Editing Genome (section)

== Definition ==
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The term "genome" usually refers to the DNA (or sometimes RNA) molecules that carry the genetic information in an organism, but sometimes it is uncertain which molecules to include; for example, bacteria usually have one or two large DNA molecules ([[chromosomes]]) that contain all of the essential genetic material but they also contain smaller extrachromosomal [[plasmid]] molecules that carry important genetic information. In the scientific literature, the term 'genome' usually refers to the large chromosomal DNA molecules in bacteria.<ref>{{cite journal |vauthors = Kirchberger PC, Schmidt ML, and Ochman H |date = 2020 |title = The ingenuity of bacterial genomes |journal = Annual Review of Microbiology |volume = 74 |pages = 815–834 |doi = 10.1146/annurev-micro-020518-115822|pmid = 32692614 |s2cid = 220699395 }}</ref>

===Nuclear genome===
Eukaryotic genomes are even more difficult to define because almost all eukaryotic species contain nuclear chromosomes plus extra DNA molecules in the [[Mitochondrion|mitochondria]]. In addition, algae and plants have [[chloroplast]] DNA. Most textbooks make a distinction between the nuclear genome and the organelle (mitochondria and chloroplast) genomes so when they speak of, say, the human genome, they are only referring to the genetic material in the nucleus.<ref name = Graur /><ref>{{ cite book |vauthors = Brown, TA |date = 2018 |title = Genomes 4 |publisher = Garland Science |place = New York, NY, USA |isbn = 9780815345084}}</ref> This is the most common use of 'genome' in the scientific literature.

===Ploidy===
Most eukaryotes are [[Ploidy|diploid]], meaning that there are two of each chromosome in the nucleus but the 'genome' refers to only one copy of each chromosome. Some eukaryotes have distinctive sex chromosomes, such as the X and Y chromosomes of mammals, so the technical definition of the genome must include both copies of the sex chromosomes. For example, the standard reference genome of humans consists of one copy of each of the 22 autosomes plus one X chromosome and one Y chromosome.<ref>{{cite web |url = https://useast.ensembl.org/Homo_sapiens/Info/Annotation |title = Ensembl Human Assembly and gene annotation (GRCh38) |publisher = Ensembl |access-date = May 30, 2022}}</ref>