Somatic mitoses of Drosophila melanogaster

Abstract

AUTHOR'S ABSTRACTThe chromosomes of D. melanogaster have been followed through the entire cycle of mitosis in ganglion cells. Primary constrictions exist at about the middle of the second and third chromosomes, a t a sub-median position in the Y, at a sub-terminal in the X. .and probably also at a sub-terminal position in the fourth chromosome. Secondary constrictions, in addition to those reported by Bridges, are sub-terminal in the arms of the second and third chromosomes. A pronounced sub-median secondary constriction in the left limb of the second chromosome offers a criterion for distinguishing between the second and third chromosomes. and between right and left arms of the second chromosome. The proximal secondary constriction i n the X-chromosome and that in the short arm of the Y are associated with the normal development of the nucleolus. Anaphase chromosomes are longitudinally double, the split following the turns of the cbromonemata. Metaphaae and ansphase chromosomes are essentially isodiametric from end to end, except at the constrictions or indentures.Somatic pairing of chromosomes reaches its maximum expression during late prophases, both in male and female ganglion cells. The four chromatids of the tightly appressed homologues open out in two planes giving distinct chiasma-like configurations.Separation of the paired chromosomes is completed in late prophase, so that a t metaphase the homologous chromosomes are not in contact.

Mapping simple repeated DNA sequences in heterochromatin of Drosophila melanogaster

“…The amount of heterochromatin in this species is estimated at 30% from cytological measurements (see HEITZ 1933;KAUFMANN 1934). Our map of defined repeated sequences therefore represents a major proportion of DNA sequences in the heterochromatin of D. melanogaster.…”

“…In Drosophila melanogaster, approximately 30% of the genome is composed of heterochromatin. Heterochromatic regions of mitotic chromosomes include the proximal half of the X, the entire Y, one-quarter of the pericentromeric region of each major autosome, and three-quarters of the dot-like chromosome 4 (HEITZ 1933;KAUFMANN 1934; HANNAH 1951). Unlike the euchromatin, heterochromatin remains condensed throughout the cell cycle, and sister chromatids remain closely associated in late prophase.…”

Structure and Mode of Formation of the Nucleolus in Meristematic Cells of Vicia faba and Allium cepa

“…Nudeolar Organizer.--Since the classical work of Heitz (19) and McClintock (21) on the role of certain segments of chromosomes in the formation of nucleoli during telophase and interphase, many reports have appeared describing small chromatin inclusions (6,(38)(39)(40)(41)(42)(43) and Feulgen-positive nucleolonemata (44) within certain nucleoli and evidence has also been presented for the existence of chromatin threads attached to and sometimes traversing nucleoli (6,(38)(39)(40)45). These different observations bear out the fact that chromatin filaments are often intimately associated with the nucleolus.…”

Interpreting crossing over data

“…The latter are then scored cytologically and correlated with the genetic exchanges that follow chiasma abortions. The apparently contradictory observations that chiasma-like structures are found in mitotic prophases, and in the spermatocytes of Drosophila (KAUFMANN, 1934;COOPER, 1949) can be explained if chiasma initiation ( Fig. lb-c) takes place and persists into diplotene, but the intermediary events that lead to crossing over ( Fig.…”

“…Chiasmata usually bear a 1 : 1 relationship to crossovers (review RI-IOADES, 1961;JOHN & LEWIS, 1965;WHITE-HOUSE, 1969), but occasionally chiasma-like structures may be observed without crossing over (KAuFMANN, 1934;COOPER, 1949). Crossing over takes place at the 4-strand stage of meiosis between any two (nonsister ?)…”