Microsatellites arise from differences in the length of a tandemly repeated sequence between two restriction sites. Differences in repeat length arise from slippage during replication and recombination. These multiallelic polymorphisms are highly informative for DNA mapping in both gene cosegregation and forensic DNA fingerprinting. In Situ Hybridization Radioactive probes can be used to localize the position of a gene on metaphase chromosomes. Mostly used in simple organisms like Drosophila , it is becoming a prelude to positional cloning in more complex species.
Chromosome painting is a grosser use of the fluorescent localization technique.
Application for Downs syndrome is an indication of the potential for this technique in diagnosis. Specific dyes for regions of a chromosome to extend fluorescent analysis to intrachromosomal rearrangements - called the chromosomal rainbow display. Replicates independently of host chromosome Has physical or chemical selection mechanism Sites for insertion of foreign DNA Plasmids Small extrachromosomal circular DNA molecules that replicate independently in bacteria. Usually express antibiotic resistance gene. New generation plasmids contain high copy number replication origin, leading to 500-700 molecules per cell, and an engineered polylinker containing multiple unique cloning sites.