A special type of DNA polymerase enzyme called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA. Telomeres act as protective caps at the end of chromosomes to prevent nearby chromosomes from fusing. The ends of the parent strands consist of repeated DNA sequences called telomeres. The ends of the linear DNA present a problem as DNA polymerase can only add nucleotides in the 5′ to 3′ direction. Another enzyme called DNA ligase joins Okazaki fragments together forming a single unified strand. Another exonuclease “proofreads” the newly formed DNA to check, remove and replace any errors. These primers are then replaced with appropriate bases. Once both the continuous and discontinuous strands are formed, an enzyme called exonuclease removes all RNA primers from the original strands. This process of replication is discontinuous as the newly created fragments are disjointed. DNA polymerase then adds pieces of DNA, called Okazaki fragments, to the strand between primers. The lagging strand begins replication by binding with multiple primers. Because replication proceeds in the 5' to 3' direction on the leading strand, the newly formed strand is continuous. In eukaryotic cells, polymerases alpha, delta, and epsilon are the primary polymerases involved in DNA replication. DNA polymerase III binds to the strand at the site of the primer and begins adding new base pairs complementary to the strand during replication. coli, polymerase III is the main replication enzyme, while polymerase I, II, IV and V are responsible for error checking and repair. There are five different known types of DNA polymerases in bacteria and human cells. Įnzymes known as DNA polymerases are responsible creating the new strand by a process called elongation. Primers are generated by the enzyme DNA primase. The primer always binds as the starting point for replication. Once the DNA strands have been separated, a short piece of RNA called a primer binds to the 3' end of the strand. The leading strand is the simplest to replicate. The two sides are therefore replicated with two different processes to accommodate the directional difference. However, the replication fork is bi-directional one strand is oriented in the 3' to 5' direction (leading strand) while the other is oriented 5' to 3' (lagging strand). This directionality is important for replication as it only progresses in the 5' to 3' direction. The 5' end has a phosphate (P) group attached, while the 3' end has a hydroxyl (OH) group attached. This notation signifies which side group is attached the DNA backbone. This area will be the template for replication to begin.ĭNA is directional in both strands, signified by a 5' and 3' end. DNA helicase disrupts the hydrogen bonding between base pairs to separate the strands into a Y shape known as the replication fork. This is performed by an enzyme known as DNA helicase. In order to unwind DNA, these interactions between base pairs must be broken. Adenine only pairs with thymine and cytosine only binds with guanine. DNA has four bases called adenine (A), thymine (T), cytosine (C) and guanine (G) that form pairs between the two strands. Before DNA can be replicated, the double stranded molecule must be “unzipped” into two single strands.
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