Though separated from one another within the cell, nuclear genes and those of mitochondria and chloroplasts can affect each other in a number of ways. Nuclear genes play major roles in the expression of chloroplast genes and mitochondrial genes. Additionally, gene products of mitochondria can themselves affect the expression of genes within the cell nucleus. This can be done through metabolites as well as through certain peptides trans-locating from the mitochondria to the nucleus, where they can then affect gene expression.