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Genetics

How does the original strand of DNA act as a template?

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04/14/2009

DNA acts as a template for two things: # DNA # mRNA transcripts This is possible because each nitrogenous base in one strand pairs only with one other base in the complementary strand as follows: Adenine (A) always pairs with Thymine (T) Guanine (G) always pairs with Cytosine (C) Note: in RNA, Uracil (U) takes the place of Thymine (T), but still always pairs with Adenine (A). So, if one looks at a single strand of DNA, one can reconstruct the sequence of the complementary strand from the original strand, because each base on one strand will pair with only one base on the other. Consider this single strand of DNA: DNA acts as a template because of one fact: each nitrogenous base in one strand pairs only with one other base in the complementary strand as follows: Adenine always pairs with Thymine (or Uracil in RNA) Guanine always pairs with Cytosine So, if one looks at a single strand of DNA, one can reconstruct the sequence of the complementary strand from the original strand, because each base on one strand will pair with only one base on the other. Consider this single strand of DNA: ATTGCAT Looking at the first base, we know that A always pairs with T, and that T always pairs with A for the second base, and so on. Therefore we can reconstruct the complementary strand is: TAACGTA The mRNA transcript would be: UAACGUA This fact is important in two ways. First, when a cell needs to replicate its DNA, it uses an enzyme, DNA Polymerase, to assemble complementary strands by adding nucleotides with the appropriate matching bases along the strand. The result is two identical, complete DNA molecules. Secondly, when the cell needs to build messenger RNA for protein synthesis, it uses an enzyme called RNA Polymerase to assemble the mRNA transcript by adding RNA nucleotides with the complementary matching bases from the DNA strand. The result is an mRNA transcript which will be translated into a protein whose amino acid sequence will reflect the codon sequence of the original DNA strand.