Human Telomeric DNA (monomer with 22 bases)
Homo sapiens (human)
DNA polymerases that replicate DNA move in the 5'-3' direction. As a result, one strand of DNA is rapidly and continuously replicated while the other strand is replicated discontinuously in fragments which are joined together later. This causes a problem in the replication of the end of the lagging strand which is solved by the presence of telomeres. Telomeres are specialized DNA sequences at the ends of eukaryotic chromosomes. They contain repeated nucleotide sequences that allow the ends of the chromosomes to be replicated correctly. These nucleotide repeat sequences (tandem repeats) are rich in the guanine (G) nucleotide and allow an enzyme, called the telomerase, to bind to the chromosome ends. The telomerase synthesizes new repeats on the lagging strand that are used by the DNA polymerase as templates to replicate the ends of the chromosomes. The telomeres also fold over themselves to prevent the ends of chromosomes from being recognized by DNA degradative enzymes. They help clearly distinguish normal chromosome ends from those of broken DNA molecules. The repeats in the telomeres can form four-stranded or quadruplex structures with the guanine bases as their centers.
The structure here shows one such G-quadruplex found at the telomeric ends of human chromosomes. This intramolecular G-quadruplex is formed from a 22 nucleotide repeat sequence in the presence of K+ ions. Multiple G-quadruplexes fold into a stack at the telomeric end which can be stabilized by specific ligands and bind to telomeric proteins.
Protein Data Bank (PDB)
author: Ashwini Patil