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Extra DNA causes Mendel's peas to pucker.

Extra DNA causes Mendel's peas to pucker

Genetic research has added a new wrinkle on an old pea. More than a century after Gregor Mendel crossed his round and wrinkled peas, British geneticists have cloned the enzyme-encoding gene that ultimately determines the shapes so painstakingly recorded by the Austrian monk. In pinpointing the gene's chromosomal location, or locus, they have discovered that the wrinkling trail stems from an extra piece of DNA, which prevents the gene from directing proper starch synthesis.

The new work, conducted at the John Innes Institute in Norwich, confirms that the chromosomal locus r houses the gene coding for the production of starch-branching enzyme 1 (SB1). This enzyme, found in all plants, converts amylose a simple starch of linear construction, into a "branched" starch called amylopectin.

Scientists have known for some time that the ratio of these starches in peas and other plants influences other elements of their composition, says Alison M. Smith, who coauthored the report in the Jan. 12 CELL. Round seeds (RR or Rr) contain a much higher ratio of amylopectin to amylose than do wrinkled seeds (rr), suggesting the enzyme doesn't function properly in wrinkled peas. when a plant's starch conversion is impaired, sucrose and water build up in the young seeds. Maturing seeds lose much of this water, and the shrinkage leaves them wrinkled.

When the researchers cloned the gene, they found it was always larger in wrinkled seeds than in round seeds. This, says Smith, indicates that wrinkled seeds carry a gene with an extra insertion of DNA that leaves the plants without an efficient means of starch conversion. The insertion occurs in the part of the gene that codes for the SBE1, thus garbling the DNA message for SBE1 production.

Now that scientists understand how the r locus influences starch production, they hope to modify the chemical structure of plant starches to improve the quality of prepackaged vegetables. Frozen-food packagers often add starch to maintain a vegetable's structure, Smith notes, but the food tends to lose water as it thaws, leaving consumers with a drippy mess.
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Title Annotation:Gregor Mendel
Author:Decker, C.
Publication:Science News
Date:Feb 10, 1990
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