Question

In the formation of a polynucleotide (a short portion of the DNA molecule), which components (sugar, base, or phosphate) on adjacent nucleotides bond to each other?

Short answer

In the formation of a polynucleotide, the sugar component of one nucleotide bonds with the phosphate group of the adjacent nucleotide, creating phosphodiester bonds that form the backbone of the polynucleotide chain. The nitrogenous bases do not bond to each other within the same chain, but they do interact with complementary bases on another polynucleotide chain via hydrogen bonding.

Step-by-step solution

Define a nucleotide

A nucleotide is the basic building block of nucleic acids such as DNA and RNA. It consists of three components: a sugar molecule, a phosphate group, and a nitrogenous base. The sugar molecule in DNA is deoxyribose, while in RNA it is ribose.

Describe the structure of a polynucleotide

A polynucleotide is formed when multiple nucleotides are joined together in a chain or sequence. The sugar molecule in one nucleotide is connected to the phosphate group of the adjacent nucleotide. This connection forms the backbone of the polynucleotide chain, while the nitrogenous bases protrude from the backbone and are available for pairing with complementary bases on another polynucleotide chain.

Identify the components on adjacent nucleotides that bond to each other

In the formation of a polynucleotide, the sugar molecule of one nucleotide (either ribose in RNA or deoxyribose in DNA) forms a bond with the phosphate group of the adjacent nucleotide. This is known as a phosphodiester bond. The nitrogenous bases do not directly bond to one another between adjacent nucleotides but are available for base pairing with complementary bases on another polynucleotide chain.

Conclusion

In the formation of a polynucleotide (a short portion of the DNA molecule), the sugar component on one nucleotide bonds with the phosphate group on the adjacent nucleotide, creating phosphodiester bonds that form the backbone of the polynucleotide chain. The nitrogenous bases do not bond to each other within the same chain, but they do interact with complementary bases on another polynucleotide chain via hydrogen bonding (not covered in this exercise).

Key concepts

Phosphodiester Bond

Understanding the formation of polynucleotides is a fundamental aspect of molecular biology. Key to this process is the creation of phosphodiester bonds. These bonds are the backbone of the structure of DNA and RNA, providing the necessary stability and integrity for the transfer of genetic information.

During polynucleotide synthesis, a phosphodiester bond is formed when the phosphate group of one nucleotide reacts with the hydroxyl group on the sugar component of another nucleotide. Specifically, this bond links the 3' carbon atom of one sugar molecule to the 5' phosphate group of the next nucleotide in the chain. This connection establishes a directional strand that has a distinct 5' end and a 3' end, terms referring to the carbon numbers in the DNA's sugar backbone. The sequence, in which the nucleotides are added, determines the genetic information contained within a polynucleotide.

Why is it called a 'diester' bond? In chemistry, an ester bond forms between an alcohol and an acid. In a phosphodiester bond, there are two ester links; one to the phosphate group and another to the sugar ring. This dual ester formation facilitates the linkage of nucleotides into long, stable chains that are essential for genetic function and replication.

Nucleotide Components

Nucleotides, the building blocks of DNA and RNA, are composed of three essential components - a sugar molecule, a nitrogenous base, and a phosphate group. Each of these components plays a vital role in nucleotide function and the overall structure of nucleic acids.

The sugar in a nucleotide is either ribose (found in RNA) or deoxyribose (found in DNA). Ribose has one more oxygen atom than deoxyribose, and this slight difference significantly impacts the structure and functionality of the resulting nucleic acid. The nitrogenous base attached to the sugar can be one of four types in DNA: adenine (A), cytosine (C), guanine (G), or thymine (T), and in RNA, uracil (U) replaces thymine. These bases are categorized into two groups based on their structures: purines (adenine and guanine) and pyrimidines (cytosine, thymine, and uracil).

The third component, a phosphate group, is negatively charged and acidic. It connects to the 5' carbon of the sugar, creating a consistent linkage point for the formation of the phosphodiester bond with another nucleotide's sugar. This attachment site is crucial for the chaining of nucleotides during the formation of polynucleotides.

DNA and RNA Structure

The structures of DNA and RNA are a topic of much interest in biology due to their roles in the storage and expression of genetic information. DNA is typically double-stranded, with two polynucleotide chains coiled around each other to form a double helix. In contrast, RNA usually exists as a single-stranded molecule that can form various secondary structures based on intramolecular base pairing.

In DNA, the two strands are held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine, and cytosine pairs with guanine. RNA also follows base pairing rules where adenine pairs with uracil and cytosine with guanine. These interactions are critical for the structure's stability and function in processes like transcription and translation.

DNA's Double Helix: The iconic DNA double helix structure provides a stable framework for the molecule. Its formation is a result of base pairing and the wrapping of the polynucleotide chains around a common axis. This configuration is critical for DNA replication and packaging into chromosomes.

RNA's Versatile Structures: RNA can fold into various shapes, such as hairpins and loops because of its single-stranded nature. These structures are vital for RNA's many functions, including its role as a messenger (mRNA), a component of the ribosome (rRNA), or as an enzyme (ribozyme).

Both DNA and RNA are crucial for maintaining the flow of genetic information from DNA to RNA to protein, commonly known as the central dogma of molecular biology.