Gene
The word "gene"shared by many disciplines, including whole organism-based or "classical" genetics, molecular genetics, evolutionary biologypopulation genetics. It has multiple uses within eachthese contexts. But inprimary sense "genes"material things that parents passoffspring during reproductionthrough whichpropagate their biological traits or characteristics. This sense, whichcommonall ofabove disciplines,alsooriginal historical meaning"gene."
FollowingdiscoveryDNA,in parallel more recently withascentbiotechnologyprojects sequencehuman genome, common usage ofwordever more instances has echoed usesmolecular biology. Inprimary, molecular sense, genessegmentsDNA within chromosomes. In particular,aresubsetsuch DNA which cells transcribe into RNAstranslate, at leastpart, into proteins.
Encodersproteins
A genethis sense specifiesprotein by wayits chemical structure. Anyfour typessequentially linked nucleotides makeDNA molecule or "strand" (more at DNA). These four representgenetic alphabet, whilevarious possible sequencesthree, called codons, representgenetic vocabulary. The sequencewhich different codons appear ingene specifiesamino-acid sequence ofprotein, andgenetic code describes which amino acids relatewhich codons. This codemore or lesssame from bacteriahumans;other words, commonall cellular life.
Throughproteins"encode," genes governcellswhichreside. In multicellular organismscontrol development ofindividual fromfertilized egg andday-to-day functions ofcells that make up tissuessorganss. The instrumental rolestheir protein products range from mechanical support ofcell structure totransportationmanufactureother moleculestoregulationother proteins' activities.
Gene activityregulation
Because itthrough proteins that genes exert their effects,because gene transcripts (which areprerequisiteprotein synthesis) degrade rapidly, genesinsense inactive when theynot actively being transcribed. Cells appearregulateactivitygenes primarily by increasing or decreasing their ratetranscription. Overshort term, this regulation occurs throughbinding or unbindingproteins known as transcription factors, which attachspecific "non-coding" DNA sequences called regulatory elements. Over longer periodstime, genes may be "silenced through DNA methylation or changes inDNA packing ofchromosomes (see histone).
Organizationgenes
In many speciesorganism, very little ofDNA inchromosomes encodes proteins. Rather,genesseparated by often vast sequencesso-called junk DNA,theysometimes fragmented internally by "non-coding" sequences called introns, which may be many times longer thangenes themselves. Intronsremoved onheelstranscription by splicing. Inprimary molecular senserepresent parts ofgene, however
Allgenesintervening DNA together make upgenomean organism, whichmany speciesdivided among several chromosomestypically presenttwo or more copies. The location or locus ofgene andchromosome on which itsituatedinsense arbitrary. Genes that appear together onchromosomesone species, such as humans, may appear on separate chromosomesanother species, such as mice. Two genes sited close together onchromosome may encode proteins that figure either insame cellular process orcompletely unrelated processes. As an example offormer, many ofgenes responsiblehuman sexual characteristics reside together onY chromosome.
Genetic variation
Duerare, spontaneous errorsDNA replication,example, mutationshence variations insequence ofgene arise withinspecies population. Variants ofsingle geneknown as alleles,differencesalleles may give risedifferencestraits,example eye color. A gene's most common allelecalledwild type,rare allelescalled mutants.
Inmany species that carry more than one copytheir genome within eachtheir somatic cells, these copiesin effect never identical. With respecteach gene,copies that an individual possessesliablebe distinct alleles, which may act either synergistically or antagonisticallygeneratetrait or phenotype (more at genetics, allele).
Genetic complexitytraitspitfallscommon usage
In common speech, "gene"often usedrefer tohereditary cause oftrait, disease or condition--as"the geneobesity." A biologist,contrast, might referan allele ormutation that had been implicated in or correlated with obesity. Based onincidenceobesity across parentsoffspring, notmention common sense, biologists know that not only genes but factors such as upbringing, culture andavailabilityfood decide whether or notpersonobese. To continue withsame example,also appears unlikely that variations withinsingle gene--or single genetic locus--determine one's genetic predispositionobesity. These aspectsinheritance--the interplay between genesenvironment,influencemany genes--appearbenormregardmanyperhaps most traits. The term phenotype refers tocharacteristics that result from this interplay, as well as fromeffectschance onmigrationpatterningcells during embryonic development.
Other aspects"gene"molecular biology
Regulatory elementsheredity
Natural variations within regulatory sequences appear alsounderlie many ofheritable characteristics seenorganisms. The influencesuch variations ontrajectoryevolution through natural selection may be as large as or larger than variationsequences that encode proteins. Thus, though regulatory elementsoften distinguished from genesmolecular biology,effectsatisfysharedhistorical sense ofword. Indeed,breeder or geneticist,followinginheritance pattern oftrait, has no immediate wayknow whether this pattern arises from coding sequences or regulatory sequences. Typically, he or she will simply attribute itvariations within"gene."
RNA genes
RNAalwaysintermediary between genesproteins, butsome gene sequences RNA moleculesactuallyend products. These molecules may be capableenzymatic function, suchRNAs known as ribozymes,ormay engageregulatory base pairing, as incase"small interfering RNAs.
The DNA sequences from which such RNAstranscribedknown as RNA genes.
More on molecular nomenclatureusage
For various reasons,relationship between genesproteinsnot so simple as "one nucleotide sequence-->one amino-acid sequence." For example, cells may splicetranscripts ofgenealternate waysproduce not one butvarietyproteins (alternative splicing). Onchromosome meanwhile,single DNA sequence may contain overlapping genes. In addition, accidents overcourseevolution may lead toduplication ofgene tosecond locus, wheremay fall under different regulation. Thoughtwo sequences may remainsame or be only slightly altered, theytypically regarded as separate genes (i.e. not as alleles ofsame gene). The sametrue when duplicate sequences appeardifferent species. Yet, thoughalleles ofgene differsequence, nevertheless theyseenrepresent one gene.
Finally,molecular biologists will often use "gene"referjustnucleotide sequence ofgene;at timessequenceonly its coding regions withoutintrons. This more abstract sensegene underliessensegenes as information. It also means that, by wayits sequence, not only DNA but RNA may be said eitherbecarrygene (see below).
RNAsgenessome viruses
Although all cell-based organisms carry their genestransmit themoffspring as DNA, many ofviruses that parasitizereproducethem carry only RNA. Becauseuse RNA, their cellular hosts may synthesize their proteins as soon as theyinfectedwithoutdelaywaitingtranscription. RNA "retroviruses, onother hand, require "reverse transcription"their genome from RNA into DNA.
"Selfish" gene
The genes that exist todaythose that have reproduced successfully inpast. This isbasis ofselfish gene view, publicised by Richard Dawkins. He points outhis book, The Selfish Gene, that all DNA existsno other purpose thanpropagate itself, even atexpense ofhost organism's welfare. AccordingDawkins,possibly disappointing answer toquestion "what ismeaninglife?" may be "the survivalperpetuationribonucleic acidstheir associated proteins".
History
The existencegenes was first suggested by Gregor Mendel, who studied inheritancepea plantshypothesizedfactor that conveys traits from parentoffspring. Although he did not useterm "gene", he explained his resultstermsinherited characteristics. Mendel was alsofirsthypothesize independent assortment,distinction between dominantrecessive traits,distinction betweenheterozygotehomozygote, anddifference between what would later be described as genotypephenotype.
Wilhelm Johannsen coined "gene"1909, based onworkGregor Mendel.
Typical numbersgenesan organism:
The following table gives typical numbersgenesgenome sizesome organisms. Estimates ofnumbergenesan organismsomewhat controversial, because itonly possiblediscovergene,no techniques currently existprove thatDNA sequence contains no gene. Nonetheless, estimatesmade based on current knowledge.
| organism | #genes | base pairs |
|---|---|---|
| Plants | <50000 | <1011 |
| Humans | 35000 | 3x109 |
| Flies | 12000 | 1.6x108 |
| Fungi | 6000 | 1.3x107 |
| Bacteria | 500-6000 | 5*105-107 |
| Mycoplasma genitalium | 500 | 580.000 |
| DNA viruses | 10-300 | 5000-200.000 |
| RNA viruses | 1-25 | 1000-23.000 |
| Viroids | 0-1 | ~500 |
| Prions | 0 | ;0 |
See also: genetics, gene expression, gene therapy, homeobox, genomics, DNA, protein, listgene families
