Understanding the Double Helix Structure of DNA

DNA Double Helix
DNA Double Helix. Credit: Andrey Prokhorov/E+/Getty Images

In biology, double helix is a term used to describe the structure of DNA. A DNA double helix consists of two spiral chains of deoxyribonucleic acid. The shape is similar to that of spiral staircase. DNA is a nucleic acid composed of nitrogenous bases (adenine, cytosine, guanine and thymine), a five-carbon sugar (deoxyribose), and phosphate molecules. The nucleotide bases of DNA represent the stair steps of the staircase and the deoxyribose and phosphate molecules form the sides of the staircase.

Why Is DNA Twisted?

DNA is coiled into chromosomes and tightly packed in the nucleus of our cells.

The twisting aspect of DNA is a result of interactions between the molecules that comprise DNA and water. The nitrogenous bases that compose the steps of the twisted staircase are held together by hydrogen bonds. Adenine is bonded with thymine (A-T) and guanine pairs with cytosine (G-C). These nitrogenous bases are hydrophobic, meaning that they lack an affinity for water. Since the cell cytoplasm and cytosol contain water-based liquids, the nitrogenous bases want to avoid contact with cell fluids. The sugar and phosphate molecules that form the phosphate and sugar backbone of the molecule are hydrophilic. This means they are water loving and have an affinity for water. DNA is arranged such that the phosphate and sugar backbone are on the outside and in contact with fluid, while the nitrogenous bases are in the inner portion of the molecule. In order to further prevent the nitrogenous bases from coming in contact with cell fluid, the molecule twists to reduce space between the nitrogenous bases and the phosphate and sugar strands.

The fact that the two DNA strands that form the double helix are anti-parallel helps to twist the molecule as well. The DNA strands run in opposite directions ensuring that the strands fit tightly together, reducing the potential for fluid to seep between the bases.

DNA Replication and Protein Synthesis

The double helix shape allows for DNA replication and protein synthesis to occur.

In these processes, the DNA unwinds and opens to allow a copy of the DNA to be made. In DNA replication, the double helix unwinds and each separated strand is used to synthesize a new strand. As the new strands form, bases are paired together until two double helix DNA molecules are formed from a single double helix DNA molecule. DNA replication is required for the processes of mitosis and meiosis to occur.

In protein synthesis, the DNA molecule is transcribed to produce an RNA version of the DNA code known as messenger RNA (mRNA). The messenger RNA molecule is then translated to produce proteins. In order for DNA transcription to take place, the DNA double helix must unwind and allow an enzyme called RNA polymerase to transcribe the DNA. RNA is also a nucleic acid, but contains the base uracil instead of thymine. In transcription, guanine pairs with cytosine and adenine pairs with uracil to form the RNA transcript. After transcription, the DNA closes and twists back to its original state.

Examples: DNA Models