This study describes an efficient microinjection procedure and a dominant genetic marker, *rol-6(su-1006)*, for *Caenorhabditis elegans* DNA transformation. The authors investigate the mechanism of DNA transformation in *C. elegans* by studying the formation, composition, and heritability of transgenic extrachromosomal elements. They find that large extrachromosomal arrays assemble directly from injected DNA molecules through homologous recombination, with the size of the array determining whether it is maintained extrachromosomally or lost. Low copy number extrachromosomal transformation can be achieved by adjusting the relative concentration of DNA molecules in the injection mixture. Integration of the injected DNA is relatively rare but can be achieved when single-stranded oligonucleotides are co-injected with double-stranded DNA. The study also explores the role of double-strand breaks in stimulating homologous recombination during array formation, and discusses the heritability and expression of transgenic sequences. The findings have practical applications for *C. elegans* transformation and provide insights into the cellular processes governing chromosome structure, replication, and partitioning.This study describes an efficient microinjection procedure and a dominant genetic marker, *rol-6(su-1006)*, for *Caenorhabditis elegans* DNA transformation. The authors investigate the mechanism of DNA transformation in *C. elegans* by studying the formation, composition, and heritability of transgenic extrachromosomal elements. They find that large extrachromosomal arrays assemble directly from injected DNA molecules through homologous recombination, with the size of the array determining whether it is maintained extrachromosomally or lost. Low copy number extrachromosomal transformation can be achieved by adjusting the relative concentration of DNA molecules in the injection mixture. Integration of the injected DNA is relatively rare but can be achieved when single-stranded oligonucleotides are co-injected with double-stranded DNA. The study also explores the role of double-strand breaks in stimulating homologous recombination during array formation, and discusses the heritability and expression of transgenic sequences. The findings have practical applications for *C. elegans* transformation and provide insights into the cellular processes governing chromosome structure, replication, and partitioning.