An organism may be either a prokaryote or a eukaryote. Multicellular organisms then evolved from the eukaryote. It is a unicellular eukaryote, whose cells are rod-shaped. Binary fission is similar to eukaryote cell reproduction that involves mitosis. Mitochondria are organelles found in all but one eukaryote. A karyotype is the characteristic chromosome complement of a eukaryote species. Mitochondria provide energy to the eukaryote cell by converting sugars into ATP. In general, the karyotype is the characteristic chromosome complement of a eukaryote species. Prasinophyte algae of the genus "Ostreococcus" are the smallest free-living eukaryote. DNA methylation was probably present at some extent in very early eukaryote ancestors. Cholesterol also occurs naturally in other eukaryote cell membranes. An rRNA analyses supports the eocyte scenario, apparently with the Eukaryote root in Excavata. The first sirtuin was identified in yeast (a lower eukaryote) and named sir2. This single cell eukaryote is nonpathogenic and easily grown and manipulated in the lab. No known eukaryote can survive over .