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

Answer: 5'-C - T - G-3'-PO4

Step-by-step solution

Understand the basic structure of DNA

DNA is a double-stranded molecule composed of nucleotide units. Each nucleotide is made up of a sugar (deoxyribose), a phosphate group, and a nitrogenous base. The bases are adenine (A), cytosine (C), guanine (G), and thymine (T). Nucleotides are connected by covalent bonds between the phosphate group of one nucleotide and the sugar of the next, forming the DNA backbone. In this exercise, we are working with a single-stranded section of DNA with the base sequence CTG.

Identify the 5' and 3' ends of the DNA strand

The 5' and 3' designations refer to the carbons of the sugar molecule in a nucleotide. The 5' end is the end of the strand where the fifth carbon of the sugar is not bonded to any other molecules, while the 3' end is the end where the third carbon of the sugar is not bonded to any other molecules. In our single-stranded DNA with the base sequence CTG, the 5' end will be at the cytosine (C) and the 3' end will be at the guanine (G).

Draw the base sequence with the proper backbone orientation

Here is the structural formula of our DNA strand with the base sequence CTG: ``` 5'-C - T - G-3' ```

Include the phosphorylation at the 3' end

Phosphorylation refers to the addition of a phosphate group to a molecule. In this case, we will add a phosphate group to the 3' end of the DNA strand. This occurs by forming a covalent bond between the phosphate group and the third carbon of the sugar at the 3' end (guanine). Here is the final structural formula of our DNA strand with the base sequence CTG and phosphorylation at the 3' end: ``` 5'-C - T - G-3'-PO4 ```