she will simply attribute it to variations within a gene.
Errors during DNA replication may lead to the duplication of a gene, which may diverge over time. Though the two sequences may remain the same, or be only slightly altered, they are typically regarded as separate genes (i.e. not as alleles of the same gene). The same is true when duplicate sequences appear in different species. Yet, though the alleles of a gene differ in sequence, nevertheless they are regarded as a single gene (occupying a single locus).
The existence of genes was first suggested by Gregor Mendel, who, in the 1860s, studied inheritance in pea plants and hypothesized a factor that conveys traits from parent to offspring. Although he did not use the term gene, he explained his results in terms of inherited characteristics. 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. Mendel's concept was finally named when Wilhelm Johannsen coined the word gene in 1909.
In the early 1900s, Mendel's work received renewed attention from scientists. In 1910, Thomas Hunt Morgan showed that genes reside on specific chromosomes. He later showed that genes occupy specific locations on the chromosome. With this knowledge, Morgan and his students began the first chromosomal map of the fruit fly Drosophila. In 1928, Frederick Griffith showed that genes could be transferred. In what is now known as Griffith's experiment, injections into a mouse of a deadly strain of bacteria that had been heat-killed transferred genetic information to a safe strain of the same bacteria, killing the mouse.
In 1941, George Wells Beadle and Edward Lawrie Tatum showed that mutations in genes caused errors in certain steps in metabolic pathways. This showed that specific genes code for specific proteins, leading to the "one gene, one enzyme" hypothesis. Oswald Avery, Collin Macleod, and Maclyn McCarty showed in 1944 that DNA holds the gene's information. In 1953, James D. Watson and Francis Crick demonstrated the molecular structure of DNA. Together, these discoveries established the central dogma of molecular biology, which states that proteins are translated from RNA which is transcribed from DNA. This dogma has since been shown to have exceptions, such as reverse transcription in retroviruses.
Richard Roberts and Phillip Sarp discovered in 1977 that genes can be split into segments. This leads to the idea that one gene can make several proteins. Recently (as of 2003-2006), biological results let the notion of gene appear more slippery. In particular, genes do not seem to sit side by side on DNA like discrete beads. Instead, regions of the DNA producing distinct proteins may overlap, so that the idea emerges that "genes are one long continuum". (Pearson, 2006)
George C. Williams first explicitly advocated the gene-centric view of evolution in his book Adaptation and Natural Selection. Also, he proposed an evolutionary concept of gene to be used when we are talking about natural selection favoring some gene. 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.
Richard Dawkins' The Selfish Gene and The Extended Phenotype defended the idea that the gene is the only replicator in living systems. This means that only genes transmit their structure largely intact and are potentially immortal in the form of copies. So, genes should be the unit of selection. In River Out of Eden, Dawkins further refined the idea of gene-centric selection by describing life as a river of compatible genes flowing through geological time. Scoop up a bucket of genes from the river of genes, and we have an organism serving as temporary bodies. A river of genes may fork into two branches representing two non-interbreeding species as a result of geographical separation.
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