Nucleic Acids: Structure and Sequence (Chapter 17 Study Notes)

Chapter 17: Nucleic Acids and Protein Synthesis

Primary Structure of Nucleic Acids

Nucleic acids are essential biomolecules that store and transmit genetic information in living organisms. Their primary structure refers to the unique sequence of nucleotide bases that encodes genetic instructions.

• Nucleic Acid Sequence: Each nucleic acid (DNA or RNA) has a specific order of bases (adenine, cytosine, guanine, thymine/uracil) that determines its genetic code.

• Genetic Information: The sequence of bases carries genetic information from one cell to the next, ensuring continuity of hereditary traits.

• Abbreviated Notation: The sequence can be written using the one-letter abbreviations for each base, e.g., A-C-G-U for RNA.

• Example: In the figure of an RNA nucleotide, the sequence is shown as A-C-G-U.

Nucleic Acid Structure

Nucleic acids are polymers made up of repeating nucleotide units. Their structure is defined by the way these nucleotides are linked together.

• Polymerization: Nucleic acids are formed by joining nucleotides through covalent bonds between the 3'-hydroxyl group of one sugar and the phosphate group attached to the 5'-carbon of the next sugar.

• Primary Structure: The unique sequence of bases in a nucleic acid is referred to as its primary structure.

• Terminal Ends: One end of the nucleic acid has a free 5' phosphate group, while the other end has a free 3' hydroxyl group.

• Directionality: Nucleic acid sequences are always read from the 5' end to the 3' end.

Phosphodiester Bond

The backbone of nucleic acids is formed by phosphodiester bonds, which link the sugars of adjacent nucleotides.

• Definition: A phosphodiester bond is a covalent bond that joins the 3'-OH group of one sugar to the phosphate group on the 5'-carbon of the next sugar.

• Formation: This bond is formed through a condensation reaction, releasing water ($H_2O$).

• Equation: $\text{Nucleotide}_1\text{ (3'-OH)} + \text{Nucleotide}_2\text{ (5'-phosphate)} \rightarrow \text{Nucleotide}_1\text{-phosphodiester-Nucleotide}_2 + H_2O$

• Example: The diagram shows the linkage between two nucleotides via a phosphodiester bond.

Nucleotides vs. Nucleosides

It is important to distinguish between nucleotides and nucleosides, as both are building blocks of nucleic acids but differ in composition.

• Nucleotide: Consists of a phosphate group, a five-carbon sugar (ribose or deoxyribose), and a nitrogenous base.

• Nucleoside: Consists of only a sugar and a nitrogenous base (no phosphate group).

• Identification: If the structure contains a phosphate-sugar-base, it is a nucleotide.

• Example: The provided structure with a phosphate group is a nucleotide.

Nucleic Acid Sequence Reading

The sequence of a nucleic acid is read from the 5' end (free phosphate group) to the 3' end (free hydroxyl group).

• Reading Direction: Always start from the sugar/base at the free 5' phosphate end at the top, moving to the sugar/base with the free 3' hydroxyl group at the bottom.

• Example: For the sequence —C G A U—, cytosine is at the free 5' phosphate end.

Table: Comparison of Nucleotide and Nucleoside

Component	Nucleotide	Nucleoside
Sugar	Present	Present
Base	Present	Present
Phosphate Group	Present	Absent

Key Terms and Definitions

• Nucleotide: The monomer unit of nucleic acids, composed of a phosphate group, a sugar, and a nitrogenous base.

• Nucleoside: A compound consisting of a sugar and a nitrogenous base, without a phosphate group.

• Phosphodiester Bond: The covalent bond that links nucleotides together in a nucleic acid chain.

• Primary Structure: The linear sequence of bases in a nucleic acid.

Summary Table: Nucleic Acid Structure Features

Feature	Description
Polymer Type	Polynucleotide
Monomer	Nucleotide
Bond Type	Phosphodiester bond
Sequence Direction	5' to 3'
Terminal Groups	5' phosphate (start), 3' hydroxyl (end)

Additional info: These notes cover the foundational aspects of nucleic acid structure, including the chemical nature of nucleotides, the formation of the nucleic acid backbone, and the conventions for reading and writing nucleic acid sequences. Understanding these principles is essential for further study of genetic information flow and protein synthesis.