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Геном



Въведение

В областите на молекулярната биология и генетиката геномът се отнася до сумофалния генетичен материал на организъм. Този генетичен материал включва ДНК или РНК (вирусна РНК).

Геномната ДНК включва кодираща ДНК и некодираща ДНК, митохондриална ДНК и хлоропластна ДНК.

Thetermgenomewascreatedin1920byHansWinkler,professorofbotanyattheUniversityofHamburg,Germany.

Науката за изучаване на генома се нарича геномика.

Класификация

Вирусен геном

Вирусният геном може да бъде съставен от РНК или ДНК. Геномът на РНК вируса съдържа едноверижна или двойноверижна РНК, както и една или повече отделни РНК молекули. ДНК вирусният геном може да бъде едноверижна или двойноверижна ДНК. Повечето ДНК вирусни геноми са съставени единична линейна ДНКмолекула, но някои са съставени от ДНКвирусни геноми, съставени от кръгови ДНКмолекули.

Prokaryoticgenome

ThegenomesofprokaryotesandeukaryotesarecomposedofDNA.ArchaeahaveaDNAgenomecomposedofcircularchromosomes.Mostbacteriaalsohaveacircularchromosome,however,somebacterialspeciescontainlinearchromosomesormultiplechromosomes.ThegenomesofmostprokaryotesdonotcontainrepetitiveDNA.Somesymbioticbacterialgenomespeciescontainahighproportionofpseudogenes.Forexample,Serratiasymbioticagenomespecieshasonlyabout40%ofitsDNAencodingproteins.Somebacterialgenomesalsocontainauxiliarygeneticmaterial,whichispresentinplasmids.Forthisreason,thetermgenomeshouldnotbeusedasasynonymforchromosome.

Theeukaryoticgenome

TheeukaryoticgenomeconsistsofoneormorelinearDNAchromosomes.Thenumberofchromosomesthatmakeupthegenomeofeukaryotesvariesgreatly.Jackjumperantsandasexualnematodeshaveonlyonepairofchromosomeseach,whilefernspecieshave720pairsofchromosomes.Humancellshave22pairsofautosomesand1pairofsexchromosomes.

Inadditiontothechromosomesinthenucleus,eukaryoticorganellessuchaschloroplastsandmitochondriahavetheirownDNAandchromosomes.Therefore,therearealsotheterms"mitochondrialgenome"and"plastidgenome".Likethebacteriafromwhichtheyoriginate,bothmitochondriaandchloroplastscontaincircularchromosomes.

Differentfromprokaryotes,eukaryoteshaveexon-intronorganizationofprotein-codinggenesandacertainamountofrepetitiveDNA.ThemajorityofmammalianandplantgenomesarecomposedofrepetitiveDNA.

Кодираща последователност

TheDNAsequencethatcarriesthegeneticinformationofthesyntheticproteinisthecodingsequence.Theproportionofcodingsequencesinthegenomevariesgreatlyindifferentspecies.Largergenomesdonotnecessarilycontainmoregenes,andtheproportionofnon-repetitiveDNAincomplexeukaryotesdecreasesasthesizeofthegenomeincreases.SimpleeukaryotessuchasCaenorhabditiselegansandDrosophilahaveahigherproportionofcodingDNAthanrepetitiveDNA,whilethegenomesofmorecomplexeukaryotesareoftencomposedofrepetitiveDNA.TheproportionofrepeatedDNAinthegenomesofsomeplantsandamphibiansexceeds80%.Similarly,only2%ofthehumangenomeencodesDNA.

Некодиращи последователности

Non-codingsequencesincludeintrons,non-codingRNAsequences,regulatoryDNAandrepetitiveDNA.98%ofthehumangenomebelongstonon-codingsequences.TherepetitiveDNAofthegenomehastandemrepetitivesequencesandscatteredrepetitivesequences.

Транспозони

Transposons(TEs)areDNAsequenceswithaspecificstructure.Theycanmovearoundthegenomewithoutafixedposition.ClassITEjumpsthroughthecopyandpastemechanism,andClassIITEisexcisedfromthegenomeandinsertedintoanewposition.

ThemovementofTEisthedrivingforcefortheevolutionofeukaryoticgenomes,becausetheirinsertioncandestroygenefunction,homologousrecombinationbetweenTEscanleadtogeneduplication,andTEcanalsocombineexonswithTheadjustmentsequenceisreorganizedtoanewposition.

Ретротранспозони

Ретротранспозоните могат да се транскрибират в РНК и след това да се копират в генома на друго място. Ретротранспозоните могат да се разделят на две категории: дълги крайни повторения (LTR) и не-дългокрайни повторения (не-LTR).

Размер на генома

Размерът на генома е общият брой двойки ДНК бази в копие на хаплоиднома.

Genomesizeispositivelycorrelatedwiththemorphologicalcomplexityofprokaryotesandlowereukaryotes.However,aftermollusksandallotherhighereukaryotesmentionedabove,thiscorrelationnolongerexists,mainlybecauseofrepeatedDNA.

Genomechanges

Allcellsoftheorganismarederivedfromthesamesinglecell,sotheyshouldhavethesamegenome.However,insomecases,differencescanoccurbetweencells.BothDNAreplicationduringcelldivisionandtheactionofenvironmentalmutagenscancausemutationsinsomaticcells.Insomecases,suchmutationscancausecancerbecausetheycausecellstodividefasterandinvadesurroundingtissues.Duringmeiosis,diploidcellsdividetwicetoproducehaploidgermcells.Inthisprocess,recombinationcausesthegeneticmaterialtobereshuffledfromhomologouschromosomes,soeachgametehasauniquegenome.

Genomeevolution

Thegenomeisnotonlythesumofthegenesoftheorganism,thegenomealsocontainsothercharacteristicsthatcanbeconsideredforspecificgenesandtheirproducts.

Replicationplaysanimportantroleintheshapingofthegenome.Thescopeofreplicationincludestheextensionofshorttandemrepeats,thereplicationofgeneclusters,thereplicationofentirechromosomesandeventheentiregenome.Thisduplicationmaybethebasisforthecreationofnewgenetictraits.

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