The Time and Mode of Duplication of Chromosomes

Abstract

DNA (deoxyribonucleic acid) synthesis is usually limited to a part of interphase, but may occur at early, middle or late inter-phase. DNA synthesis and chromosome duplication, as measured by radiation breakage, occur simultaneously. By utilizing tritium-labeled thymidine, the original and a new DNA of a duplicating chromosome were shown to be distinct entities. The new DNA is equally distributed to the 2 daughter chromosomes and it must be assumed that the original DNA is likewise equally distributed. When these labeled chromosomes duplicate in the absence of labeled precursors, they produce one daughter chromosome with all of the isotope and one completely free of label except for an occasional sister strand exchange. This behavior supports the idea that replication of DNA follows the scheme proposed by Watson and Crick. A model chromosome with an axis composed of a double ribbon, with DNA double helices attached only as side chains, is proposed. The double helices are attached so that the two chains of each double helix are attached to the edges of different ribbons in the axis. These ribbons with their attached DNA chains separate during duplication and are the units revealed by the tritium labeling.