CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 181 



A brief summary of the principal current hypotheses will indicate the 

 present state of the knowledge. 



1. The multistrand hypothesis proposes that the chromosome is a 

 bundle of fibrils, each about 100 A in diameter, and associated in pairs 

 of pairs, so that the interphase chromosome before duplication consists 

 of 32 fibrils. The 100 A unit itself, which has been seen in cross section 

 by many investigators, is considered to consist of 2 or 4 DNA double 

 helices with associated protein. X-ray diffraction studies of intact sperm 

 heads and isolated nuclei have indicated the presence of a nucleo- 

 protein unit some 20 A in diameter. How many of these units are present 

 in the 100 A fibril is not established experimentally as yet. 



Doubleness of strands has been reported by cytologists at all levels 

 between the 100 A fibril and the prophase chromosome, but some of 

 these observations may represent chance associations. 



Support for the multistrand hypothesis comes not only from cytologi- 

 cal observations but also from the logistic problem of packing tremen- 

 dous lengths of DNA into short, fat chromosomes. Objections come 

 first from genetic evidence, in which the segregation of new mutations 

 and the 1 : 1 segregation of alleles are difficult to explain on a multistrand 

 hypothesis, and from experiments with tritium-labeling of chromosomes 

 (p. 191) in which the segregation of DNA subunits can be most simply 

 visualized in terms of a duplex rather than a multiplex structure. 



2. According to the single-strand hypothesis, the chromosome consists 

 of a linear array of DNA molecules (about 10'^ in bacteria; as many as 

 10^ in some chromosomes) combined with various proteins. When fully 

 extended, it is at most 100 A in diameter, and millimeters or even centi- 

 meters in length. It is capable of tremendous contraction by coiling 

 or folding. 



Cytological evidence in support of a single-strand hypothesis comes 

 from correlated light and electron microscope studies of "lampbrush" 

 chromosomes, found in meiotic prophase in amphibian eggs. These 

 giant chromosomes, which attain lengths of more than a millimeter, con- 

 sist of a central axis, from which pairs of loops project at right angles. 

 The central axis consists of a row of granules (chromomeres) held to- 

 gether by a fine strand. This axis is Feulgen-positive, readily dispersed 

 by the enzyme DNA-ase, but relatively resistant to dilute saline, ribo- 

 nuclease, and the proteolytic enzymes, pepsin and trypsin. The loops 

 also contain DNA and are heavily coated with RNA and proteins, which 

 can be removed enzymatically, leaving a delicate core less than 500 A 

 in diameter. Figure 7.1 shows a drawing of a section of lampbrush 

 chromosome as seen with the light microscope, together with a schematic 



