Analysis of the C-Value Paradox by Molecular Hybridization

Abstract

Poly(A)-containing RNA was isolated from ovaries of Xenopus laevis laevis and Triturus cristatus carnifex and used as a template for the synthesis of radioactive complementary DNA with RNA-dependent DNA polymerase. When annealed with an excess of homologous DNA, the complementary DNA is rendered double-stranded with kinetics that suggest that the coding sequences are singlecopy in both these organisms. In Triturus, these sequences are distinct from the majority of the genome, which consists of repeated sequences, and distinct from the ribosomal cistrons, which are present in proportion to the increase in C-value relative to the Xenopus genome. Moreover, the number of different poly(A)-containing molecules in the ovary (sequence complexity) is the same in Xenopus and in Triturus.There exists an enormous variation in C-value (DNA content per haploid genome) among eukaryotic organisms (1 (3,4).On the basis of this range of DNA values and cytological evidence from various sources, Callan proposed that chromosomal DNA, and in particular genetically significant DNA, is arranged in tandemly repeated sequences (5). These repeated sequences consist of a "master" sequence and "slave" sequences; the "master" sequence would be subject to mutational and recombinational events while the "slave" sequences would be corrected once per life-cycle to reflect the sequence of the "master" sequence. The formation of extended lampbrush loops during meiotic prophase was proposed as the stage at which this correction process takes place.In washed in Barth X medium. Nucleic acid was prepared by the method of Kirby, with the addition of 0.5% sodium lauryl sulfate to the aqueous phase, as described (7). Highmolecular-weight RNA was prepared by homogenizing the pelleted nucleic acid in 3 M NaOAc (pH 6) to solubilize DNA, transfer RNA, and 5S RNA (8). The RNA precipitate was resuspended in "binding" buffer [0.4 M NaCl, 1 mM EDTA, 0.1% sodium lauryl sulfate, 10 mM Tris * HCl (pH 7.4), 10% glycerol] and passed over oligo(dT)-cellulose (9). The poly-(A)-containing RNA was eluted in "elution" buffer [10 mM Tris * HCl pH 7.4, 0.1% sodium lauryl sulfate], and the RNA spectrum was measured in the Unicam SP800A spectrophotometer. The amount of poly(A) in this eluate was routinely assayed by hybridization with radioactive poly(U), as described (7). By comparing the amount of poly (A) (Fig. 1). About 75% of

Transposable Elements, Epigenetics, and Genome Evolution

“…The C value, which is the DNA content per haploid genome, varies widely among closely related organisms of apparently comparable complexity (Fig. 1); this has for some time been attributed to the repetitive portion of the genome (17). Such variation is especially striking in angiosperms, whose highest and lowest C values differ by a factor of 2000 (18,19).…”

Structural characteristics of genome organization in amphibians: Differential staining of chromosomes and DNA structure

“…This fact points to the possible participation of repetitive DNA in the regulation of transcription. Nevertheless, the major part of transcribed DNA in amphibian oocytes, of course, consists of unique sequences (Rosbash et al 1974). It is interesting to add, that heterogeneous nuclear RNA size increases during development (gastrula and tailbud RNAs of Rana pipiens were compared) and this is correlated to an increasing degree of turnover of this RNA (Shepherd and Flikinger 1979).…”

“…The pioneer works on the genome structure in amphibian had pointed out that genome size increases as the amount of repetitive DNA increases (Straus 1971;Morescalchi and Serra 1974;Rosbash et al 1974). Experimental studies of the kinetics of DNA reassociation revealed that in Andreas ]aponicus, 2n=60, 2C=93 pg (the most primitive of the urodeles studied), and in Necturus maculosus, a species with a very high DNA content (2C=165 pg), DNA reassociates very quickly, due to a great number of repetitive DNA fractions in their genomes (Morescalchi and Serra 1979).…”

Genome Size and the Role of Transposable Elements

“…Fungal genomes, which are generally from 10 to 60 Mbp in size, tend to occupy the lower end of this range, topping off at about 200 Mbp (Gregory et al 2007;Schulman and Wicker 2013). The lack of correlation between genome size (C-value) and organismal complexity in terms of tissue and organ numbers was early on dubbed the "C-value Paradox" (Gaut and Ross-Ibarra 2008;Rosbash et al 1974). Weighing in with the largest genomes among the angiosperms are two diploids, the monocot Trillium hagae (1C ¼ 129,536 Mbp) and the eudicot Viscum album (Santalaceae) with 100,636 Mbp (Zonneveld 2010), and the octoploid Paris japonica (1C ¼ 148,881 Mbp; (Bennett and Leitch 2011)).…”