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function when present in transgene arrays, whether they are integrated or extrachro-
mosomal. The inclusion of complex DNA in the injection mixtures was found to
overcome this problem ( Kelly et al., 1997 ), although such transgene strains require
careful maintenance to avoid silencing. Microparticle bombardment, a technique
used for many years to make transgenic plant cells ( Sanford, 1989 ), was found to
Fig. 1 Basic strategies for marking transgenics and delivering DNA to C. elegans. (A) Transgenic
animals can be marked by an induced gain-of-function phenotype in a wild-type background, such as by
the presence of the rol-6D allele in the transgene array, or through rescue of a mutant to a wild-type
phenotype, as with rescue of unc-119 mutants (B). (C) Delivery of transgenes is achieved primarily by
microinjection, but also by microparticle bombardment and a modification of injection, Mos Single Copy
Insertion (MosSCI). Each approach produces a different spectrum of extrachromosomal and/or integrated
transgene types. Higher copy number arrays (generated by strategies in A and B) give higher transgene
expression, but can undergo silencing (particularly for maternally expressed genes), while lower copy
number transgenes (generated by strategies in C) show weaker expression that is less prone to silencing.
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