Deoxyribonucleic acid (DNA) serves as the instruction manual for all living organisms. It holds the genetic information defining everything from the simplest bacterium to complex humans. Its iconic double helix structure is central to its function.
Understanding the Double Helix
The DNA double helix is like a twisted ladder. Its “side rails” are formed by alternating sugar and phosphate groups, forming the sugar-phosphate backbone. Pairs of nitrogenous bases connect these two backbones, like the “rungs” of the ladder. There are four types of these bases: adenine (A), thymine (T), guanine (G), and cytosine (C).
Adenine always pairs with thymine, and guanine always pairs with cytosine. These pairings are held together by weak chemical bonds called hydrogen bonds. This precise pairing ensures that the two DNA strands are complementary, meaning the sequence of bases on one strand dictates the sequence on the other. The entire structure coils around a central axis in a right-handed spiral, with roughly 10 base pairs per complete turn, and a diameter of approximately 20 Å (2 nanometers).
The Purpose of Unwinding
The double helix, while stable, must temporarily unwind for the cell to access its genetic information. One primary reason for this is DNA replication, the process by which a cell makes exact copies of its DNA. This copying is necessary before cell division, ensuring each new daughter cell receives a complete and identical set of genetic instructions. Without unwinding, the machinery responsible for copying DNA cannot read the individual strands.
Unwinding is also necessary for gene expression, specifically during transcription. Transcription involves reading specific DNA segments, known as genes, to create messenger RNA (mRNA) molecules. These mRNA molecules then carry the genetic code out of the nucleus to be translated into proteins, which perform most of the work in cells. Both replication and transcription require the DNA strands to be separated to serve as templates for new molecule synthesis.
How the Double Helix Unwinds
The unwinding of the DNA double helix is carried out by a specialized enzyme called helicase. Helicase acts like a molecular zipper, moving along the DNA molecule and breaking the hydrogen bonds that hold the complementary base pairs together. This action effectively “unzipps” the two DNA strands, creating a Y-shaped structure known as a replication fork.
The helicase enzyme utilizes energy, often derived from the breakdown of adenosine triphosphate (ATP), to power its movement and the breaking of these bonds. While the general principle involves helicase pulling the DNA strands apart, the exact molecular details of how helicases accomplish this unwinding are still being studied.
What Happens Next
Once the DNA double helix has been unwound by helicase, the separated single strands become accessible for further biological processes. These individual strands serve as templates for the synthesis of new molecules. During DNA replication, each original strand guides the building of a new, complementary DNA strand, resulting in two identical double helix molecules, each containing one old and one new strand.
Similarly, during gene expression, an unwound DNA strand serves as a template for creating an RNA molecule. This RNA molecule carries the genetic information from a specific gene, allowing it to be expressed as a protein. The unwound state is temporary, providing necessary access for the cell’s machinery to read and utilize the genetic information.