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.
Unwinding of Double Helix
The first step of DNA replication is the unwinding parent double helix molecule so that each strand acts as a template for the new strand. Unwinding mechanism is very complex. Hydrogen bonds between two strands are broken. This is achieved by enzymes called DNA helicases which move along DNA and separate the strands.
DNA helicases bind to the lagging strand template. Strand separation create topological stress like the one produced if the two ends of a coiled rope are pulled apart fc- separation. This forms supercoils in the unreplicated double helix in front of the replication fork. This stress is removed and strands are separated by the action of DNA topoisomerase Topoisomerase I introduces cut or nick in one DNA strand.
Topoisonmerase II remove supercoils by causing double stranded breaks. Some parts of DNA molecule have circle linked as a chain. This structure is called catenane. DNA gyrase is capable of decatenating two circles, thus separating them. In this way these enzymes open up and unwind the DNA helix.
In order to prevent the formation of hydrogen bonds again between two separated strand- the two single strands are coated by Single Strand Binding Proteins (SSB proteins), which stabilize the separated strands.
