Úvod
V oblastech molekulární biologie a genetiky se genomer vztahuje k souhrnnému genetickému materiálu fanorganismu. Tyto genetické materiály zahrnují DNA nebo RNA (virální RNA).
GenomickáDNAzahrnuje kódujícíDNAa nekódujícíDNA,mitochondriálníDNAachloroplastovouDNA.
Thetermgenomewascreatedin1920byHansWinkler,professorofbotanyattheUniversityofHamburg,Germany.
Věda o studiu genomu se nazývá genomika.
Klasifikace
Viralgenom
Virální genom může být složen z RNA nebo DNA. Genomeof RNA viru obsahuje jednovláknovou nebo dvouvláknovou RNA a také jednu nebo více jednotlivých molekul RNA. úhlově lineární molekuly DNA, ale některé jsou složeny z virových genomů DNA složených z kruhových molekul DNA.
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.
Posloupnost kódování
TheDNAsequencethatcarriesthegeneticinformationofthesyntheticproteinisthecodingsequence.Theproportionofcodingsequencesinthegenomevariesgreatlyindifferentspecies.Largergenomesdonotnecessarilycontainmoregenes,andtheproportionofnon-repetitiveDNAincomplexeukaryotesdecreasesasthesizeofthegenomeincreases.SimpleeukaryotessuchasCaenorhabditiselegansandDrosophilahaveahigherproportionofcodingDNAthanrepetitiveDNA,whilethegenomesofmorecomplexeukaryotesareoftencomposedofrepetitiveDNA.TheproportionofrepeatedDNAinthegenomesofsomeplantsandamphibiansexceeds80%.Similarly,only2%ofthehumangenomeencodesDNA.
Nekódovací sekvence
Non-codingsequencesincludeintrons,non-codingRNAsequences,regulatoryDNAandrepetitiveDNA.98%ofthehumangenomebelongstonon-codingsequences.TherepetitiveDNAofthegenomehastandemrepetitivesequencesandscatteredrepetitivesequences.
Transpozony
Transposons(TEs)areDNAsequenceswithaspecificstructure.Theycanmovearoundthegenomewithoutafixedposition.ClassITEjumpsthroughthecopyandpastemechanism,andClassIITEisexcisedfromthegenomeandinsertedintoanewposition.
ThemovementofTEisthedrivingforcefortheevolutionofeukaryoticgenomes,becausetheirinsertioncandestroygenefunction,homologousrecombinationbetweenTEscanleadtogeneduplication,andTEcanalsocombineexonswithTheadjustmentsequenceisreorganizedtoanewposition.
Retrotranspozony
Retrotranspozony lze přepsat do RNA a poté zkopírovat do genomu na jiné místo. Retrotranspozony lze rozdělit do dvou kategorií: dlouhodobé opakování (LTR) a dlouhodobé opakování (bez LTR).
Velikost genomu
Velikost genomu je celkový počet párů bází DNA v kopii haploidgenomu.
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.