In multicellular organisms, every cell has a definite life span. The old worn out cells have to be continuously replaced by the new cells. For this reason, cell divisions occur incessantly in the body. During each cell division, the DNA must also replicate and divide. DNA or deoxyribonucleic acid is the storehouse of genetic information that helps the body to develop and function properly.

Chemically, DNA comprises of two long strands of polymers that run in opposite direction to each other, and form the double helix that is the characteristic feature of DNA. Each strand is made up of four types of nucleotides — adenine, cytosine, thymine and guanine. In the nucleotide, a triphosphate and base are attached to deoxyribose sugar; they together form triphosphate deoxyribonucleoside.

As the result of the chemical interaction between the nucleotides phosphodiester linkages are formed, and in this manner the backbone of the DNA strand is created. Between the two strands, the nucleotides form base pair through hydrogen bonds, Adenine with thymine and cytosine with guanine.

As stated above, each DNA strand has a definite directionality. The two ends of a single strand are called 3’end and 5’end. The term denotes the carbon atom of the ribose sugar to which the next phosphate is attached. The directionality helps in DNA synthesis, while the sequence of nucleotides in one strand ascertains the sequence of nucleotides in the new strand of DNA during the replication process.

For the cell to divide successfully, the DNA should replicate. DNA has specific points called origins where the replication process begins. The replication initiator proteins recognize the origins and instruct other proteins to separate the two strands and form replication fork. After the formation of the replication fork, the RNA primers are produced on the template strands. These primers are used by the DNA polymerase enzyme to synthesize the entire new strand of DNA.

The replication process comes to an end when two replication forks meet one another. Multicellular organisms have linear chromosomes, and DNA replication usually doesn’t synthesize the end of the chromosome called telomere. Due to this, after a few cycles of DNA replication, the telomere shortens substantially. Thereafter, the cell stops dividing to prevent further loss of DNA.





In the germ line cells, as the DNA has to be passed to the next generation, therefore DNA loss has to be prevented. This is done by the enzyme called Telomerase, which in each replication cycle extends the telomere region and prevents DNA degradation. Sometimes, the telomerase enzyme becomes active in the somatic cells and this lead to the formation of cancer.