The human genome is made up of 23 pairs of chromosomes, the biological blueprints that make humans … well, human. But it turns out that some of our DNA — about 8% — are the remnants of ancient viruses ...

The images show the inside of the cell nucleus at an 8-cell stage (left) and at a later stage (right). Chromosome 19 is shown in red whereas the nucleolus containing the gene-silencing machinery is in ...