DNA replication is the process of creating two identical copies from one original DNA molecule. DNA is composed of two strands and each strand of the original DNA molecule serves as template for the production of the complementary strand. Cellular proofreading and error-checking mechanisms ensure near perfect DNA replication. In a cell, DNA replication begins at specific locations, or origins of replication, in the genome. Unwinding of DNA at the origin and synthesis of new strands results in replication forks growing bidirectional from the origin. A number of proteins are associated with the replication fork which helps in terms of the initiation and continuation of DNA synthesis. Most prominently, DNA polymerase synthesizes the new DNA by adding complementary nucleotides to the template strand.
DNA polymerases are a family of enzymes that carry out all forms of DNA replication. DNA polymerases in general cannot initiate synthesis of new strands, but can only extend an existing DNA or RNA strand paired with a template strand. To begin synthesis, a short fragment of RNA, called a primer, must be created and paired with the template DNA strand.
DNA polymerase synthesizes a new strand of DNA by extending the 3′ end of an existing nucleotide chain, adding new nucleotides matched to the template strand one at a time via the creation of phosphodiester bonds. In general, DNA polymerases are highly accurate. In addition, some DNA polymerases also have proofreading ability; they can remove nucleotides from the end of a growing strand in order to correct mismatched bases. Finally, post-replication mismatch repair mechanisms monitor the DNA for errors that had occurred during DNA replication, being capable of distinguishing mismatches in the newly synthesized DNA strand from the original strand sequence.
Function in DNA replication
|DNA Helicase||Unwinds the DNA double helix at the Replication Fork.|
|DNA Polymerase||Builds a new duplex DNA strand by adding nucleotides in the 5′ to 3′ direction. Also performs proof-reading and error correction|
|Single-Strand Binding Proteins||Bind to ssDNA and prevent the DNA double helix from re-annealing after DNA helicase unwinds it thus maintaining the strand separation.|
|Topoisomerase||Relaxes the DNA from its super-coiled nature.|
|DNA Gyrase||Relieves strain of unwinding by DNA helicase; this is a specific type of topisomerase|
|DNA Ligase||Re-anneals the semi-conservative strands and joins Okazaki Fragments of the lagging strand.|
|Primase||Provides a starting point of RNA (or DNA) for DNA polymerase to begin synthesis of the new DNA strand.|
|Telomerase||Lengthens telomeric DNA by adding repetitive nucleotide sequences to the ends of eukaryotic chromosomes.|