Question

How are the nucleotides held together in a nucleic acid polymer?

Short answer

Nucleotides are held together by phosphodiester bonds formed through dehydration synthesis.

Step-by-step solution

Understand the Structure of Nucleotides

Nucleotides are the building blocks of nucleic acids, like DNA and RNA. Each nucleotide consists of a phosphate group, a pentose sugar, and a nitrogenous base.

Identify the Bond Between Nucleotides

Nucleotides are held together by covalent bonds called phosphodiester bonds. These bonds form between the phosphate group of one nucleotide and the hydroxyl group on the 3' carbon of the pentose sugar of another nucleotide.

Formation of Phosphodiester Bonds

During the formation of nucleic acid polymers, an enzyme facilitates the removal of a water molecule, forming a phosphodiester bond. This process is known as dehydration synthesis or condensation reaction.

Direction of the Polymer Chain

The nucleic acid polymer has directionality, meaning it has a 5' end with a free phosphate group and a 3' end with a free hydroxyl group. The polymer grows by adding nucleotides to the 3' end.

Key concepts

Nucleotides

Nucleotides are the essential building blocks of nucleic acids, such as DNA and RNA.
Each nucleotide is composed of three main components:

• A phosphate group
• A pentose sugar (deoxyribose in DNA and ribose in RNA)
• A nitrogenous base (adenine, thymine, guanine, cytosine in DNA, and uracil in RNA instead of thymine)

These three parts form a nucleotide through covalent bonds, creating the fundamental unit of nucleic acid structure.

Phosphodiester Bonds

The backbone of nucleic acid polymers is formed through phosphodiester bonds. These bonds link the nucleotides together in a chain.
A phosphodiester bond is a covalent bond that forms between the phosphate group of one nucleotide and the hydroxyl group on the 3' carbon of the pentose sugar of another nucleotide.
The strong covalent nature of these bonds provides stability to the nucleic acid structure, making it resilient against breakdown.
The repeating pattern of phosphate and sugar units forms a sturdy backbone, with the nitrogenous bases extending from this core structure.

Dehydration Synthesis

The process of forming phosphodiester bonds involves a reaction known as dehydration synthesis. This reaction is also called a condensation reaction.
During dehydration synthesis, an enzyme helps to remove a water molecule (H₂O) from the reactants.
This occurs when a hydroxyl group (OH) from the 3' carbon of one nucleotide's sugar and a hydrogen atom (H) from the phosphate group of another nucleotide combine and are released as a water molecule.
The resulting bond between the nucleotides is a phosphodiester bond, contributing to the extension of the nucleic acid polymer.
This process repeats, enabling the formation of long chains of nucleic acids.

Polymer Directionality

Nucleic acid polymers have a specific directionality, which is crucial for their function.
This directionality is defined by the ends of the polymer chain:

• The 5' end, which has a free phosphate group
• The 3' end, which has a free hydroxyl group

Nucleotides are added to the growing chain at the 3' end, enabling the polymer to lengthen.
This 5' to 3' directionality is essential for processes such as DNA replication and RNA transcription.
Understanding polymer directionality helps in comprehending how genetic information is accurately copied and expressed in biological systems.