DNA has been extracted from 18 million-year-old fossilized leaves dating to the Miocene period, suggesting that it is an inherently stable molecule. This presents the possibility that DNA molecules from dead organisms or viruses in the environment can recombine with nuclear DNA, causing changes in genome structure and function. Therefore, early in evolutionary history organisms must have developed defense mechanisms to detect sequence duplication and to protect nuclear DNA from unpredictable and unscheduled changes. The introduction of transgenes has led to the recognition that cells have multiple mechanisms that monitor the arrangement and content of the genome, detect sequence duplications and silence them in vegetative or sexual phases, or both. A great advantage offered by fungi is the feasibility of constructing heterokaryons. The experimental results with heterokaryons suggest one possible silencing mechanism—a diffusible transgene product, presumably aberrant RNA, is recognized by an RNA-dependent RNA polymerase, leading to the production of antisense RNA, formation of double stranded RNAs and RNA degradation.
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