The End of Genes: Routine Test Reveals Unique Divergence in Genetic Code
Published:04 Jan.2024 Source:Earlham Institute
Scientists testing a new method of sequencing single cells have unexpectedly changed our understanding of the rules of genetics. Dr Jamie McGowan, a postdoctoral scientist at the Earlham Institute, analysed the genome sequence of a microscopic organism -- a protist -- isolated from a freshwater pond at Oxford University Parks. The work was intended to test a DNA sequencing pipeline to work with very small amounts of DNA, such as DNA from a single cell. But, when researchers looked at the genetic code, the protist Oligohymenophorea sp. PL0344 turned out to be a novel species with an unlikely change in how its DNA is translated into proteins.
Oligohymenophorea sp. PL0344 is a ciliate. These swimming protists can be seen with a microscope and are found almost anywhere there is water. Ciliates are hotspots for genetic code changes, including reassignment of one or more stop codons -- the codons TAA, TAG, and TGA. In virtually all organisms, these three stop codons are used to signal the end of a gene. Variations in the genetic code are extremely rare. Among the few variants of the genetic code reported to date, the codons TAA and TAG virtually always have the same translation, suggesting that their evolution is coupled. DNA is like a blueprint of a building. It does not do anything in and of itself -- it provides instructions for work to be done. In order for a gene to have an impact, the blueprint must be "read" and then built into a molecule which has a physical effect.
For DNA to be read, it is first transcribed into an RNA copy. This copy is taken to another area of the cell where it is translated into amino acids, which are combined to make a three-dimensional molecule. The translation process starts at the DNA start codon (ATG) and finishes at a stop codon (normally TAA, TAG, or TGA). In Oligohymenophorea sp. PL0344, only TGA functions as a stop codon. "This is extremely unusual," Dr McGowan said. "We're not aware of any other case where these stop codons are linked to two different amino acids. It breaks some of the rules we thought we knew about gene translation -- these two codons were thought to be coupled. Scientists attempt to engineer new genetic codes -- but they are also out there in nature. There are fascinating things we can find, if we look for them.