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In contrast, chemical or physical mutagenesis can generate simple sequence mutations, such as point mutations or small deletions and insertions, and could thus be used to dissect all functional domains of proteins by producing a variety of different phenotypes associated with a specific genetic locus.The major drawback in creating mutations by chemical or physical means is the ability to identify the locus responsible for the mutant phenotype because locus identification, unless limited to a few candidate genes (Mc Callum et al., 2000), entails a laborious process involving map-based cloning or chromosome walking.

The mutation in Now that the genome of Arabidopsis has been sequenced, this cruciferous plant has emerged as the preferred reference organism for understanding the function of unknown genes in higher plants.Gene duplications are believed to be responsible for generation of these two activation pathways, because C2 and Bf as well as C4 and C3 are homologous proteins.After identification of C3 c DNA from , a Japanese ascidian, reverse transcriptase–PCR amplification of hepatopancreas RNA was performed using primers encoding highly conserved amino acid sequences of the vertebrate Bf and C2 serine protease domain.The most common method for elucidating the function of unknown genes is to use various mutagenesis procedures, such as insertional mutagenesis (Feldmann, 1991; Koncz et al., 1992; Bancroft and Dean, 1993; Aarts et al., 1995), gene silencing (Baulcombe, 1996; Kooter et al., 1999), and physical or chemical mutagenesis (Redei and Koncz, 1992).Gene disruption by insertional mutagenesis can be accomplished by either transposon mutagenesis (Bancroft and Dean, 1993; Aarts et al., 1995) or Agrobacterium-mediated T-DNA transformation (Feldmann, 1991; Koncz et al., 1992).In a more sophisticated use of restriction endonucleases as tools to generate molecular markers, genomic DNA is digested with a restriction enzyme followed by linker addition and amplification with random sequence-tagged primers to yield amplified fragment length polymorphisms (AFLPs) (Thomas et al., 1995; Alonso-Blanco et al., 1998).

Multiple markers can be analyzed in a single lane of a sequencing gel.

Acknowledgement: based on the Catalog of Star-Forming Regions in the Galaxy ( Avedisova, V.

S., Astronomy Reports, Volume 46, Issue 3, March 2002, pp.193-205 [2002ARep...46..193A]). For more information, read The Avedisova catalog: A real Hitchhiker's Guide to the Galaxy?

To elucidate further the components and function of the pre-vertebrate complement system, we attempted to isolate an ascidian (urochordata) C3 convertase.

The C3 convertases of the classical and alternative pathways are composed of C2 and C4, Bf and C3, respectively.

Recently, a c DNA clone with significant sequence identity to vertebrate C3, C4, and C5 has been identified in the sea urchin, a marine invertebrate (4), placing the phylogenetic origin of the complement system in the invertebrate line.