As part of their study, the authors examined working genes encoded in ribosomal DNA (rDNA). These genes encode instructions for the synthesis of proteins that form ribosomes, the cellular structures that ensure that all proteins needed by the organism are synthesised according to instructions encoded in RNA molecules. To synthesise a sufficient number of ribosomes, a cell needs a large amount of rDNA. Most genes in an organism are restricted to a single genetic localisation, but rDNA genes are repeated many times in different regions of the genome. In humans, for example, five chromosomes have regions of rDNA genes, with each region containing hundreds of repetitive copies. This genetic repetition allows the cell to synthesise enough protein material to form ribosomes. However, this multiplicity comes with its own set of challenges. During the process of cell division, cells can make mistakes when copying repetitive sections of DNA. As a result, some copies are lost, and with each division cycle they actually fall out of the chromosome. Previously, this mechanism was attributed exclusively to aging in single-celled yeast. However, the role of rDNA in the aging of multicellular organisms has so far remained a mystery. The authors analysed rDNA genes in stem cells of the seminal glands of Drosophila flies. These cells, known as germinal stem cells, are able to divide indefinitely to form a copy of the mother cell and sperm cell. In Drosophila, the rDNA gene chains are located on the X- and Y-chromosomes. The researchers found that older males have fewer rDNA genes on the Y chromosome than younger males. Moreover, it appears that this lack of rDNA is passed from generation to generation. Old males (older than 40 days) passed on Y-chromosomes with a reduced number of rDNA genes to their offspring. As a result, their male offspring had fewer copies of rDNA than individuals born from eggs fertilised by young males. However, the authors subsequently uncovered an unexpected mechanism. In many cases, the lost portion of rDNA spontaneously regenerated. By about 10 days of age, the number of rDNA genes in young males of both groups became comparable. This rejuvenation of rDNA in the germline stem cells of young males may be a critical mechanism to ensure that these cells retain their potential from generation to generation. To date, it is not yet clear whether something similar occurs in the ovaries. Although much further research is needed, Dr Yamashita suggests that a similar ‘reset’ may also occur in certain types of human cells, such as stem cells and cancer cells. Evgenia Ryabtseva, Eternal Youth portal http://vechnayamolodost.ru based on the article Lu et al. Transgenerational dynamics of rDNA copy number in Drosophila male germline stem cells, published in the journal eLife.