During DNA replication, nucleotides are specifically matched to their complementary base pair. According to PBS, DNA resembles a long spiraling ladder consisting of a sugar-phosphate backbone and nucleotide pairs making up the rungs.
There are four different nucleotides used to sequence the genetic code of DNA: thymine, adenine, cytosine and guanine. The order of these nucleotides within DNA sequences determines the genetic code that is expressed. During DNA replication, special enzymes unzip the DNA strand as if a ladder were being cut down the middle. New nucleotides are then matched up to each of these DNA halves to produce two identical DNA strands. Cytosine always pairs with guanine, and adenine always pairs with thymine. These two strands of DNA each contain one side of the original DNA and another side made of new nucleotides.