Understanding Genomes: Structure, Variation, Evolution

Definition and scope of genome

In molecular biology and genetics, a genome is the complete set of genetic information carried by an organism. It is typically defined as the DNA molecules (or RNA in RNA viruses) that contain the organism’s genetic instructions, including protein-coding genes, non-coding genes, regulatory regions, and often a large fraction of repetitive or “junk” DNA whose function may not be fully understood. The scope of the term can vary depending on the organism and what molecules are included. For example, bacteria usually have one or two large chromosomal DNA molecules that contain essential genetic material, but they may also carry additional extrachromosomal plasmids; in scientific usage, “genome” often refers mainly to the large chromosomal DNA. In eukaryotes, genomes are more complex to define because nuclear chromosomes coexist with additional DNA in organelles such as mitochondria (mitochondrial genome) and chloroplasts (chloroplast genome). As a result, many references to “the human genome” or other eukaryotic genomes commonly mean the nuclear genome only. Because many eukaryotes are diploid, the “genome” may be interpreted as one copy of each chromosome (even though cells contain two copies), and technical definitions must account for both sex chromosomes when present. Genome scope also extends to how genomes are studied: genome sequencing produces the complete nucleotide list for all chromosomes in an individual or species, and genome projects and annotations help characterize genetic diversity and structure.

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Origin of the term genome

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