Everything about Paramutation totally explained
In
epigenetics,
paramutation is an interaction between two
alleles of a single
locus, resulting in a heritable change of one allele that's induced by the other allele. Paramutation violates Mendel’s first law, which states that in the process of the formation of the gametes (egg or sperm) the allelic pairs separate, one going to each gamete, and that each gene remains completely uninfluenced by the other. In paramutation an allele in one generation heritably affects the other allele in future generations, even if the allele causing the change is itself not transmitted. What may be transmitted in such a case are RNAs such as
piRNAs,
siRNAs,
miRNAs or other regulatory
RNAs. These are packaged in egg or sperm and cause paramutation upon transmission to the next generation. This means that RNA is a molecule of inheritance, just like DNA.
Paramutation was first discovered and studied in
maize (
Zea mays) by
R.A. Brink at the
University of Wisconsin-Madison in the
1950s. Brink noticed that specific weakly expressed alleles of the
red1 (r1) locus in maize, which confers red pigment to
corn kernels, can heritably change specific strongly expressed alleles to a weaker expression state. The weaker expression state adopted by the changed allele is heritable and can, in turn, change the expression state of other active alleles in a process termed
secondary paramutation. Brink showed that the influence of the paramutagenic allele could persist for many generations. However, since paramutation is an
epigenetic phenomenon, it can be attenuated over successive generations by dilution of the causative RNA. This contrasts sharply with ordinary mutations in
DNA, which don't fade away.
Interestingly, paramutation can result in a single allele of a gene controlling a spectrum of phenotypes. At
r1 in maize, for example, the weaker expression state adopted by an allele following paramutation can range from completely colorless to nearly fully-colored kernels. This is an exception to the rule that
continuous variation is controlled by many genes (see
multi-genic traits).
Allelic interactions similar to paramutation have since been reported in other organisms, including
tomato,
pea, and
mice.
The molecular basis of paramutation is being unraveled. Paramutation may share common mechanisms with other
epigenetic phenomena, such as
gene silencing,
genomic imprinting, and
transvection (genetics). Alleman (2006) proposed that "paramutation is RNA-directed. Stability of the chromatin states associated with paramutation and transposon silencing requires the mop1 gene, which encodes an RNA-dependent RNA polymerase." This polymerase is required to maintain a threshold level of the repeat RNA, which causes the paramutation. Exactly how the RNA does this isn't understood, but like other epigenetic changes, it involves a covalent modification of the DNA and/or the DNA-bound histone proteins without changing the DNA sequence of the gene itself.
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