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To date, studies on G-quadruplexes of genomic sources have mostly been focused on intramolecular G-quadruplexes, and the biogenesis of G-quadruplexes in physiological processes in cells remains largely elusive. A dimeric intermolecular G-quadruplex can form as long as the two participating strands can supply a sum of four G-tracts ( 9). For example, an intramolecular G-quadruplex requires a strand that carries at least four tandem G-tracts. A G-quadruplex can form intramolecularly or intermolecularly depending on the number of G-tracts available in participating strands. G-quadruplex formation requires four G-tracts ( Figure 1A). Information on the formation of G-quadruplexes in these processes and their structural forms is important for understanding the physiological function of G-quadruplexes. Genomic DNA undergoes dynamic changes in structural organization during transcription and replication. Using engineered antibody, a recent work explicitly provides substantive evidence for the presence of G-quadruplex structures in the genome of mammalian cells ( 8). Arrow in (B) indicates the 7-nitrogen.įor many years, interest in G-quadruplex has drawn attention to the question of whether G-quadruplexes are actually present in cells. Structure of ( A) an intramolecular G-quadruplex composed of three G-quartet layers with four G-tracts connected by three loops, ( B) a G-quartet with four guanines connected by eight Hoogsteen hydrogen bonds (dashed lines) and ( C) a DNA:RNA hybrid G-quadruplex (HQ) with two G-tracts from the non-template DNA strand and two from an RNA transcript. It was predicted that G-quadruplexes might modulate transcription and translation because of their enrichment near transcription and translation start sites ( 7). In the past few years, bioinformatic analyses have revealed large numbers of potential intramolecular G-quadruplex sequences in the genomes of various organisms ( 5, 6), ranging from bacteria to animals. Thus, targeting G-quadruplexes has been considered as a promising therapeutic strategy against cancer and other diseases ( 3, 4). Interest in the biological significance of G-quadruplexes was initially evoked by telomeric DNA that forms intramolecular G-quadruplex and inhibits its extension by telomerase ( 2), a reverse transcriptase expressed in cancerous but not in normal somatic cells. A G-quadruplex is characterized by a stack of planar G-quartets ( Figure 1B), each of which comprises four guanines connected by eight Hoogsteen hydrogen bonds ( 1). G-quadruplexes are four-stranded structures ( Figure 1A) formed by guanine-rich (G-rich) nucleic acids. Besides the conventional Watson–Crick double helix, DNA can also adopt other forms of higher order structures, such as G-quadruplexes.
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