One may wonder how a single cell becomes two cells, and why this is ultimately important to life. For this essay, it would be much too difficult to discuss the process of cell division, which is the biological basis of life. That being said, this essay will examine more closely the precursor to cell division, also known as DNA replication. DNA replication is the process of copying a double-stranded DNA strand on a chromosome within a cell. The process, in its totality, is a sequence of three basic steps: initiation, elongation, and termination.
To understand the process of DNA replication, you much first be familiar with the structure of DNA. Resembling a twisted ladder or a zipper, DNA is a double helix formed with nucleotides (structural units of DNA containing a base, a sugar, and one or more phosphates), a phosphate and sugar backbone, and nitrogenous bases. There are four bases, and each one will only bond with its complement. Adenine will only bond with Thymine, and Cytosine with only Guanine. The double helix is ant parallel, meaning each strand runs in a different direction.
Initiation, the first step to DNA replication is the unwinding of the double helix with the help of an enzyme called helicase. Once unwound, the DNA unzips and the bonded nucleotides are separated. In the next stage, elongation, an enzyme called DNA polymerase is important. The DNA polymerase attaches to each strand at the replication origins, reading the nucleotides and finding the complement base. Because the leading strand runs in the same direction that it was unzipped, the DNA polymerase moves with ease down the length of the strand.
Because human DNA is so very long (with up to 80 million base pairs in a chromosome) it unzips at multiple places along its length so that the replication process is going on simultaneously at hundreds of places along the length of the chain. Eventually these areas run together to form a complete chain. The lagging strand, however, runs in the opposite direction that it was unzipped, making it harder for the DNA polymerase to move down the strand, so it keeps returning to the strand until the job is done.
Proof reader enzymes then run down the strand detecting and fixing errors. To complete the replication, the new strands are paired with their corresponding original strands. This is called semi-conservative because there is one new and one old strand combined. The strands are rewound into a double helix with the help of enzymes. The cell then prepare for mitosis where a copy of the DNA will be sent to each nucleus, then cells will go through ctytokinesis, where a nucleus will be distributed to each cell.
This process has provided the daughter cell with an exact copy of DNA from its parent. DNA polymerase makes very few errors, and most of those that are made are quickly corrected by DNA polymerase and other enzymes that “proofread” the nucleotides added into the new DNA strand. If a newly added nucleotide is not complementary to the one on the template strand, these enzymes remove the nucleotide and replace it with the correct one. With this system, a cell’s DNA is copied with less than one mistake in a billion nucleotides.
