DNA stands for Deoxyribonucleic acid. It's a template for passing essential information related to an organism's functioning and development from one generation of cells to the next. The chemical constitution of DNA strand may be described as a long polymer of simple units called nucleotides. DNA molecules have a double helix structure with sugar and phosphate groups, joined by ester bonds, as its backbone. The genetic code is represented using four bases viz. adenine (A), thymine (T), guanine (G) and cytosine(C). Each sugar molecule has one of the four bases attached to it.
How does DNA replicate?
The DNA molecule consists of two strands held together by hydrogen bonds. DNA replication starts with the breaking of these bonds, by the action of the enzyme helicase, in order to yield two parent strands. This structure, of unpaired DNA, is called the DNA fork. Since the base sequence of the two strands is complementary to each other, they need to be replicated by two different mechanisms i.e. the leading chain mechanism and the lagging chain mechanism. In the leading chain mechanism and enzyme called DNA polymerase reads the sequence of the parent strand and then adds the corresponding base to give rise to a new DNA molecule. However, the lagging strand mechanism is slightly more complex. This is because the enzyme DNA polymerase can read the DNA strand only in one direction. Thus a facilitator called RNA primase is required for generating the second DNA molecule. The RNA primers, created by RNA primase, attach itself to the lagging strand at various points. The DNA polymerase to reads the lagging strand through these primers. Thus the lagging strand can also be synthesized. An enzyme called DNA ligase removes all the unwanted RNA primers in order to yield a complete DNA molecule. Thereafter enzymes like nucleases verify and correct errors on the new DNA strand to give a high degree of fidelity.
