A Cycloid Model for the Chromosome

1967 ◽  
Vol 2 (1) ◽  
pp. 9-22
Author(s):  
H. L. K. WHITEHOUSE
Keyword(s):  
Salivary Gland ◽  
Messenger Rna ◽  
Nucleolar Organizer ◽  
Duplicate Genes ◽  
Crossing Over ◽  
Lampbrush Chromosomes ◽  
Circular Dna ◽  
Dna Molecule ◽  
Amphibian Oocyte ◽  
Identical Nucleotide

Investigations of lampbrush chromosomes and the nucleolar organizer have suggested that each gene may be duplicated many times in consecutive linear series within one DNA molecule. This conclusion is in direct conflict with recombination data which indicate, not only that each gene is represented only once per chromatid, but that different genes are contiguous. This paradox is resolved by postulating that the chromosome has the form of a cycloid. Each loop of the cycloid would correspond to a set of copies of a gene forming a chromomere. It is suggested that at meiosis the copies of the gene are detached as a result of intrachromatid crossing-over between the first and last members of the series. The master copy remaining in the chromatid would then be in a position to undergo crossing-over with a homologous chromatid, while the duplicate copies in the detached chromomere would all be included in a single circular DNA molecule. They could subsequently be restored to the chromatid by crossing-over between one of their number and the master copy. This intrachromatid crossing-over would imply that the chromosome can alternate between two states with each set of duplicate genes either detached as a circle or integrated with the DNA axis. Callan's model for matching slave genes against a master copy so that all acquire identical nucleotide sequences is modified to facilitate coiling and uncoiling of nucleotide chains, by postulating breakage of the matching chains at one end of the gene. Matching of only one chain of the slaves against the master is proposed or, if necessary, subsequent matching of the second slave chain to the first. It is suggested that matching may regularly precede the synthesis of messenger RNA. Investigations of dipteran salivary gland chromosomes and amphibian oocyte nucleoli have established that the chromomere is the unit of replication of the chromosome. On the cycloid model the replicons would be adjacent to one another, and each would comprise a master gene and all the copies. It is suggested that the replicator may correspond to the operator of the master copy of the gene. This hypothesis provides an explanation for several previously unexplained features of crossing-over, including its occurrence at the four-strand stage.

The role of Lampbrush Chromosomes in the formation of Nucleoli in Amphibian Oocytes

1965 ◽  
Vol s3-106 (75) ◽  
pp. 215-228
Author(s):  
H. C. MACGREGOR
Keyword(s):  
Phase Contrast ◽  
Nucleolar Organizer ◽  
Plethodon Cinereus ◽  
Oocyte Nucleus ◽  
Lampbrush Chromosomes ◽  
Triturus Cristatus ◽  
Amphibian Oocyte ◽  
Ambystoma Jeffersonianum ◽  
And Function

Theories concerning the mode of origin of peripheral nucleoli in amphibian oocytes have been examined and tested. In Triturus cristatus the giant fusing loops of the 3 shortest lampbrush bivalents resemble nucleoli when viewed in phase contrast and may be considered as possible sites of production of nucleoli. Giant fusing loops, however, differ from peripheral nucleoli in certain important respects, and animals lacking giant fusing loops on their lampbrush chromosomes nevertheless have normal peripheral nucleoli. Therefore, similarity in appearance between objects attached to lampbrush chromosomes and free peripheral nucleoli may not be significant. In oocytes of T. c. carnifex, T. c. karelinii, and T. c. danubialis, peripheral nucleoli do not increase in number during the lampbrush phase of oogenesis, except by division of pre-existing nucleoli towards the end of oogenesis. There are about 1,000 nucleoli per oocyte nucleus in each of these sub-species. In T. c. cristatus there are more nucleoli in large oocytes than in small ones, and it seems likely that in this sub-species the giant fusing loops add to the existing population of nucleoli in an oocyte by successively growing and shedding new nucleoli. A similar situation probably holds in Plethodon cinereus. Hexaploid oocytes from triploid females of Ambystoma jeffersonianum have 3 times as many nucleoli as diploid oocytes from diploid females of the same species. The number of nucleoli in an amphibian oocyte nucleus is therefore related to the number of sets of chromosomes in the cell. In yolky oocytes from hypophysectomized newts most peripheral nucleoli are firmly attached to the inner surface of the nuclear membrane; whereas in similar oocytes from unoperated or gonadotrophin-treated animals none of the nucleoli is so attached. On the basis of these observations 2 mechanisms are suggested for the formation of amphibian oocyte nucleoli. The first of these mechanisms probably operates in T. c. carnifex, where all peripheral nucleoli are formed before or soon after the chromosomes assume the lampbrush form, and no part of a lampbrush chromosome is involved in a process which adds to the existing population of nucleoli. The second mechanism probably operates in T. c. cristatus, where most of the peripheral nucleoli are formed before the lampbrush phase of oogenesis but a nucleolar organizer on the lampbrush chromosomes continues to grow and detach nucleoli throughout oogenesis. Both these mechanisms are discussed in terms of what is known of the chemical composition and function of peripheral nucleoli.

scholarly journals SMALL GRANULES IN THE AMPHIBIAN OOCYTE NUCLEUS AND THEIR RELATIONSHIP TO RNA

1956 ◽  
Vol 2 (4) ◽  
pp. 393-396 ◽  
Author(s):  
Joseph G. Gall
Keyword(s):  
Endoplasmic Reticulum ◽  
Salivary Gland ◽  
Nuclear Envelope ◽  
Oocyte Nucleus ◽  
Balbiani Ring ◽  
Lampbrush Chromosomes ◽  
Small Particles ◽  
Amphibian Oocyte

Small particles (100 to 300 A in diameter) are seen in sections of nucleoli, the loops of the amphibian lampbrush chromosomes, and the Balbiani-ring regions of dipteran salivary-gland chromosomes. All of these structures contain cytochemically demonstrable RNA. Furthermore, the annuli seen on the nuclear envelope are composed of small particles which are similar to or identical with those commonly associated with the endoplasmic reticulum. It seems likely that ribonucleoproteins are organized as small particulates in the nucleus as well as in the cytoplasm.

scholarly journals MULTIPLE GENE SITES FOR 5S AND 18 + 28S RNA ON CHROMOSOMES OF GLYPTOTENDIPES BARBIPES (STAEGER)

1974 ◽  
Vol 62 (1) ◽  
pp. 132-144 ◽  
Author(s):  
Wu-Nan Wen ◽  
Pedro E. León ◽  
Donald R. Hague
Keyword(s):  
Drosophila Melanogaster ◽  
Salivary Gland ◽  
Nucleolar Organizer ◽  
Balbiani Ring ◽  
5S Rna ◽  
Multiple Gene ◽  
Main Site ◽  
Chromosome Iii ◽  
The Right

Ribosomal RNAs (28 + 18S and 5S) and 4S RNA extracted from the chironomid Glyptotendipes barbipes were iodinated in vitro with 125I and hybridized to the salivary gland chromosomes of G. barbipes and Drosophila melanogaster. Iodinated 18 + 28 S RNA labeled three puffed sites with associated nucleoli on chromosomes IR, IIL, and IIIL of G. barbipes and the nucleolar organizer of Drosophila. Labeled 5S RNA hybridized to three sites on chromosome IIIR, two sites on chromosome IIR and one site in a Balbiani ring on chromosome IV of Glyptotendipes. Most of the label produced by this RNA was localized seven bands away from the centromere on the right arm of chromosome III, and we consider this to be the main site complementary to 5S RNA in the chironomid. This same RNA preparation specifically labeled the 56 EF region of chromosome IIR of Drosophila which has been shown previously to be the only site labeled when hybridized with homologous 5S RNA. Hybridization of G. barbipes chromosomes with iodinated 4S RNA produced no clearly localized labeled sites over the exposure periods studied.

The salivary gland chromosomes of seven taxa in the subgenus Stegopterna (Diptera, Simuliidae, Cnephia)

1969 ◽  
Vol 47 (1) ◽  
pp. 115-119 ◽  
Author(s):  
D. P. Madahar
Keyword(s):  
Salivary Gland ◽  
North American ◽  
Nucleolar Organizer ◽  
Close Relation ◽  
Standard Sequence ◽  
Y Chromosomes ◽  
Chromosome Ii ◽  
Chromosome I

The salivary gland chromosomes are described for seven taxa in the subgenus Stegopterna Enderlein of Cnephia Enderlein. All taxa have the typical blackfly complement with n = 3. The widely distributed North American Cnephia mutata Malloch has the nucleolar organizer in IL and the standard sequence in IS. All other taxa display a shift of the nucleolus to IS and homozygosity for the inversion IS-1. The Scandinavian C. richteri Enderlein apparently exhibits no further changes, the northern North American C. emergens Stone is characterized by the additional inversions, IS-2, IIIS-1, IIIL-1. The remaining four taxa, designated here as C. "Y", C. "X", C. "O", C. "W" are western North American, presumably undescribed and close to C. mutata. Their close relation to one another is indicated by the sharing of some inversion polymorphisms. They differ in certain floating autosomal inversions, and C. "O" and C. "W" in having distinctive and complex Y-chromosomes, based on chromosome I (C. "O") or chromosome II (C. "W"). A diagram illustrating the cytophylogenetic relations of the seven taxa is presented.

Karyology of Neurergus strauchii (Steindachner, 1887) (Urodela: Salamandridae) and Cytotaxonomic Considerations on Palearctic Salamandrids

1987 ◽  
Vol 8 (2) ◽  
pp. 101-110 ◽  
Author(s):  
Stefania Bucci-Innocenti ◽  
Giorgio Mancino ◽  
Matilde Ragghianti
Keyword(s):  
Structural Feature ◽  
Evolutionary Biology ◽  
Nucleolar Organizer ◽  
Lampbrush Chromosomes ◽  
Chromomycin A3 ◽  
Fluorescent Band ◽  
Nucleolar Organizers

AbstractThe karyotype of Neurergus strauchii shows pericentric and subterminal C-bands heterochromatin and nucleolar organizers on chromosome IX. This site is revealed by both a C-band and a chromomycin A3 fluorescent band; the GC-specific fluorochrome also highlights additional sites of fluorescence. The lampbrush chromosomes have a general morpho-structural feature known for Triturus lampbrush chromosomes, but giant loops are lacking. Nucleolar organizer and sphere sites are the main cytogenetic markers in the lampbrush karyotype of Neurergus. The implication of these results to our understanding of cytotaxonomy and evolutionary biology of Palearctic Salamandrids is discussed.

FINE STRUCTURE OF NUCLEOLI AND RNA-CONTAINING CHROMOSOME REGIONS OF SALIVARY GLAND CHROMOSOMES OF CHIRONOMIDS AND THEIR INTERRELATIONSHIP

1964 ◽  
Vol 42 (6) ◽  
pp. 1147-1155 ◽  
Author(s):  
V. I. Kalnins ◽  
H. F. Stich ◽  
S. A. Bencosme
Keyword(s):  
Fine Structure ◽  
Salivary Gland ◽  
Electron Microscope ◽  
Light Microscopy ◽  
Messenger Rna ◽  
Balbiani Ring ◽  
Electron Microscopic ◽  
Chironomid Species ◽  
Balbiani Rings ◽  
Chromosome Regions

Electron microscope studies of salivary gland nuclei of four chironomid species have shown that the RNA-containing chromosome regions and associated structures, which by light microscopy exhibit a great variety of structures such as bands, granules, micronucleoli, nucleoli, puffs, and Balbiani rings, consist of only few basic units: pars amorpha, nucleolonema, and Balbiani ring granules. The fine structure of the nucleoli and spherical micronucleoli located at various chromosome regions appears to be identical, consisting of pars amorpha, which contains fibers of varying diameters, and strands of nucleolonema composed of fibers and ribosome-like granules. The arrangement of pars amorpha and nucleolonema of nucleoli and spherical micronucleoli follows a consistent pattern. Chromosome fibers are closely associated with pars amorpha, whereas strands of nucleolonema border only the surfaces of pars amorpha. Balbiani ring granules, which have a diameter of 300 Å to 500 Å and are characterized by a particular structure, accumulate in Balbiani rings, in many chromosome regions, and in nuclear sap. In the Balbiani ring these granules seem to be attached to 100 Å chromosome fibers. They are absent in nucleoli and micronucleoli. The possible correlation between our electron microscopic observations and the present-day concept of ribosomal and messenger RNA production is discussed.

scholarly journals BOTH NUCLEOLAR ORGANIZERS ARE REPLICATED IN DIPTERAN POLYPLOID TISSUES: A STUDY AT THE LEVEL OF INDIVIDUAL NUCLEI

Genetics ◽  
1985 ◽  
Vol 111 (2) ◽  
pp. 325-336
Author(s):  
Esther J Belikoff ◽  
Kathy Beckingham
Keyword(s):  
Salivary Gland ◽  
Nurse Cell ◽  
Nuclear Dna ◽  
Organizer Region ◽  
Nucleolar Organizer Region ◽  
Nucleolar Organizer ◽  
Single Pair ◽  
Cell Nuclei ◽  
Calliphora Erythrocephala ◽  
Nontranscribed Spacer

ABSTRACT Working with the Dipteran Calliphora erythrocephala, we have tested the hypothesis that only one nucleolar organizer region (NO) is replicated during polyploidization. NO replication was examined in two very different highly polyploid nuclear types: salivary gland nuclei and nurse cell nuclei. Two strains of the organism containing NO regions with highly diagnostic nontranscribed spacer (NTS) polymorphisms were prepared and reciprocal single pair-matings between members of the strains were performed. The representation of the two distinguishable NOs in diploid and polyploid DNAs of individual F1 progeny from each cross was then examined. DNA from a total polyploid nuclear DNA preparation and from individual polyploid nuclei of both tissue types was analyzed. Our results show conclusively that both genomic NOs are replicated in individual polyploid nuclei of both types. Further, evidence for variation in the relative replication of cistrons from the two NOs by individual nuclei was obtained. The cistron types present in the NOs of both strains showed differential replication upon polyploidization. In general, the patterns of differential cistron replication seen in salivary gland and nurse cell nuclei were similar.

Sign in / Sign up

Export Citation Format

Share Document