Question

Draw a structural formula for the section of DNA that contains the base sequence CTG and is phosphorylated at the \(3^{\prime}\) end only.

Short answer

Question: Draw the structural formula for a DNA section containing the base sequence CTG, with only the \(3^{\prime}\) end phosphorylated. Explain the orientation of the DNA strands and the key components. Answer: The structural formula for a DNA section containing the base sequence CTG can be obtained by drawing nucleotides with the bases C, T, and G connected to the deoxyribose sugar and phosphate groups. The complementary strand will have the sequence GAC, following the base pairing rule. The two strands will be oriented in opposite directions, with one strand having the \(5^{\prime}\) end on one side and the \(3^{\prime}\) end on the other side, and the complementary strand having the opposite orientation. The phosphorylation at the \(3^{\prime}\) end is indicated by an extra phosphate group on one of the strands. The key components of the DNA structure are deoxyribose sugar, phosphate group, and nitrogenous bases (CTG for one strand and GAC for the complementary strand).

Step-by-step solution

Identify the components of DNA structure

In this step, identify that DNA is made up of two main components: deoxyribose sugar, phosphate group, and nitrogenous bases (A, T, C, and G). The bases pair with their complementary bases on the opposite strand, with adenine (A) pairing with thymine (T) and cytosine (C) pairing with guanine (G), forming a double helix structure.

Draw the nucleotide structure

Begin by drawing a nucleotide, which consists of a phosphate group, deoxyribose sugar, and one of the bases (A, T, C, or G). The phosphate group is connected to the 5' carbon of the sugar, while the nitrogenous base is connected to the 1' carbon.

Draw the complementary strand

Now that we know the desired base sequence is CTG, draw the complementary base sequence for the opposite strand using the base pairing rule. In this case, the complementary strand will have the sequence GAC.

Draw the \(5^{\prime}-3^{\prime}\) orientation of DNA strands

Draw the DNA strands in the correct orientation. DNA strands run in opposite directions, so one strand will have the \(5^{\prime}\) end on one side and the \(3^{\prime}\) end on the other side. The complementary strand should have the opposite orientation, meaning the \(5^{\prime}\) end is opposite the \(3^{\prime}\) end of the first strand.

Phosphorylate the \(3^{\prime}\) end

The exercise states that only the \(3^{\prime}\) end of the DNA sequence is phosphorylated. To indicate this, add an extra phosphate group to the \(3^{\prime}\) end of one of the strands. By following these steps, you should have a structural formula for the DNA section containing the base sequence CTG and phosphorylated only at the \(3^{\prime}\) end.