Nucleic Acids and Protein Synthesis: Components and Structure

Chapter 17: Nucleic Acids and Protein Synthesis

Components of Nucleic Acids

Nucleic acids are essential biomolecules responsible for the storage and transmission of genetic information. There are two main types: DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid). Both are polymers composed of repeating units called nucleotides.

• Nucleotide Structure: Each nucleotide consists of three components:

  • Nitrogen-containing base

  • Pentose sugar (ribose in RNA, deoxyribose in DNA)

  • Phosphate group

• Types of Bases:

  • DNA: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)

  • RNA: Adenine (A), Guanine (G), Cytosine (C), Uracil (U) (Thymine is replaced by Uracil)

Bases of DNA and RNA

The nitrogen-containing bases in nucleic acids are derivatives of pyrimidine or purine. These bases are classified by their ring structures:

• Pyrimidines: Single-ring structures

  • Cytosine (C) – found in both DNA and RNA

  • Thymine (T) – found only in DNA

  • Uracil (U) – found only in RNA

• Purines: Double-ring structures

  • Adenine (A) – found in both DNA and RNA

  • Guanine (G) – found in both DNA and RNA

Example: The structure of adenine (a purine) contains two fused rings, while cytosine (a pyrimidine) contains a single ring.

DNA and RNA Bases: Identification

DNA contains the bases A, G, C, and T; RNA contains A, G, C, and U. Students should be able to identify these bases from their chemical structures.

• Pyrimidine Bases: Cytosine (C), Thymine (T), Uracil (U)

• Purine Bases: Adenine (A), Guanine (G)

DNA and RNA Sugars

The pentose sugar forms the backbone of nucleic acids. Both DNA and RNA contain five-carbon sugars, but with a key difference:

• RNA: Contains ribose, which has a hydroxyl group (-OH) on carbon 2'.

• DNA: Contains deoxyribose, which lacks the hydroxyl group on carbon 2' (hence 'deoxy').

• The carbon atoms in the sugars are numbered with primes (1', 2', 3', etc.) to distinguish them from atoms in the bases.

Nucleosides (Sugar + Base)

A nucleoside is formed when a nitrogen-containing base bonds to the 1' carbon of a pentose sugar (ribose or deoxyribose) via an N-glycosidic bond.

• Naming:

  • Purine nucleosides end in -osine (e.g., adenosine, guanosine)

  • Pyrimidine nucleosides end in -idine (e.g., cytidine, uridine)

• Example: Adenine + ribose = adenosine

Nucleotides (Sugar + Base + Phosphate)

A nucleotide is a nucleoside with one or more phosphate groups attached to the 5' carbon of the sugar. Nucleotides are the monomeric units of nucleic acids.

• Formation: Phosphate group bonds to the 5' -OH of ribose or deoxyribose, forming a phosphate ester.

• Backbone: The sequence of phosphate-sugar-base units forms the backbone of DNA and RNA.

Nucleosides and Nucleotides in DNA and RNA

The nucleosides and nucleotides differ between DNA and RNA due to the sugar and base composition.

Base	Nucleoside (DNA)	Nucleotide (DNA)
Adenine (A)	Deoxyadenosine	Deoxyadenosine monophosphate (dAMP)
Guanine (G)	Deoxyguanosine	Deoxyguanosine monophosphate (dGMP)
Cytosine (C)	Deoxycytidine	Deoxycytidine monophosphate (dCMP)
Thymine (T)	Deoxythymidine	Deoxythymidine monophosphate (dTMP)

Base	Nucleoside (RNA)	Nucleotide (RNA)
Adenine (A)	Adenosine	Adenosine monophosphate (AMP)
Guanine (G)	Guanosine	Guanosine monophosphate (GMP)
Cytosine (C)	Cytidine	Cytidine monophosphate (CMP)
Uracil (U)	Uridine	Uridine monophosphate (UMP)

Review: Components in DNA and RNA

Component	DNA	RNA
Bases	A, G, C, T	A, G, C, U
Sugar	Deoxyribose	Ribose
Nucleoside	Base + deoxyribose	Base + ribose
Nucleotide	Base + deoxyribose + phosphate	Base + ribose + phosphate
Nucleic Acid	Linear chain of deoxyribose nucleotides	Linear chain of ribose nucleotides

Learning Check: Purine vs. Pyrimidine

Identify the following bases as a purine or pyrimidine:

• Adenine: Purine

• Cytosine: Pyrimidine

Learning Check: DNA and RNA Base Components

Identify each base as a component of DNA, RNA, or both:

• Guanine: DNA and RNA

• Adenine: DNA and RNA

• Thymine: DNA only

• Uracil: RNA only

Sample Problem: Nucleotide Identification

For each nucleotide, identify the components and whether it is found in DNA only, RNA only, or both:

• Deoxyguanosine monophosphate (dGMP): Contains deoxyribose, guanine, and a phosphate group; found only in DNA.

• Adenosine monophosphate (AMP): Contains ribose, adenine, and a phosphate group; found only in RNA.

Key Equations and Structures

• Nucleotide General Structure: $\text{Nucleotide} = \text{Base} + \text{Sugar} + \text{Phosphate}$

• Nucleoside Formation: $\text{Nucleoside} = \text{Base} + \text{Sugar}$

• Phosphodiester Bond Formation: $\text{Nucleotide}_n + \text{Nucleotide}_{n+1} \rightarrow \text{Nucleic Acid} + H_2O$

Additional info: These notes cover the foundational chemical structures and nomenclature of nucleic acids, which are essential for understanding genetic information and protein synthesis in biological systems.