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 a structural formula for a DNA section containing the base sequence CTG with a phosphate group at the 3' end only. Include the complementary strand in your representation and indicate the hydrogen bonding between the bases. Answer: The DNA section containing the base sequence CTG with a phosphate group at the 3' end and its complementary strand can be represented as: `5'-GAC-3'` `3'-CTG-5' (phosphate)` With hydrogen bonds between G and C (3 hydrogen bonds) and A and T (2 hydrogen bonds).

Step-by-step solution

Draw the backbone structure with 3' end phosphorylation

To start, draw the sugar-phosphate backbone of the DNA section, which consists of the sugar (deoxyribose) and phosphate groups connected together. Since the \(3^{\prime}\) end is phosphorylated, add a phosphate group to the \(3^{\prime}\) end of the sugar.

Add the nitrogenous bases

Now add the nitrogenous bases to the sugar component of the backbone. The sequence we are given is CTG, which stands for Cytosine (C), Thymine (T), and Guanine (G). Connect Cytosine to the first sugar from the \(3^{\prime}\) phosphate group, Thymine to the second sugar, and Guanine to the third sugar.

Include hydrogen bonding

Once the bases are attached to the sugar components, represent hydrogen bonding between the bases. For the given sequence, Cytosine (C) will hydrogen-bond to Guanine (G) on the complementary strand with three hydrogen bonds, while Thymine (T) will hydrogen-bond to Adenine (A) on the complementary strand with two hydrogen bonds.

Draw the complementary strand

Now, we need to draw the complementary strand of DNA. The complementary bases on the opposite strand should be in the following order: G for C, A for T, and C for G. In other words, the complementary sequence will be GAC. Therefore, the complete DNA section is represented as: `5'-GAC-3'` `3'-CTG-5' (phosphate)` Note that the phosphate group is only present at the \(3^{\prime}\) end of the CTG sequence, as stated in the exercise.

Key concepts

Phosphorylation at the 3' End

In the structure of DNA, phosphorylation refers to the addition of a phosphate group to the molecule. The phosphate group is an integral part of the DNA structure, linking individual nucleotides to form the sugar-phosphate backbone. When we say a DNA strand is phosphorylated at the 3' end, it means that a phosphate group is attached to the third carbon atom in the deoxyribose sugar at one end of the strand.

For a visual, imagine the sugar molecule as a pentagon, with each corner representing a carbon atom numbered 1' to 5'. The phosphate group would be connected to the 3' carbon, hence the term '3' end phosphorylation’. This is significant in DNA synthesis because enzymes involved in DNA replication recognize the phosphorylated ends to initiate the process.

In exercises that ask you to depict this, you'll draw the phosphate group attached to the outside of the last sugar molecule (at the 3' position), indicating that this is the end of the chain and that no more nucleotides can be added at this point without further enzymatic action.

Nitrogenous Bases

The nitrogenous bases in DNA play a critical role in storing genetic information. There are four types of nitrogenous bases found in DNA: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). Adenine pairs with Thymine via two hydrogen bonds, and Guanine pairs with Cytosine through three hydrogen bonds.

These nitrogenous bases attach to the 1' carbon of the deoxyribose sugar in the DNA backbone. Their specific sequence forms the genetic code used by cells to produce proteins. Each base pairs specifically with its complement, ensuring the fidelity of genetic information during DNA replication. The order of nitrogenous bases constitutes the genetic code; for example, the sequence CTG mentioned in the exercise stands for Cytosine, Thymine, and Guanine, unique identifiers within the DNA for specific genetic instructions.

Complementary Strands

DNA consists of two strands running in opposite directions, each serving as a template for the other. These are known as complementary strands because each nitrogenous base on one strand pairs with a specific complementary base on the other. As a rule, Adenine pairs with Thymine, and Guanine pairs with Cytosine.

The complementary nature of DNA strands is crucial for many biological processes, especially DNA replication and transcription. During these processes, the strands separate, and each serves as a template for creating a new complementary strand. This ensures that the genetic information is accurately copied and transferred. For instance, if one strand has the sequence CTG as in the given exercise, its complementary strand would be GAC.

Hydrogen Bonding in DNA

Hydrogen bonding is the force that holds the complementary nitrogenous bases of DNA together. These weak bonds are essential for the double helix structure, as they enable the strands to separate and rejoin, which is crucial during replication and transcription processes.

In DNA, Adenine and Thymine form two hydrogen bonds between them, while Guanine and Cytosine form three. Although these bonds are weaker than covalent bonds, their sheer number across the double helix provides sufficient strength to keep the strands together. However, they are weak enough to be broken by enzymes that mediate replication and other cellular processes when necessary. In a diagram, you would typically represent these hydrogen bonds with dotted lines or symbols between the nitrogenous bases to illustrate this essential aspect of DNA strand pairing.