Learning Objectives
- Compare purines and pyrimidines, nucleosides and nucleotides, DNA and RNA.
- Describe the structure and composition of nucleic acids.
- Explain DNA directionality and complementarity.
- Describe DNA packaging in the nucleus.
- Relate nucleic acids to plant growth, breeding, and biotechnology.
7.1 Overview of Nucleic Acids
Nucleic acids are biological macromolecules that store, transmit, and express genetic information. They exist in two major forms:
- Deoxyribonucleic acid (DNA) – the primary genetic material.
- Ribonucleic acid (RNA) – involved in gene expression and regulation.
In most cells, RNA constitutes a larger proportion of nucleic acids than DNA due to its diverse functional roles.
Functions of Nucleic Acids
- Storage of genetic information.
- Replication and inheritance.
- Protein synthesis through transcription and translation.
- Genetic variation through mutation and recombination.
- Regulation of cellular metabolism and development.
Applied Plant Science:
Understanding nucleic acids is fundamental to plant breeding, genetic engineering, molecular diagnostics, and crop improvement.
7.2 Components of Nucleic Acids
The basic building block of nucleic acids is the nucleotide, composed of three components:
- Phosphate group
- Five-carbon sugar (ribose in RNA or deoxyribose in DNA)
- Nitrogenous base
Nitrogenous Bases
Nitrogenous bases are classified into two groups:
- Purines: Adenine (A), Guanine (G)
- Pyrimidines: Cytosine (C), Thymine (T), Uracil (U)
| Base Type |
Bases |
Occurrence |
| Purines |
A, G |
DNA and RNA |
| Pyrimidines |
C, T, U |
T in DNA, U in RNA |
Difference Between Ribose and Deoxyribose
| Feature |
Ribose |
Deoxyribose |
| Carbon-2′ position |
–OH group present |
–H (no –OH group) |
| Occurs in |
RNA |
DNA |
Nucleosides and Nucleotides
- Nucleoside: Sugar + nitrogenous base.
- Nucleotide: Nucleoside + phosphate group(s).
7.3 DNA and RNA: Structure and Differences
| Characteristic |
DNA |
RNA |
| Structure |
Double-stranded helix |
Single-stranded (usually) |
| Sugar |
Deoxyribose |
Ribose |
| Bases |
A, T, C, G |
A, U, C, G |
| Location |
Nucleus, mitochondria, chloroplasts |
Nucleus and cytoplasm |
| Function |
Genetic information storage |
Gene expression |
Major Types of RNA
- mRNA (messenger RNA): carries genetic code for protein synthesis.
- tRNA (transfer RNA): transports amino acids to ribosomes.
- rRNA (ribosomal RNA): structural and catalytic component of ribosomes.
7.4 DNA Structure
Primary Structure
The linear sequence of nucleotides linked by phosphodiester bonds.
Secondary Structure
The double helix formed by two antiparallel strands with complementary base pairing:
- A pairs with T (or U in RNA)
- C pairs with G
Directionality of DNA
DNA strands have 5′ → 3′ directionality based on sugar-phosphate backbone orientation.
Forms of DNA
- Linear DNA – nuclear DNA of eukaryotes.
- Circular DNA – mitochondrial, chloroplast, and bacterial DNA.
- Plasmid DNA – extra-chromosomal genetic elements.
7.5 DNA Packaging and Chromatin Organization
In eukaryotic cells, DNA is packaged into chromatin to fit inside the nucleus:
- DNA wraps around histone proteins forming nucleosomes.
- Nucleosomes coil to form chromatin fibers.
- Chromatin condenses into chromosomes during cell division.
Applied Plant Biotechnology:
DNA packaging influences gene expression, epigenetics, stress adaptation, and crop productivity.
7.6 Properties of DNA
- Replication and inheritance.
- Genetic stability and variability.
- Mutation and recombination.
- Gene expression and regulation.
Factors Affecting DNA Stability
- Temperature (denaturation and renaturation)
- Salt concentration
- Chemical denaturants
- GC content
Horticultural Relevance:
Molecular understanding of nucleic acids underpins marker-assisted selection, genetic engineering, genome editing (CRISPR), and conservation of plant genetic resources.