Barbara McClintock’s report that bits of DNA could jump around and integrate themselves into new loci in DNA was so dramatic and arcane that many thought the phenomenon was either a one-off, or not real! Only with the subsequent discovery of transposons in bacteria (and in other eukaryotes) were McClintock’s jumping genes finally recognized for what they were! As we describe her experiments, keep in mind that McClintock’s research and intuitions about gene regulation and epigenetic inheritance came long before molecular technologies made it possible to prove and give names to these phenomena. To begin our tale of transposons, look at the illustration of maize reproduction below.
The different colors of corn seeds (ker nels) result from anthocyanin pigments that are expressed differentially by cells of the aleurone tissue. Mclintock was studying the inheritance of color variation, which ranged from colorless (white or yellow due to an absence of anthocyanins) to brown, purple, spotted or streaked.
The mosaic of kernel colors are vividly shown the corncobs in the photograph below
Clearly, kernel color is inherited. The inheritance of colorless and purple seed color did indeed follow Mendelian rules, but the genetics of mosaicism did not. Mosaic color patterns after genetic crosses were not consistent, implying that the mutations responsible for kernel color were not due to mutations in germ cells. Rather, genes controlling anthocyanin synthesis must be undergoing mutations in somatic cells that would become (or already were) the ones in which the pigments were produced.
Credit: to owner
Comments