DNA is a deceptively simple macromolecule that contains information to guide the development of the vast majority of living organisms on Earth. If the structure of an organism's DNA is corrupted, then the subsequent development of the organism will be disturbed, so DNA needs to be resistant to environmental influences. The double helix structure provides that resistance, sheltering the information-carrying nucleotides on the inside of the helix, away from outside influences. But those information-carrying segments need to be exposed when the time comes to pass that information on. There are a few different mechanisms that can split the double helix.
Why DNA Needs to Break Apart
DNA is composed of an ordered string of four different bases: adenine, thymine, cytosine and guanine -- usually referred to as A, T, C and G. The bases chemically connect strongly together in a strand of DNA, and bases in one strand of DNA weakly connect to bases in a complementary strand, creating the double helix. The order of the bases controls the formation of all proteins within an organism, the timing and quantity of protein formation and the generation of new cells. The only way it can perform those functions is if the bases are available to be sensed by other molecules. That's why the DNA strands must be separated from each other.
One of the most important roles of DNA is to replicate -- produce a copy of itself. In the human body, for example, those copies must be perfectly produced literally trillions of times. The first step in replication is to unwind the strands of DNA so the bases can be exposed and read. The molecule that does the job is an enzyme called DNA helicase. DNA helicase is a circular molecule that wraps around one of the DNA strands and slides down it, separating the two strands. Another enzyme, DNA polymerase, then grabs one of the strands and starts reconstructing a double helix, building up a new strand base by base.
DNA contains the instructions for building proteins, but DNA doesn't participate directly in the manufacture of proteins. Instead, DNA produces an intermediary molecule, RNA. RNA then serves as the template to produce proteins. To make RNA, the two strands of DNA must be separated to allow the bases to be accessed. Again, an enzyme, a large molecule called RNA polymerase II, does the bulk of the work. Unlike the DNA helicase, RNA Pol II doesn't completely separate the strands. It just unzips the DNA strands for about 17 base pairs, then zips the opening back up again. RNA nucleosides enter the region where the DNA has been separated. RNA Pol II links the nucleosides together to create a complementary strand of messenger RNA.
DNA is a molecule with a very stable, protected configuration -- as long as it's in its native environment. If it gets too hot, however, DNA will "melt" -- separate into two separate strands. That happens at about 95 degrees Celsius (about 200 degrees Fahrenheit). Although that temperature won't be reached in a living organism, it can easily be reached in the lab. That's the first step in a process called the polymerase chain reaction, or PCR. PCR is a fundamental laboratory method at the heart of virtually all DNA analysis and genetic engineering, making DNA melting an important mechanism for breaking apart the double helix of DNA.