Genome
In biology,genomean organism iscomplete DNA sequenceone setchromosomes;example, one oftwo sets thatdiploid individual carriesevery somatic cell. When people say thatgenome ofsexually reproducing species has been "sequenced," typically theyreferring todetermination ofsequencesone setautosomesoneeach typesex chromosome, which together represent both ofpossible sexes. Evenspecies that existonly one sex, whatdescribed as "a genome sequence" may becomposite fromchromosomesvarious individuals.
Most biological entities more complex thanvirus sometimes or always carry additional genetic material besides that which residestheir chromosomes. In some contexts, such as sequencinggenome ofpathogenic microbe, "genome"meantinclude this auxiliary material, whichcarriedplasmids. In such circumstances then, "genome" describes all ofgenesnon-coding DNA that havepotentialbe present.
In vertebrates such as humans, however, "genome" carriestypical connotationonly chromosomal DNA. So although human mitochondria contain genes, these genesnot considered part ofgenome. In fact, mitochondriasometimes saidhave their own genome, often referredas"mitochondrial genome".
Note thatgenome does not capturegenetic diversity orgenetic polymorphism ofspecies. For example,human genome sequenceprinciple could be determined from just halfDNAone cell from one individual. To learn what variationsDNA underlie particular traits or diseases requires comparisons across individuals. This point explainscommon usage"genome" (which parallelscommon usage"gene")refer notany particular DNA sequence, but towhole familysequences that sharebiological context.
Although this concept may seem counter intuitive,issame concept that says thereno particular shape that isshape ofcheetah. Cheetahs vary,so dosequencestheir genomes. Yet bothindividual animalstheir sequences share commonalities, so one can may learn something about cheetahs"cheetah-ness" fromsingle exampleeither.
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2 Comparison ofsizesdifferent genomes 3 Genome evolution |
Genome projects
The Human Genome Project was organizedmap andsequencehuman genome. Other genome projects include mouse, rice,plant Arabidopsis,puffer fish, bacteria like E. coli, etc.Compare: proteome
Comparison ofsizesdifferent genomes
| Organism | Genome size (base pairs) |
|---|---|
| Phage λ | 5×104 |
| E. coli | 4×106 |
| Yeast | 2×107 |
| C. elegans | 8×107 |
| Drosophila melanogaster | 2×108 |
| Human | 3×109 |
Genome evolution
Genomesmore thansuman organism's geneshave traits that may be measuredstudied without reference todetailsany particular genestheir products. Researchers compare traits such as chromosome number, chromosome size, gene order, codon usage bias,G-C contentdetermine what mechanisms could have producedgreat varietygenomes that exist today.Duplications playmajor roleshapinggenome. Duplications may range from extensionshort tandem repeats,duplication ofclustergenes,allwayduplicationsentire chromosomes or even entire genomes. Such duplicationsprobably fundamental tocreationgenetic novelty.
Horizontal gene transferinvokedexplain how thereoften extreme similarity between small portions ofgenomestwo organisms thatotherwise very distantly related. Horizontal gene transfer seemsbe common among many microbes. Also, eukaryotic cells seemhave experiencedtransfersome genetic material from their chloroplastmitochondrial genomestheir nuclear chromosomes.
See also: molecular evolution - evolution - listgene families
