Sequence-specific transitions of the torsion angle gamma change the polar-hydrophobic profile of the DNA grooves: implication for indirect protein–DNA recognition

Variations of the shape and polarity of the DNA grooves caused by changes of the DNA conformation play an important role in the DNA readout. Despite the fact that non-canonical trans and gauche- conformations of the DNA backbone angle γ (O5′–C5′–C4′–C3′) are frequently found in the DNA crystal structures, their possible role in the DNA recognition has not been studied systematically. In order to fill in this gap, we analyze the available high-resolution crystal structures of the naked and complexed DNA. The analysis shows that the non-canonical γ angle conformations are present both in the naked and bound DNA, more often in the bound vs. naked DNA, and in the nucleotides with the A-like vs. the B-like sugar pucker. The alternative angle γ torsions are more frequently observed in the purines with the A-like sugar pucker and in the pyrimidines with the B-like sugar conformation. The minor groove of the nucleotides with non-canonical γ angle conformation is more polar, while the major groove is more hydrophobic than in the nucleotides with the classical γ torsions due to variations in exposure of the polar and hydrophobic groups of the DNA backbone. The propensity of the nucleotides with different γ angle conformations to participate in the protein–nucleic acid contacts in the minor and major grooves is connected with their sugar pucker and sequence-specific. Our findings imply that the angle γ transitions contribute to the process of the protein–DNA recognition due to modification of the polar/hydrophobic profile of the DNA grooves.     

A study of DNA denaturation in the ultracentrifuge

The melting transition of DNA in alkaline CsCl can be followed in the analytical ultracentrifuge. Equilibrium partially denatured states can be observed. These partially denatured DNA bands have bandwidths of up to several times those of native DNA. Less stable molecules melt early and are found at heavier densities in the melting region. An idealized ultracentrifuge melting transition is described. The melting transition of singly nicked PM-2 DNA resembles the idealized curve. The DNA profile is a Gaussian band at all points in the melt. DNA's from mouse, D. Melanogaster, M. lysodeikticus, T4, and T7 also show equilibrium bands at partially denatured densities, some of which are highly asymmetric. Simple sequence satellite DNA shows an all-or-none transition with no equilibrium bands at partially denatured densities. The temperature at which a DNA denatures is an increasing function of the (G + C) content of the DNA. The T_m does not show a molecular-weight dependence in the range 1.2 × 10⁶–1.5 × 10⁷ daltons (single strand) for mouse, M. lysodeikticus, or T4 DNA. The mouse DNA partially denatured bands do not change shape as a function of molecular weight. The T4 DNA intermediate band develops a late-melting tail at low molecular weight. M. lysodeikticus DNA bands at partially denatured densities become broader as the molecular weight is decreased. Mouse DNA is resolved into six Gaussian components at each point in the melting transition.     

Satellite DNA properties of the germ line limited DNA and the organization of the somatic genomes in the nematodesAscaris suum andParascaris equorum

During the early cleavage divisions in some Ascarids, parts of the chromosomes are eliminated from the somatic blastomeres (chromatin diminution, Boveri, 1887) while the chromosomes in the germ line cells maintain their integrity. To characterize the germ line and soma genome, DNA was isolated from gametes and embryonic somatic cells of two Ascarid species,Parascaris equorum var. univalens andAscaris suum. It was shown that the germ line limited DNAs of these species have the same density and almost identical reassociation kinetics: in CsCl the predominant component of the germ line limited DNA ofP. equorum andA. suum has the buoyant density of 1.697g/cm³, while soma DNA of both species bands at 1.700 g/cm³. InP. equorum there is a small additional germ line limited satellite DNA component with the density of 1.690 g/cm³, identical to that of mitochondrial DNA of both organisms. Comparison of the reassociation kinetics of germ line and soma DNA demonstrates for both species that the eliminated DNA sequences are highly repetitive. In contrast to these similarities between the germ line limited DNAs ofP. equorum andA. suum the analysis of their base composition revealed differences (40% guanine plus cytosine inP. equorum and 36% inA. suum). The only very fast reassociating DNA sequences which we could isolate from soma DNA was demonstrated to be foldback DNA. The reassociation kinetics of totalA. suum soma DNA was investigated by hydroxylapatite chromatography. Least squares analysis of the data revealed about 10% of intermediate repetitive DNA sequences. Their interspersion between single copy DNA sequences was analyzed by comparing the reassociation kinetics of DNA fragments 0.35 and 7.2 kilobases long. Thus the DNA sequence arrangement ofAscaris does not follow the short period interspersion pattern observed in most organism.     

Cdk2 Activity Is Dispensable for the Onset of DNA Replication during the First Mitotic Cycles of the Sea Urchin Early Embryo

Earlier work reported the important role of Cdk2 as a regulator of DNA replication in somatic cells and inXenopusextracts. In the present report we analyzein vivothe involvement of Cdk2 in DNA replication during early embryogenesis using the first mitotic cycles of sea urchin embryos. UnfertilizedSphaerechinus granulariseggs are arrested after the second meiotic cytokinesis. Fertilization resumes the block and induces DNA replication after a short lag period, making sea urchin early embryo a good model for studyingin vivothe onset of DNA replication. We show that Cdk2 as well as its potential partner cyclin A are present in the nucleus in G1 and S phase and therefore available for DNA replication. In accordance with data obtained inXenopusegg extracts we observed that Cdk2 kinase activity is low and stable during the entire cycle. However, in contrast with thisin vitrosystem in which Cdk2 activity is required for the onset of DNA replication, the specific inhibition of Cdk2 kinase by microinjection of the catalytically inactive Cdk2-K33R or the inhibitor p21^Cip1does not prevent DNA replication. Because olomoucine, DMAP, and emetine treatments did not preclude DNA synthesis, neither cyclin A/Cdk1 nor cyclin B/Cdk1 kinase activities are necessary to replace the absence of Cdk2 kinase in promoting DNA replication. These data suggest that during early embryogenesis Cdks activities, in particular Cdk2, are dispensablein vivofor the initiation step of DNA replication. However, the specific localization of Cdk2 in the nucleus from the beginning of M phase to the end of S phase suggests its involvement in other mechanisms regulating DNA replication such as inhibition of DNA re-replication and/or that its regulating role is achieved through a pathway independent of the kinase activity. We further demonstrate that even after inhibition of Cdk activities, the permeabilization of the nuclear membrane is required to allow a second round of DNA replication. However, in contrast toXenopusegg extracts, re-replication can take place in the absence of DMAP-sensitive kinase.     

Deoxyribonucleic acid reassociation in the taxonomy of the genus Chlorella

DNA homologies of 14 strains of Chlorella protothecoides were determined. All strains are related by a high degree of DNA similarity (96–102% D) with the exception of strain 211-11 a which proved to belong to C. kessleri. There is, however, no detectable DNA homology with strains of the genus Prototheca which is supposed to have evolved from C. protothecoides by loss of photosynthetic pigments. Even within Prototheca the low degree of DNA similarity indicates a heterogeneity similar to that observed in the genus Chlorella.     

Properties of the nonlethal recombinational repair deficient mutants of bacteriophage T4

Summary The rate at which ³H thymidine is incorporated into DNA is increased in T4w-infected cells compared to wild-type when measured late in infection under conditions of low thymidine concentration. This increased DNA synthesis is sensitive to hydroxyurea but not to mitomycin C, and can be prevented by the addition of chloramphenicol early in infection. Also, DNA replicative intermediates isolated from T4w-infected cells late in infection sediment significantly faster than those isolated from wild-type-infected cells. In contrast, DNA replicative intermediates isolated from T4x-or T4y-infected cells sediment more slowly than those produced by wild-type T4. Cells coinfected with wild-type T4⁺ and T4x, y or w; or T4w and T4x or y, produce wild-type DNA replicative intermediates. Cells coinfected with T4x and T4y produce more slowly sedimenting DNA replicative intermediates. Cells coinfected with T4w and wild-type T4 show wild-type rates of DNA synthesis while cells coinfected with T4w and T4x or T4y show increased rates of DNA synthesis over that observed with wild-type alone.     

The development of the macronucleus in the ciliated protozoan Stylonychia mytilus

Ciliated protozoa are characterized by generative micronuclei and vegetative polyploid macronuclei. Micronuclei of Stylonychia mytilus contain 1 600 times as much DNA per haploid genome as E. coli. Most of this DNA is shown to be repetitive. The development of the macronucleus involves, as demonstrated by cytology, only 1/3 of the chromosomes which in a first replication phase are polytenized in probably 5 replication steps and appear as giant chromosomes. At this developmental stage considerable amounts of repetitive DNA are still present in the chromosomes. During the subsequent disintegration phase more than 90% of the DNA are eliminated from the macronucleus anlage. The remainder is further replicated five times and composes the final macronucleus. Since this DNA reassociates with a reaction rate almost identical to an ideal second order reaction its kinetic complexity can be determined by comparison with the kinetic complexity of E. coli DNA. Macronuclear DNA reassociates with a kinetic complexity of 26 times the kinetic complexity of E. coli DNA (corrected for GC content) which indicates that macronuclear DNA sequences exist at a ploidy level of 4 096 C. We assume that macronuclear DNA may be present only once per haploid genome. In this case it represents only 1.6% of the DNA in micronuclei or 10% of the DNA in the giant chromosome stage.     

Structural insights into the interaction of the crenarchaeal chromatin protein Cren7 with DNA

Cren7, a newly found chromatin protein, is highly conserved in the Crenarchaeota. The protein shows higher affinity for double‐stranded DNA than for single‐stranded DNA, constrains negative DNA supercoils in vitro and is associated with genomic DNA in vivo. Here we report the crystal structures of the Cren7 protein from Sulfolobus solfataricus in complex with two DNA sequences. Cren7 binds in the minor groove of DNA and causes a single‐step sharp kink in DNA (～53°) through the intercalation of the hydrophobic side chain of Leu28. Loop β3‐β4 of Cren7 undergoes a significant conformational change upon binding of the protein to DNA, suggesting its critical role in the stabilization of the protein–DNA complex. The roles of DNA‐contacting amino acid residues in stabilizing the Cren7–DNA interaction were examined by mutational analysis. Structural comparison of Cren7‐DNA complexes with Sac7d‐DNA complexes reveals significant differences between the two proteins in DNA binding surface, suggesting that Cren7 and Sul7d serve distinct functions in chromosomal organization.     

Studies of He-T DNA sequences in the pericentric regions of Drosophila chromosomes

He-T DNA is a complex set of repeated DNA sequences with sharply defined locations in the polytene chromosomes of Drosophila melanogaster. He-T sequences are found only in the chromocenter and in the terminal (telomere) band on each chromosome arm. Both of these regions appear to be heterochromatic and He-T sequences are never detected in the euchromatic arms of the chromosomes (Young et al. 1983). In the study reported here, in situ hybridization to metaphase chromosomes was used to study the association of He-T DNA with heterochromatic regions that are under-replicated in polytene chromosomes. Although the metaphase Y chromosome appears to be uniformly heterochromatic, He-T DNA hybridization is concentrated in the pericentric region of both normal and deleted Y chromosomes. He-T DNA hybridization is also concentrated in the pericentric regions of the autosomes. Much lower levels of He-T sequences were found in pericentric regions of normal X chromosomes; however compound X chromosomes, constructed by exchanges involving Y chromosomes, had large amounts of He-T DNA, presumably residual Y sequences. The apparent co-localization of He-T sequences with satellite DNAs in pericentric heterochromatin of metaphase chromosomes contrasts with the segregation of satellite DNA to alpha heterochromatin while He-T sequences hybridize to beta heterochromatin in polytene nuclei. This comparison suggests that satellite sequences do not exist as a single block within each chromosome but have interspersed regions of other sequences, including He-T DNA. If this is so, we assume that the satellite DNA blocks must associate during polytenization, leaving the interspersed sequences looped out to form beta heterochromatin. DNA from D. melanogaster has many restriction fragments with homology to He-T sequences. Some of these fragments are found only on the Y. Two of the repeated He-T family restriction fragments are found entirely on the short arm of the Y, predominantly in the pericentric region. Under conditions of moderate stringency, a subset of He-T DNA sequences cross-hybridizes with DNA from D. simulans and D. miranda. In each species, a large fraction of the cross-hybridizing sequences is on the Y chromosome.     

Conditions and mutations affecting the maintenance and replication of plasmid DNA in Escherichia coli 15

Summary An Escherichia coli 15 strain has been constructed which contains, in addition to the plasmids inherent to E. coli 15 (P 1-like DNA and minicircular DNA), the colicinogenic factor E₁ (Col E₁). Whereas the P 1-like DNA of E. coli 15 is unaffected by the uptake of the colicin plasmid, the number of copies of minicircular DNA of E. coli 15 decreases and an equivalent amount of Col E₁ DNA becomes established in the cell. The ratio between these two small plasmids is dependent on the growth temperature. The mode of replication of minicircular DNA and Col E₁ DNA is very similar, but is different in various respects from that of the P 1-like plasmid: 1. Both small plasmids continue to replicate in the presence of chloramphenicol, whereas the replication of P 1-like DNA stops like the chromosomal DNA. 2. Rifampicin inhibits the synthesis of both small plasmids rather rapidly. The replication of P 1-like DNA continues during the remaining replication cycle of the chromosome in the presence of rifampicin. 3. The replication of Col E₁ DNA and of the minicircular DNA still proceeds at elevated temperatures (45–50°C), whereas little or no incorporation of ³H-thymidine into P 1-like DNA is observed at these temperatures. 4. Mutants have been obtained, which show altered properties in the maintenance and replication of the plasmids without being affected in the replication of the chromosomal DNA. In all these mutants the replication and (or) maintenance of the minicircular DNA of E. coli 15 and Col E₁ DNA is affected in the same way, but not that of the P 1-like plasmid.     

DNA Polymerase C of the Thermophilic Bacterium Thermus aquaticus: Classification and Phylogenetic Analysis of the Family C DNA Polymerases

Bacterial family C DNA polymerases (DNA pol IIIs), the major chromosomal replicative enzymes, have been provisionally classified based on primary sequences and domain structures into three classes: class I (Escherichia coli DNA pol C-type), class II (Bacillus subtilis DNA pol C-type), and class III (cyanobacterial DNA pol C-type), respectively. We have sequenced the structural gene encoding the DNA pol C catalytic subunit of the thermophilic bacterium Thermus aquaticus. This gene, designated the Taq DNA pol C gene, contains a 3660-bp open reading frame which specifies a polypeptide of molecular weight of 137,388 daltons. Comparative sequence analyses revealed that Taq DNA pol C is a class I family C DNA polymerase. The Taq DNA pol C is most closely related to the Deinococcus radiodurans DNA pol C. Although a phylogenetic tree based on the class I family C DNA pols is still in the provisional stage, some important conclusion can be drawn. First, the high-G+C and the low-G+C Gram-positive bacteria are not monophyletic. Second, the low-G+C Gram-positive bacteria contain multigenes of family C DNA pols (classes I and II). Third, the cyanobacterial family C DNA pol, classified as class III because it is encoded by a split gene, forms a group with the high-G+C Gram-positive bacteria. Received: 7 October 1998 / Accepted: 12 January 1999     

A linear DNA molecule of 5.9 kilobase-pairs is highly homologous to the chloroplast DNA in the green alga Chlamydomonas moewusii

Summary The unicellular green alga Chlamydomonas moewusii contains small DNA species of unknown cellular location. We report that the most abundant of these DNAs, here designated low-molecular-weight DNA (LMW DNA), is a linear molecule of 5.9 kilobase pairs (kbp). Southern blot hybridization and restriction enzyme analysis revealed that the LMW DNA sequence also exists as an integrated sequence in a discrete region of the chloroplast genome. We have confirmed earlier reports that small DNA species related to the LMW DNA are absent from Chlamydomonas eugametos, an alga which is interfertile with C. moewusii. In the C. eugametos chloroplast genome we found only remnants of the LMW DNA sequence.     

Virus DNA packaging: the strategy used by phage λ

Phage λ, like a number of other large DNA bacterio-phages and the herpesviruses, produces concatemeric DNA during DNA replication. The concatemeric DNA is processed to produce unit-length, virion DNA by cutting at specific sites along the concatemer. DNA cutting is coordinated with DNA packaging, the process of translocation of the cut DNA into the preformed capsid precursor, the prohead. A key player in the λ DNA packaging process is the phage-encoded enzyme terminase, which is involved in (i) recognition of the concatemeric λ DNA; (ii) initiation of packaging, which includes the introduction of staggered nicks at cosN to generate the cohesive ends of virion DNA and the binding of the prohead; (iii) DNA packaging, possibly including the ATP-driven DNA translocation; and (iv) following translocation, the cutting of the terminal cosN lo complete DNA packaging. To one side of cosN is the site cosB, which plays a role in the initiation of packaging; along with ATP, cosB stimulates the efficiency and adds fidelity to the endo-nuclease activity of terminase in cutting cosN. cosB is essential for the formation of a post-cleavage complex with terminase, complex I, that binds the prohead, forming a ternary assembly, complex II. Terminase interacts with cosN through its large subunit, gpA, and the small terminase subunit, gpNul, interacts with cosB. Packaging follows complex II formation. cosN is flanked on the other side by the site cosQ, which is needed for termination, but not initiation, of DNA packaging. cosQ is required for cutting of the second cosN, i.e. the cosN at which termination occurs. DNA packaging in λ has aspects that differ from other λ DNA transactions. Unlike the site-specific recombination system of λ, for DNA packaging the initial site-specific protein assemblage gives way to a mobile, translocating complete, and unlike the DNA replication system of λ, the same protein machinery is used for both initiation and translocation during λ DNA packaging.     

Cloning of the carbohydrate-binding portion of the toxin A gene ofClostridium difficile

A portion of the toxin A gene ofClostridium difficile was cloned into pBR322 withEscherichia coli Chi 1776 as the host. Five identical clones, each containing a 4.7-kbPstI restriction endonuclease fragment and producing toxin A antigens, were detected with affinity-purified, monospecific antibodies against toxin A. Digestion of the cloned DNA withPstI revealed as internal restriction site that resulted in two fragments (2.1 and 2.6 kb in size). Probe DNA from either of these fragments hybridized with DNA in the 4.7 kb region ofPstI-digested, high-molecular-weight DNA from the sourceC. difficile strain, indicating that the internalPstI site is protected. The probe DNA also hybridized with restriction-digested DNA from five additional toxigenic strains, but it did not hybridize with DNA from four nontoxigenic strains. In addition, a DNA fragment from a toxigenic strain ofClostridium sordellii, whose toxin cross-reacts with antibody toC. difficile toxin A, hybridized with the clonedC. difficile DNA. Unlike native toxin A, the cell lysate from the recombinant clone was not cytotoxic to Chinese hamster ovary cells or enterotoxic in hamsters. It did agglutinate rabbit red blood cells, a characteristic of toxin A. The cell lysate also exhibited a line of partial identity when compared with purified toxin A in Ouchterlony assays, and it reacted with monoclonal antibody to toxin A in an enzyme-linked immunosorbent assay. The cloned DNA appears to code for a nontoxic binding portion of toxin A, which is responsible for binding to galactose-1-3-galactose-1-4-N-acetylglucosamine.A preliminary report of this work has been presented by S.B. Price and J.L. Johnson, Abstracts of the Annual Meeting of the American Society for Microbiology, 1986:67.     

Deoxyribonucleic acid reassociation in the taxonomy of the genus Chlorella

Four species of the unicellular green alga Chlorella, C. vulgaris, C. luteoviridis, C. minutissima, and C. zofingiensis, were characterized with respect to DNA similarities as determined by quantitative DNA hybridization procedures. In contrast to previous DNA hybridization procedures. In contrast to previous results, C. vulgaris turned out to be a homogeneous species with the exception of strain 211-11c of the Göttingen collection, which was shown to belong to C. kessleri. Similary, C. luteoviridis and C. minutissima represent well defined species in terms of phenotypic and genotypic features. Whitin C. zofingiensis on strain is clearly different with respect to DNA base composition and DNA hybridization data even though it shares phenotypic characteristics with the other strains of C. zofingiensis.     

On the identity of dnaP and dnaF genes of Bacillus subtilis

Summary The dnaP strains of Bacillus subtilis are altered in the initiation of DNA replication at high temperature (Riva et al., 1975). Fine mapping of the gene shows that it is located very close to the dnaF gene, described by Karamata and Gross (1970) and mapped by Love et al. (1976) in the polC region. The phenotype of both mutants is indistinguishable: the DNA synthesis stops at non permissive temperature after synthesizing an amount of DNA equivalent to the completion of the rounds of replication already initiated; at permissive temperature they are abnormally sensitive to MMS and are reduced in the ability to be transformed. Both mutants are to be considered as belonging to the dnaF locus.The dnaF gene is very close to the polC gene, which specifies the DNA polymerase III of B. subtilis. The DNA polymerase III of the dnaF mutants is not temperature sensitive in vitro, however, the level of this enzyme is lower by a factor of 4 or 5 in the dnaF mutants, at the permissive temperature. Following shift of dnaF cultures to the non permissive temperature, the level of DNA polymerase III activity specifically decreases further by a factor of at least 10 in the mutant, whereas the DNA polymerase I level is unaffected.The possible roles of the dnaF gene in the control of the cellular level of the DNA polymerase III, and the possibility of a regulatory role of DNA polymerase III in the initiation of DNA replication in bacteria are discussed.Abbreviations and symbols HPUra 6-(p-hydroxyphenylazo)-uracil; mic, minimum inhibitory concentration - MMS methyl-methanesufonate - Pol I Pol II and Pol III: DNA polymerase I, II and III respectively - PCMB parachloro-mercuri-benzoate     

Cotransposition of a highly repetitive DNA element with flanking sequences in the genome of the midgeChironomus thummi

Summary A family of highly repetitive DNA elements, the Cla-elements, is present in the genomes of the two sibling speciesChironomus th. thummi andCh. th. piger. These Cla-elements are organized in large tandem repetitive clusters as well as occuring as interspersed monomeric elements, in both subspecies. The analysis of a monomeric Cla-element and several kilobases of its flanking sequences fromCh. th. piger revealed that the short Cla-elements are cotransposed together with adjacent DNA. We found the same association of Cla-elements with specific flanking DNA in clones obtained from the rDNA ofCh. th. thummi and from nonribosomal Cla-DNA ofCh. th. piger. The Cla-element-flanking DNA is clearly also repetitive, but mainly of inter-spersed organization.     

Evidence for the wide distribution of repetitive DNA sequences in the genusStreptomyces

Summary Repeated DNA sequences were detected as rapidly reannealing sequences in the chromosomal DNA of 13 out of 14Streptomyces species using either hypochromicity measurements or hydroxyapatite chromatography. These sequences made up between approximately 4% and 11% of the total DNA of these species; only inStreptomyces rimosus were repeated DNA sequences not detected. The repeated sequences fall into a number of distinct percentage G+C (%G+C) classes, many being of rather low %G+C. Analytical density ultracentrifugation of the DNA of these species indicated satellite bands of low %G+C, and high-resolution thermal denaturation profiles indicated the presence of blocks of DNA of low G+C content too. No such satellite band could be found inStreptomyces coelicolor and no low-%G+C DNA could be detected in its thermal denaturation profile. The possible relationship of this repeated DNA, an unusual occurrence in a procaryote, to genetic instability and genetic control mechanisms inStreptomyces is discussed.     

The location of highly repetitious DNA in the somatic chromosomes of Drosophila melanogaster

In situ hybridization of Drosophila melanogaster somatic chromosomes has been used to demonstrate the near exact correspondence between the location of highly repetitious DNA and classically defined constitutive heterochromatin. The Y chromosome, in particular, is heavily labeled even by cRNA transcribed from female (XX) DNA templates (i.e., DNA from female Drosophila with 2 Xs and 2 sets of autosomes). This observation confirms earlier reports that the Y chromosome contains repeated DNA sequences that are shared by other chromosomes. In grain counting experiments the Y chromosome shows significantly heavier label than any other chromosome when hybridized with cRNA from XY DNA templates (i.e., DNA from male Drosophila with 1 X and 1 Y plus 2 sets of autosomes). However, the preferential labeling of the Y is abolished if the cRNA is derived from XX DNA. We interpret these results as indicating the presence of a class of Y chromosome specific repeated DNA in D. melanogaster. The relative inefficiency of the X chromosome in binding cRNA from XY and XYY DNA templates, coupled with its ability to bind XX derived cRNA, may also indicate the presence of an X chromosome specific repeated DNA.     

DNA endoreplication in the clustered supramedullary neurons of the pufferfish Diodon holacanthus L. (osteichthyes)

DNA contents, ranging from 4C to more than 500C, were determined by a quantitative microfluorimetric assay in supramedullary neuron nuclei of the pufferfish Diodon holacanthus. The distribution of C values after ethidium bromide staining indicates an inter- and intra-individual variation in DNA contents which do not correspond to duplications of the total genome, suggesting that DNA replication is specific for particular genes (endoreplication). Moreover, the DNA content appears to be correlated with nuclear size. A relationship between the DNA amounts and the presence of AT- and GC-rich sequences has been shown.This work demonstrates, for the first time, DNA endoreplication in a specific neuronal type in vertebrates.