Only half of a chromosome is DNA, study finds. DNA makes up only half of the material inside chromosomes – far less than was previously thought – a study has revealed. Only half of a chromosome is DNA, study finds. Up to 47 per cent of their structure is a mysterious sheath that surrounds the genetic material, researchers say. While the precise function of this sheath is unknown, researchers suggest it may keep chromosomes isolated from one another during the key process of cell division.