Cytological evidence for switches in polarity of chromosomal DNA.

Abstract

From the types of ring chromosomes induced in x-irradiated Chinese hamster ovary (CHO) cells, we deduce the existence of switches in the polarity of chromosomal DNA; if there is a continuous DNA double helix along the full length of the chromosome then the polarity switches imply 3'-3' and 5'-5' phosphodiester linkages. The resolution of the method is such that we estimate that there is one polarity switch for every 109 normal 3'-5' phosphodiester bonds.In a previous communication (1) we presented data that we interpreted as indicating the presence of infrequent switches in the polarity of chromosomal DNA, i.e., the equivalent of 3'-3' opposite 5'-5' linkages. This was based on the types of ring chromosomes induced by x-irradiating Chinese hamster ovary (CHO) cells in the Gi stage of the cell cycle. When broken chromosomes rejoin, the way in which the polynucleotide strands of DNA can rejoin is restricted according to the polarity (3'-5') of the strands (2, 3 sulting tetraploid metaphases (1). In addition to the paired rings and single rings formed by the orthodox rejoining of the two broken ends of a chromosome fragment with each other (Fig. 1A and C), single rings may also be formed by isolocus rejoining, i.e., reunion of complementary subunits of the DNA molecule (Fig. 1B). The essence of harlequin staining is that when two rounds of DNA replication are carried out in the presence of bromodeoxyuridine (BrdUrd), one chromatid is unifilarly substituted and stains darkly with fluorescence plus Giemsa whereas its sister chromatid is bifilarly substituted and stains lightly. In such material sister chromatid exchanges can be identified clearly as exchanges between light and dark chromatids. Thus, it is possible not only to determine whether ring chromosomes are paired or single, but also to ascertain whether sister chromatid exchange is responsible for the conformations observed.Because the centromere provides a reference point that is readily identifiable, we confine our attention to centric rings; furthermore, most rings scored were probibly derived from the three large metacentric chromosomes in the CHO complement. Consider a centric fragment in which the broken ends are in regions of like polarity and rejoin in the orthodox manner to form a ring (Fig. 1A). If no sister chromatid exchange occurs, a pair of monocentric rings will be found in the tetraploid descendent cell (Fig. la); one chromatid of each ring will be darkly stained (Fig. 2a). A sister chromatid exchange during the first post-irradiation round of DNA replication gives rise to a single dicentric ring with a twin exchange. This ring will be a symmetrical dicentric, i.e., will have centromeres that are diametrically opposite one another. The twin exchange can be recognized because there are two label switches so that one half of each chromatid is darkly stained, and the positions of the two exchanges are trans-symmetrical with respect to the centromeres (Figs. la and 2a). If the centric fragment has broken ends in regions of opposite polari...

Classification of loops of lampbrush chromosomes according to the arrangement of transcriptional complexes

“…Spring et al 1974;Angelier & Lacroix, 1975). This may indicate either transcription from different DNA strands in different regions of the loop, or, in our opinion less likely, inversion of polarity within one loop (as to the occurrence of such switches in polarity, see Wolff, Lindsley & Peacock, 1976). (f) Interspersed, more or less extended, apparent spacer regions in the multigenic loops; these are found in a large proportion of loops (Figs.…”

Problems and paradigms: Chromosome reproduction: Units of DNA for segregation

“…Wolff et a1. (39) reported data from experiments with CHO cells in culture in which bromodeoxyuridine labeling was used to obtain high resolution staining by the FPG method. They found the ratio of twins to singles to be 34: 78.…”

Preuve Cytogenetique de la Production de Chromosomes Lineaires Remanies a Partir D'Un Chromosome Annulaire ChezPetunia HybridaHort

“…'evolution theorique de ces chromosomes particuliers ont d'ailleurs fait l'objet d'etudes mathematiques approfondies (LEJEUNE 1967(LEJEUNE , 1968. Par ailleurs, PEACOCK et al (1972) et WoLFF et al (1976 ont choisi des chromosomes annulaires, induits aux rayons X dans des cultures de cellules d'ovaires de Hamster chinois, pour realiser de tres fines etudes de genetique moleculaire concernant notamment les echanges entre chromatidessoeurs et les inversions de polarite dans Ia synthese du DNA. [Caryologia, Vol.…”