Mitochondria also had a similar origin, but chloroplasts are found only in plants and protista. In green plants, chloroplasts are surrounded by two lipid-bilayer membranes. They are thought to correspond to the outer and inner membranes of the ancestral cyanobacterium. Chloroplasts have their own genome, which is much smaller than that of free-living cyanobacteria. The DNA which remains shows clear similarities with the cyanobacterial genome. Plastids may contain 60–100 genes whereas cyanobacteria often contain more than 1500 genes. Many of the missing genes are encoded in the nuclear genome of the host. 