An extrachromosomal array is a method for

mosaic analysis Mosaicism or genetic mosaicism is a condition in multicellular organisms in which a single organism possesses more than one genetic line as the result of genetic mutation. This means that various genetic lines resulted from a single fertilized e ...

genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar worki ...

. It is a

cosmid A cosmid is a type of hybrid plasmid that contains a Lambda phage ''cos'' sequence. They are often used as a cloning vector in genetic engineering. Cosmids can be used to build genomic libraries. They were first described by Collins and Hohn in 1 ...

, and contains two functioning (

wild-type The wild type (WT) is the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard "normal" allele at a locus, in contrast to that produced by a non-standard, "m ...

) closely linked

genes In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

: a ''gene of interest'' and a ''mosaic marker''. Such an array is injected into

germ line In biology and genetics, the germline is the population of a multicellular organism's cells that pass on their genetic material to the progeny (offspring). In other words, they are the cells that form the egg, sperm and the fertilised egg. They ...

cells, which already contain mutant (specifically,

loss of function In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosi ...

)

alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...

of all three genes in their

chromosomal DNA A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...

. The cosmid, which is not packed correctly during

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maint ...

, is occasionally present in only one daughter cell following cell division. The daughter cell containing the array expresses the gene of interest; the cell lacking the array does not. The ''mosaic marker'' is a gene which exhibits a visible

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prop ...

change between the functioning and non-functioning alleles. For example, ''ncl-1'', located in chromosomal DNA, exhibits a larger

nucleolus The nucleolus (, plural: nucleoli ) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis, which is the synthesis of ribosomes. The nucleolus also participates in the formation of s ...

than the wild-type allele, which is in the array. Thus, cells which exhibit larger nucleoli have usually not retained the extrachromosomal array. The ''gene of interest'' is the target of the mosaic analysis. Cells lacking the extrachromosomal array also lack the functional gene of interest. Cells which develop normally without the array do not require the gene of interest for normal function. Cells which do not develop normally are said to require the gene. In this way, those cell lineages which require a specific gene can be identified. Extrachromosomal arrays replace an earlier technique involving a duplicated piece of chromosome called a ''free duplication''. The latter technique required that the gene of interest and the mosaic marker be closely linked on the duplication; the former allows free choice of mosaic marker and target gene.

References

Miller LM, Waring DA, Kim SK (1996). Mosaic analysis using a ''ncl-1'' (+) extrachromosomal array reveals that ''lin-31'' acts in the Pn.p cells during ''Caenorhabditis elegans'' vulval development. ''Genetics 143'' (3): 1181-1191. Genetics {{genetics-stub