Showing papers on "Ribosomal DNA published in 1971"

Increasing the multiplicity of ribosomal RNA genes in Drosophila melanogaster

Abstract: In wild-type Drosophila melanogaster females there are about 250 ribosomal RNA genes in each nucleolus organizer region of the two X chromosomes. When this same nucleolus organizer region is present in flies in only a single dose, the number of ribosomal RNA genes increases to approximately 400. This increase is most easily explained by disproportionate replication of these genes.

DNA content, ribosomal gene multiplicity, and cell size in fish.

Abstract: The diploid nuclear DNA contents in eight species of fish have been found to differ over a 178-fold range. Among these, lungfish have the highest values, and teleosts have the lowest. Shark and coelacanth have intermediate values, which are close to the highest teleost values. Such great variation in DNA contents among vertebrates indicates that fundamental differences in chromosome structure and DNA function have arisen during evolution. In order to gain insight into the organization of genes in organisms with different DNA contents, the relative proportion of the genome coding for ribosomal RNA was determined for five species, winter flounder, goldfish, rainbow trout, dogfish shark, and Australian lungfish. The proportion of ribosomal DNA (r-DNA) was found to vary over a six-fold range from flounder to lungfish. In general, species with higher DNA contents have lower proportions of r-DNA. The r-DNA proportion of trout is consistent with the view that salmonids are recent tetraploids. If the goldfish is also considered a tetraploid, then its r-DNA proportion indicates a loss of ribosomal genes subsequent to polyploidization. Measurement of the ribosomal genes in fish DNA's was based on hybridization with yeast ribosomal RNA, which was assumed to be equally homologous with the r-DNA of all species studied. The activity of multiple copies of genes was studied by measuring the volumes of blood cells in five species. It was found that erythrocyte volume correlated both with ribosomal gene multiplicity and with total DNA content. The size of goldfish and trout erythrocytes suggests that cell volume in these tetraploids has declined to the ancestral diploid level. It is concluded that the multiplicity of specific repetitive genes varies out of proportion with the DNA content, contrary to expectation based on simple polyteny. This variation in the proportions of specific genes and the large variations in total DNA content among fish are thought to be due to a combination of tetraploidy and extensive linear replication of specific chromosomal regions during evolution.

The nucleolus organizer region of maize (Zea mays L.): Chromosomal site of DNA complementary to ribosomal RNA

Abstract: A maize genetic marker strain (designated as the 2NOR strain) was found to carry a cytologically-visible “duplication” of the nucleolar organizer region (NOR). The 2 NOR condition is a simply inherited cytological marker which is transmitted normally through mega- and microsporogenesis and is associated when homozygous with a nucleolus approximately 64% greater in volume than that of two representative normal inbred lines of maize.—Utilizing DNA · rRNA hybridization techniques and the 2 NOR strain plus two unrelated inbred lines with normal NOR's, the chromosomal site of DNA complementary to rRNA was shown to be localized in the NOR. To our knowledge, this is the first report on the localization of rDNA cistrons in the NOR of plants. An estimate was made of 17,000 rDNA cistrons per diploid nucleus for normal maize.—The 2 NOR strain was shown to possess approximately twice as many rDNA cistrons (34,000) as normal maize. The usefulness of this 2 NOR condition for plant protein improvement programs is being investigated.

Ribosomal DNA satellite of Euglena gracilis chloroplast DNA

Abstract: A chloroplast of Euglena gracilis contains between 1.2 X 10-r’ [l] and 1 X lo-i4 g [2] ofdouble stranded DNA. Electron microscopic studies have shown this DNA to be in the form of circular molecules of about 40 I.tm in contour length in situ with an estimated molecular weight of 8.3 X 10’ daltons [3]. From a study of chloroplast DNA renaturation rates [4] we concluded that the kinetic complexity was in the range of 1.8 X 1 O* daltons which, if corrected according to Wetmur and Davidson [S] for the low GC content, is equivalent to a molecular weight of 9 X 10’ daltons. The results from the electron microscopic study and the renaturation data coincide within acceptable limits, suggesting that the circular molecule carries all the genetic information located in the chloroplasts. However, from DNA/RNA hybridization experiments it was known [6, 71 that chloroplast ribosomal RNA (rRNA) anneals with approximately 1 to 1.2% (wt/wt) of the chloroplast DNA. Relating this value to the size of the circular molecule one obtains a nucleotide sequence equivalent to 0.53 and 0.64 rRNA cistrons, respectively*. Obviously, this means that either the circular DNA molecule does not contain all the genetic information or the reported hybridization data are too small. In this report we shall investigate the second possibility and show that the DNA/rRNA hybridization