Scientists discover role for 'junk' DNA

Scientists have determined how satellite DNA, previously considered to be "junk", plays a crucial role in holding the genome together.

The findings, published in the journal eLife, indicate that this genetic "junk" performs the vital function of ensuring that chromosomes bundle correctly inside the cell's nucleus, which is necessary for cell survival.

This function appears to be conserved across many species, according to the researchers at the University of Michigan and the Howard Hughes Medical Institute in the US.

This pericentromeric satellite DNA consists of a very simple, highly repetitive sequence of genetic code.

Although it accounts for a substantial portion of our genome, satellite DNA does not contain instructions for making any specific proteins.

Its repetitive nature is thought to make the genome less stable and more susceptible to damage or disease. Until recently, scientists believed this so-called "junk" or "selfish" DNA did not serve any real purpose.

"But we were not quite convinced by the idea that this is just genomic junk," said Yukiko Yamashita, research professor at the Michigan and lead author on the study.

"If we don't actively need it, and if not having it would give us an advantage, then evolution probably would have gotten rid of it. But that hasn't happened," said Yamashita.

Researchers decided to see what would happen if cells could not use this pericentromeric satellite DNA.

Since it exists in long, repetitive sequences, they could not simply mutate or cut the entire satellite DNA out of the genome. Instead, they approached the question through D1, a protein known to bind to satellite DNA.

The researchers removed D1 from the cells of a commonly used model organism, Drosophila melanogaster (fruit flies).

The team quickly noticed that germ cells - the cells that ultimately develop into sperm or eggs - were dying.

Further analysis revealed that the dying cells were forming micro-nuclei, or tiny buds, outside the nucleus that included pieces of the genome, researchers said.

Without the entire genome encapsulated in the nucleus, the cells could not survive, they said.

The researchers believe that the D1 protein binds onto the satellite DNA to pull all of the chromosomes together in the nucleus.

If the D1 protein cannot grab the satellite DNA, the cell loses its ability to form a complete nucleus and ultimately dies.