Pol delta is part of a large complex of proteins, which includes an arm that reaches in front of the unzipping DNA ladder and grasps encircling it, like a doughnut the still-zipped region. In this way, it can better hang on to the DNA ladder as it moves along making copies.
Pol delta has high fidelity; meaning can make new strands that are 30, nucleotides long before falling off. Discontinuous DNA synthesis is the copying of a DNA template strand in short fragments and then combining them into one long strand. These short fragments are called Okazaki fragments, and in one mammalian cell cycle, 50 million fragments are made.
DNA Pol delta is responsible for making these fragments, which are only around nucleotides long. Another major difference between continuous and discontinuous synthesis is the number of primers for each. Primers are attached to unzipped DNA by enzymes called primases. Since continuous DNA synthesis makes only one strand, it needs only one primer to start it off. Discontinuous DNA synthesis, however, produces 50 million Okazaki fragments.
Since each Okazaki fragment needs a primer to start it off, discontinuous synthesis would need just as many primers as there are Okazaki fragments. Discontinuous DNA synthesis results in many Okazaki fragments that need to be merged. Thus, this type of synthesis requires more enzymes than continuous synthesis. An enzyme called primase lays down a primer that is RNA nucleotides long. In order to merge the fragments, enzymes called Fen1, Rnase H, and Dna2 replace the RNA primers at the start of each Okazaki fragment, and then ligate, or fuse, the fragments into one long strand.
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The sides of the double helix DNA molecules are made up of phosphate and sugar groups while the rungs are made up of nitrogenous bases. By convention, the carbon atoms in the carbon chains or rings of organic molecules are numbered in sequence. The carbon atoms in the nitrogenous bases are numbered 1, 2, 3, etc. To distinguish the numbered carbon atoms of the sugar groups, these carbons are numbered using a prime symbol, i.
There are five carbon atoms in the sugar groups, numbered 1' to 5'. The 5' atom has a phosphate group attached to it while the 3' carbon links to an OH group.
To form the sides of the helix, the 5' phosphate on one side of the sugar group links to the 3' OH of the next nucleotide. The sequence of this strand is 5' to 3'. The rungs of the helix molecule are formed from linked nitrogenous bases. The A and T bases can form a link, and G and C can link. This means one side of the helix runs in the 5' to 3' direction while the other side runs in the 3' to 5' direction.
DNA synthesis can only take place when the two strands of the double helix are separated. The strand running in the 5' to 3' direction is called the leading strand while the other strand, with a 3' to 5' sequence, is the lagging strand.
The polymerase can only copy DNA in the 5' to 3' direction. This means that it can continuously replicate the leading strand as it moves from the initial point of separation along the strand.
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