im J. HHRUHRT TAYLUK 



3 or 4 .u. A larjio ohromosomo would likewijio bo of a size so tliat one or 

 two setc? of coils in the ribbon formed by the ladder-like arrangement 

 of DXA helices would produce the structure seen with the light micro- 

 scope (Taylor. 1959d: Taylor ct ai. 1957 1. 



In G: each chromatid should consist of a single axial element made o\ 

 alternating H linkei-s and .V linkei's. DXA extracted at this stage should 

 be a very poor primer for DXA polymerase unless the chains in replicated 

 units were broken to provide growing points. By some stage of prophase 

 when half-chromatid exchanges can be induced the new set of H linkers 

 may be assumed to have formed and become st^ibilized so that each chro- 

 matid has two axial elements. By late anaphase or early telophase some 

 chromatids become separable into two subunits (in most chromosomes 

 which are twisted the plectonemic coiling of the two axial elements, 

 half-chromatids, prevents complete separation^ The separation can be 

 brought about by agents which would be expected to break hydrogen 

 bonds and perhaps denature proteins, but would not be expected to break 

 covalent bonds. This is the basis for the idea that the 3'R linkei-s may 

 be opened at this time and presumably replaced by regular 3' linkers at 

 a later st^*ige. If that is the correct interpretiition. the DX^A might now 

 be converted to a potentially active primer if the enzyme for opening 

 the regular 3' linkers were available. However, there is no assurance 

 that this is the time of the change even if the model is correct. We know 

 that diromosomes may undergo duplication without passing through 

 anaphase. However, in the cases studied there is an endomitotic reorgani- 

 zation of some sort before a second duplication occui"s. 



The model can be used to account for most or all of the presently 

 knouii properties of chromosomes. It is based on morphological and 

 biochemical evidence drawn from a variety of sources where information 

 in all cases is at best fragmentary. The morphological counterparts are 

 the lampbrush chromosomes and the structures found by Pappas and 

 Brandt (1960: and personal communication^ in the interphase nucleus 

 of amoeba (Fig. W. The structures consist of remarkably well- 

 preserved helical filaments clustered as if attached by one end to an 

 axial element. I interpret these to be G._. chromosomes of the type shown 

 in the model (Fig. lOV The helical filaments have a g\'re diameter of 

 about 3(XV3oO A and an over-all length of 3000-4000 A. The fiber wound 

 into this helix is about 120 A in diameter and has a periodicity indicating 

 that a smaller fiber of about 40 A is wound into a helix to form this 

 r20-A fiber. At certain loci the coiled filaments are separated enough to 

 show that some at least are double, i.e.. consists of two strands of lo- 

 20 A (Fig. 12V Measurements indicate that the ultimate helical filaments 

 would have a contour length of 3-4 «. Each of these two associated 



