68 J. HERBERT TAYLOR 



1)1)11(18 as well us (k'nalui'c pi'otcins, hut would not he expected to break 

 ('o\'alent bonds. 



At the end of anajjhase theic may be a further eondensation of the 

 chromosomes but foUowinji; this phase they expand somewhat in telophase 

 and the coiled chromonemata become less tightly coiled. The chromo- 

 somes then ai)pear to fuse and nearly fill the refoi'iued inteiphase 

 nucleus. As tiie nuclei grow, and particularly in cells that are not to 

 divide again, the chromosomes may not continue to fill the nucleus. Their 

 disposition in most interphase nuclei is difficult to follow either by the 

 use of the light microscope or the electron microscope. 



In dividing cells there is typically a gap (G'l) of a few hours before 

 DNA replication begins (see Chapter IV for further details). Synthesis 

 (*S period) then occurs over a period of several hours followed by 

 another gap (G2) before the next division. 



By the earliest pro])hase stages at which chromosomes can be dis- 

 tinguished, they can be seen to consist of two chromatids which continue 

 to shorten during prophase. The chromonemata can be seen to be helically 

 coiled in some types of cells. The gyres increase in diameter and their 

 number apparently decreases during prophase as the chromatids shorten. 



The coils usually show best in meiotic chromosomes. In addition, the 

 chromomeres of the classical cytologists, which have sometimes been 

 equated with poorly resolved coils, are most striking at the prophase of 

 the first meiotic division. Although chromomere patterns along the 

 chromonemata vary in cells at different stages of development or in 

 different cells in a species (Lima-de-Faria et al., 1959), the mapping of 

 pachytene and dijilotene chromosomes indicates that chromomeres are 

 significant structural features comparable to the bands along the giant 

 chromosomes of Diptera. The ultimate shape and dimensions of chro- 

 monemata, in other words, the packing pattern of the nucleoprotein in 

 the chromosomes, is still largely unknown. Investigation is difficult 

 because the structural details are generally below the level of resolution 

 of the light microscope and often above the size level at which the thin 

 sections required for resolution in electron microscopy can give a com- 

 plete picture. An additional difficulty appears to be an instability of any 

 tertiary structure in the nucleoprotein com))onents of chi'omosomes. 



B. GIANT CHROMOSOMES 



From a study of structures such as prophase and anaphase chro- 

 mosomes one would easily reach the conclusion that there are many 

 strands along their axes at the molecular level. A chromonema with a 

 diameter of 0.1-0.3 /j, is several orders of magnitude larger than the 

 extended nucleohistone fiber of 30 A. However, in a few special cases 



