Chromosomes are made of long thin molecules of DNA coiled around packaging proteins called histones.The histones bind to and condense the DNA molecule to maintain its integrity.The DNA and histone complex is in the nucleus and is called chromatin.Normally, chromosomes are visible under a light microscope only during cell division, where all chromosomes are in their condensed form. Before this stage occurs, each chromosome is duplicated in the S phase of interphase, and the resulting two sister chromatids are joined by a centromere.Prokaryotes do not have nuclei. Instead, their DNA is organised into a structure called the nucleoid. Some bacteria also contain plasmids or other extrachromosomal DNA. These are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer.Bacterial chromosomes tend to be tethered to the plasma membrane of the bacteria. Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally supercoiled.