Researchers at the Institute of Life Sciences at the University of Michigan and Howard Hughes Medical Institute have identified how satellite DNA, considered "junk DNA," plays a critical role in genome assembly. The findings, published in the journal eLife, show that genetic junk plays a vital role in ensuring that chromosomes are correctly positioned within the cell nucleus, which is essential for cell survival. And this function seems to have been preserved in many species.
Why is junk DNA needed?
Pericentromeric satellite DNA consists of a very simple and often repetitive sequence of genetic code. And although it accounts for a significant portion of our genome, satellite DNA does not contain any specific instructions for making any specific proteins. Moreover, its repetitive nature was thought to make the genome less stable and more vulnerable to injury or disease. Until recently, scientists believed that "junk" or "selfish" DNA had no role or purpose.
“We couldn't just come to terms with the idea that this was just genomic junk,” says Yukiko Yamashita, a research professor at LSI and lead author of the study. “If we don’t need it and we don’t get anything from it, evolution would surely get rid of it. But that did not happen.
Yamashita and her colleagues decided to see what would happen if these cells did not use pericentromeric satellite DNA. Because it exists as long, repetitive sequences, scientists cannot simply mutate or cut out all of the DNA from the genome. Instead, they assigned the D1 protein, which binds to satellite DNA.
Scientists have removed D1 from the cells of a widely used test organism, Drosophila melanogaster (fruit fly). And they immediately discovered that the germ cells - which usually turn into sperm or eggs - die.
Further analysis showed that the dying cells formed micronuclei, or tiny buds outside the nucleus, that included parts of the genome. Without the entire genome encapsulated in the nucleus, cells could not survive.
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Scientists believe that the D1 protein binds to satellite DNA, assembling all chromosomes in the nucleus. If the D1 protein is unable to capture satellite DNA, the cell loses its ability to form a complete nucleus and dies. Further testing showed that satellite DNA is essential for cell survival in a variety of species that incorporate DNA into the nucleus, including humans.
Ilya Khel
