When a cell divides, it is important that each daughter cell receives an identical copy of the DNA. This is accomplished by the process of DNA replication. The replication of DNA occurs before the cell begins to divide into two separate cells.

The discovery and characterization of the structure of the double helix provided a hint as to how DNA is copied. Recall that adenine nucleotides pair with thymine nucleotides, and cytosine with guanine. This means that the two strands are complementary to each other. For example, a strand of DNA with a nucleotide sequence of 3′-AGTCATGA-5′ will have a complementary strand with the sequence 5′-TCAGTACT-3′ (Figure 1). Recall that the two strands of the double helix are anti-parallel as you look at the order of bases in these two sequences.

Because of the complementarity of the two strands, having one strand means that it is possible to recreate the other strand. During DNA replication, the two strands are separated and the missing bases are filled in on each side (Figure 2). Each strand of DNA serves as a template to produce the missing strand. The new strand will be complementary to the parental or “old” strand. Each new double helix consists of one parental strand and one new daughter strand. This is known as semiconservative replication. When two DNA copies are formed, they have an identical sequence of nucleotide bases and are divided equally into two daughter cells.

References

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OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:2ousESf0@5/DNA-Replication