The nucleus houses the chromosomes which contain the genes, which direct protein synthesis. Eukaryotic chromosomes are composed of chromatin, a composite of protein and DNA. Within each chromosome a single strand of DNA is tightly bound to proteins called histones. The basic central dogma of gene expression begins with the transcription of DNA into mRNA. In eukaryotes the mRNA moves from the nucleus into the cytoplasm where translation occurs either free in the cytoplasm or on the rough ER. A ribosome only binds to the rough ER once it begins to synthesize a protein marked with a signal sequence, an address tag, which destines for sorting within the endomembrane system. The membrane of the rough endoplasmic reticulum is continuous with the outer layer of the nuclear envelope.