CBSE Class 10 Answered

DNA copying or DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance.

The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule. Each strand of the original double-stranded DNA molecule serves as template for the production of the complementary strand, a process referred to as semiconservative replication.

In a cell, DNA replication begins at specific locations in the genome, called origins Unwinding of DNA at the origin, and synthesis of new strands, forms a replication fork. In addition to DNA polymerase (the enzyme that synthesizes the new DNA by adding nucleotides matched to the template strand) a number of other proteins are associated with the fork and assist in the initiation and continuation of DNA synthesis.

Several enzymes and proteins are involved with the replication of DNA. At a specific point, the double helix of DNA is caused to unwind possibly in response to an initial synthesis of a short RNA strand using the enzyme helicase. Proteins are available to hold the unwound DNA strands in position. Each strand of DNA then serves as a template to guide the synthesis of its complementary strand of DNA. DNA polymerase III is used to join the appropriate nucleotide units together. 

It is very important that the cells duplicate the DNA genetic material exactly, that the sequence of newly synthesized nucleotides is checked by two different polymerase enzymes. The second enzyme can check for and actually correct any mistake of mismatched base pairs in the sequence. The mismatched nucleotides are hydrolyzed and cut out and new correct ones are inserted.