Deoxyribonucleic acid sequence organization in the genome of the dinoflagellate Crypthecodinium cohnii

Details of the general DNA sequence organization in the dinoflagellate Crypthecodinium cohnii have been obtained by using hydroxylapatite binding experiments, S1 nuclease digestion .and electron microscopy of reassociated DNA. It has been found that roughly half of the genome is made up of unique sequences interspersed with repeated sequence elements with a period of approximately 600 nucleotides. This class represents roughly 95% of the total number of interspersed unique elements in the genome. The remaining 5% are uninterrupted by repeated sequences for at least 4000 nucleotide pairs. The interspersed repeated elements are narrowly distributed in length with 80% under 300 nucleotide pairs in length. About half of the repeated DNA (20-30% of the genome) is not interspersed among unique sequences. The close spacing of the short repeats interspersed throughout much of the genome is consistent with the occurrence of the huge network structures observed in the electron microscope for low Cot reassociation of moderately long fragments. An unusual class of heteroduplexes was detected in the electron microscope which is believed to derive from the reassociation of repeated sequences from different families which are frequently found adjacent to one another in different locations in the genome. The occurrence of this novel arrangement of repeated sequences may reflect the unusual organization of the dinoflagellate nucleus. However, in most respects the sequence arrangement in this unicellular alga is very typical of higher plants and animals.