Scanning electron micrograph of budding cells of baker's yeast, Saccharomyces cerevisiae. The picture shows cells about 4 microns long, most of which are in process of producing a bud (orange). S. cerevisiae is eukaryotic - each cell contains a membrane-bound nucleus, the site of the chromosomes containing the cell's DNA. Cell division (mitosis) involves the replication of the chromosomes, then their separation to form two nuclei. As this takes place, a bud develops from the mother cell wall, as here. When sufficiently large, the bud receives one of the new nuclei. The pore between mother and bud closes, and the bud separates to become an independent cell. S. cerevisiae is a model organism for genetic studies. It was the first eukaryotic organism to have its genome sequenced; comprising just over 12 million base pairs and 6000 genes. It is widely used in genetic studies of ageing.