Nucleic Acids and Carbohydrates: Structure and Function

Nucleic Acids

Introduction to Nucleic Acids

Nucleic acids are essential biomolecules that store and transmit genetic information in all living organisms. The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These molecules serve as blueprints for proteins and regulate the life of a cell.

• DNA: Stores genetic instructions for protein synthesis.

• RNA: Involved in protein synthesis and regulation of gene expression.

Nucleotide Structure

Nucleic acids are polymers made up of monomers called nucleotides. Each nucleotide consists of three components:

• Five-carbon sugar (pentose): Ribose in RNA, deoxyribose in DNA.

• Phosphate group: Provides the acidic property and links nucleotides together.

• Nitrogenous base: Encodes genetic information.

The general structure of a nucleotide can be represented as:

• Phosphate group – attached to the 5' carbon of the sugar.

• Sugar – a pentose (ribose or deoxyribose).

• Nitrogenous base – attached to the 1' carbon of the sugar.

Nitrogenous Bases

Nitrogenous bases are classified into two families:

• Pyrimidines (single ring): Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)

• Purines (double ring): Adenine (A), Guanine (G)

DNA contains A, T, C, G. RNA contains A, U, C, G.

Comparison of DNA and RNA Sugars

	DNA	RNA
Sugar	Deoxyribose	Ribose
Presence of 2' OH group	No (H at 2')	Yes (OH at 2')

Polynucleotide Synthesis

Nucleotides are joined together by phosphodiester bonds to form polynucleotide chains. This process occurs via dehydration synthesis (condensation reaction), where a water molecule is removed to form the bond.

• The phosphate group of one nucleotide links the 3' carbon of one sugar to the 5' carbon of the next sugar.

• This forms the sugar-phosphate backbone of nucleic acids.

Phosphodiester linkage:

$\text{Nucleotide}_1\text{–}3'\text{OH} + \text{Nucleotide}_2\text{–}5'\text{phosphate} \rightarrow \text{Nucleotide}_1\text{–}3'\text{O–PO}_4\text{–}5'\text{Nucleotide}_2 + \text{H}_2\text{O}$

Structure of DNA

DNA is typically a double helix composed of two antiparallel strands held together by hydrogen bonds between complementary base pairs:

• Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)

• Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds)

The backbone is formed by alternating sugar and phosphate groups, while the bases project inward and pair specifically.

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information:

• DNA → RNA → Protein

This means genetic information is transcribed from DNA to RNA and then translated into proteins.

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double-stranded	Single-stranded (usually)
Function	Genetic information storage	Protein synthesis, regulation

Carbohydrates

Introduction to Carbohydrates

Carbohydrates are the most abundant biological molecules and serve as energy sources and structural components. They are composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio (general formula: $(CH_2O)_n$).

• Functions: Energy storage (e.g., sugars, starches), structural support (e.g., cellulose).

Classification of Carbohydrates

• Monosaccharides: Simple sugars with 3–7 carbon atoms (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides linked by a covalent bond (e.g., sucrose, lactose).

• Oligosaccharides: 3–100 monosaccharide units.

• Polysaccharides: Hundreds to thousands of monosaccharides linked together (e.g., starch, glycogen, cellulose).

Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as building blocks for more complex sugars.

• Can exist in linear or ring forms (5 or 6 carbons).

• Common examples: Glucose, Fructose, Galactose.

• Isomers: Same chemical formula, different structures.

Disaccharides

Disaccharides are formed by dehydration synthesis (removal of water) between two monosaccharides, resulting in a glycosidic bond.

• Sucrose = Glucose + Fructose

• Lactose = Glucose + Galactose

• Maltose = Glucose + Glucose

Polysaccharides

Polysaccharides are long chains of monosaccharide units and serve as energy storage or structural molecules.

• Starch: Energy storage in plants; composed of glucose monomers.

• Glycogen: Energy storage in animals; highly branched glucose polymer.

• Cellulose: Structural component of plant cell walls; glucose monomers in a linear, unbranched chain.

All three are polymers of glucose but differ in the type of glycosidic bonds and branching, resulting in different properties.

Other Biological Carbohydrates

• Chitin: Structural polysaccharide in the exoskeleton of arthropods and cell walls of fungi; contains nitrogen.

• Peptidoglycan: Structural component of bacterial cell walls; consists of sugar chains cross-linked by peptides.

Summary Table: Major Polysaccharides

Polysaccharide	Monomer	Function	Occurrence
Starch	Glucose	Energy storage	Plants
Glycogen	Glucose	Energy storage	Animals
Cellulose	Glucose	Structural	Plant cell walls
Chitin	Modified glucose (with N)	Structural	Fungi, arthropods
Peptidoglycan	Modified sugars + peptides	Structural	Bacteria

Key Review Questions

• Describe the structure of nucleotides and nucleic acids.

• Compare and contrast DNA and RNA.

• Describe the structure and functions of carbohydrates, including their monomers and polymers.