DNA

Cell division exists to produce more cells for growth and repair processes. It is essential that all cells of the body have the same genetic material (2004). For this reason, an important event always precedes cell division: the exact duplication of the genetic material in a process called DNA replication. Only after this has occurred can mitosis take place (2000).

The primary function of DNA replication is understood to be the provision of progeny with the genetic information possessed by the parent. Thus, the replication of DNA must be complete and carried out with high fidelity to maintain genetic stability within the organism and the species. The process of DNA replication is complex and involves many cellular functions and several verification procedures to ensure fidelity in replication (2000). About 30 proteins are involved in the replication of the E Coli chromosome, and this process is almost certainly more complex in eukaryotic organisms.

DNA in eukaryotic cells is associated with a variety of proteins, resulting in a structure called chromatin (2000). The double-stranded DNA helix in the chromatin of each eukaryotic chromosome has a length that is thousands of times the diameter of the cell nucleus. The precise trigger for DNA synthesis is unknown, but once it starts, it continues until all the DNA has been replicated (2004).

Eukaryotic DNA in nondividing cells is located in chromosomes, which typically are associated as identical pairs. DNA replication begins as the DNA helix uncoils and gradually separates into its two nucleotide chains.   The DNA in each chromosome is exactly replicated according to the rules of base pairing during the S phase of the cell cycle. Each strand of the double helix is replicated simultaneously but by somewhat different mechanisms. Nucleotides join in a complementary way: adenine always bonds to thymine, and guanine always bonds to cytosine (2004). Hence, the order of the nucleotides on the template strand also determines the order on the new strand.

A complex of proteins replicates the leading strand continuously in the 5’ to 3’ direction. The lagging strand is replicated discontinuously, in short pieces of 150-250 nucleotides, in the 3’ to 5’ direction (2000). The principal enzymes for replicating DNA are a complex of multiple enzymes called DNA polymerase which attaches and moves along the DNA template strand (2000). These are joined by DNA ligase. DNA ligase causes bonding of successive DNA nucleotides to one another, using high-energy phosphate bonds to energize these attachments ( 2000). DNA replication occurs at several sites, called replication bubbles, in each chromosome (2000). The entire process takes about 9 hours in a typical cell. The end result is that two DNA molecules are formed that are identical to the original DNA helix, and each consists of one old and one newly assembled nucleotide strand. Each newly formed strand of DNA remains attached by loose hydrogen bonding to the original DNA strand that has been used as its template ( 2000).This new set of nucleotide strand then serves as a template, or set of instructions, for building a new nucleotide strand.