DNA molecules are extremely long, thin and delicate. They are packaged so that they can fit in the nucleus and be protected from damage. DNA is attracted to clusters of histone proteins. A portion of the DNA wraps around a cluster twice. This occurs at regular intervals along the molecule. This shortens the DNA so it can fit within the nucleus. The chromosomes are still fairly long and their movement during cell division is facilitated by condensation. Following replication they loop and fold back on themselves forming tight packages of DNA and histone. This shortens and thickens the chromosomes so they are visible under a microscope at the beginning of prophase.