Deoxyribonucleic Acid (DNA) is the highly stable, double helix molecule that comprises the genetic material of life. The reason DNA is so stable is that it is made of two complementary strands and the bases that connect them. DNA’s twisted structure arises from sugar phosphate groups joined by strong covalent bonds, and thousands of weaker hydrogen bonds that join the nucleotide base pairs of adenine and thymine, and cytosine and guanine, respectively.
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The enzyme helicase can separate the tightly bound DNA double helix molecule, allowing for replication of DNA.
The Need to Separate DNA Strands
These tightly bound strands can be physically pulled apart, but they would rejoin into a double helix again due to their bonds. Similarly, heat can cause the two strands to separate or “melt.” But in order for cells to divide, DNA needs to be replicated. This means there needs to be a way of separating DNA to reveal its genetic code, and making new copies. This is called replication.
The Job of DNA Helicase
Prior to cell division, DNA replication begins. Initiator proteins begin to unfurl part of the double helix, almost like a zipper being unzipped. The enzyme that can perform this job is called a DNA helicase. These DNA helicases unzip the DNA where it needs to be synthesized. The helicases do this by breaking the nucleotide base pair hydrogen bonds that hold the two strands of DNA together. It is a process that uses the energy of adenosine triphosphate (ATP) molecules, which power all cells. The single strands are not permitted to return to a supercoiled state. In fact, the enzyme gyrase steps in and relaxes the helix.
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Once the base pairs are revealed by the DNA helicase, they can only bond with their complementary bases. Therefore each polynucleotide strand provides a template for a new, complementary side. At this point, the enzyme known as primase kickstarts replication on a short segment, or primer.
At the primer segment, the enzyme DNA polymerase polymerizes the original DNA strand. It works at the area where DNA is unwinding, called the replication fork. The nucleotides are polymerized starting at one end of the nucleotide chain, and synthesis proceeds in only one direction of the strand (the “leading” strand). New nucleotides join the revealed bases. Adenine (A) joins with thymine (T), and cytosine (C) joins with guanine (G). For the other strand, only short pieces can be synthesized, and these are called Okazaki fragments. The enzyme DNA ligase enters and completes the “lagging” strand. Enzymes “proofread” the replicated DNA and remove 99 percent of any errors found. The new strands of DNA contain the same information as the parent strand. This is a remarkable process, constantly occurring in many millions of cells.
Because of its strong bonding and stability, DNA cannot simply break apart on its own, but rather conserves genetic information to be passed on to new cells and descendants. The highly efficient enzyme helicase makes possible the breaking apart of the tremendously coiled DNA molecule, so that life can continue.