Introduction
Nucleic acids are complex biomolecules that store, transmit, and express genetic information in all living organisms. They are essential for heredity, protein synthesis, cell division, and the regulation of cellular activities. The two major types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA acts as the genetic material in most organisms, while RNA plays a crucial role in gene expression and protein synthesis.
What are nucleic acids?
Nucleic acids are polymers of nucleotides. Each nucleotide consists of three components:
A pentose sugar
A nitrogenous base
A phosphate group
Nucleotides are linked together by phosphodiester bonds to form long polynucleotide chains.
Components of a nucleotide
Pentose sugar
There are two types of sugars found in nucleic acids.
| Nucleic acid | Sugar |
|---|---|
| DNA | Deoxyribose |
| RNA | Ribose |
The absence of one oxygen atom at the 2′ carbon of deoxyribose makes DNA more chemically stable than RNA.
Nitrogenous bases
Nitrogenous bases are of two types.
Purines
Adenine (A)
Guanine (G)
Pyrimidines
Cytosine (C)
Thymine (T) – present only in DNA
Uracil (U) – present only in RNA
Phosphate group
The phosphate group links adjacent nucleotides through 3′–5′ phosphodiester bonds, forming the sugar-phosphate backbone.
Nucleoside and nucleotide
A nucleoside consists of a sugar and a nitrogenous base, while a nucleotide consists of a nucleoside plus one or more phosphate groups.
Examples:
Adenosine = adenine + ribose
Adenosine monophosphate (AMP) = adenosine + phosphate
Structure of DNA
The DNA structure was proposed by James Watson and Francis Crick (1953) based on X-ray diffraction studies by Rosalind Franklin and Maurice Wilkins.
Key features of the DNA double helix
DNA consists of two antiparallel polynucleotide strands
The strands are twisted into a right-handed double helix
Sugar-phosphate backbones are located on the outside
Nitrogenous bases face inward
Base pairing
Complementary base pairing occurs through hydrogen bonds.
| Base pair | Hydrogen bonds |
|---|---|
| A–T | 2 |
| G–C | 3 |
Guanine-cytosine pairs are more stable due to the presence of three hydrogen bonds.
Dimensions of B-DNA
Diameter: 2 nm
Distance between adjacent base pairs: 0.34 nm
One complete turn: 3.4 nm
Number of base pairs per turn: 10
Chargaff’s rules
Erwin Chargaff discovered that:
A = T
G = C
Total purines = total pyrimidines
These observations provided crucial evidence for complementary base pairing.
Types of DNA
DNA may exist in different conformations.
| Form | Helix type | Base pairs per turn |
|---|---|---|
| B-DNA | Right-handed | 10 |
| A-DNA | Right-handed | 11 |
| Z-DNA | Left-handed | 12 |
B-DNA is the most common form found in cells.
Structure of RNA
RNA is generally single-stranded, although it may fold into complex secondary and tertiary structures.
RNA contains:
Ribose sugar
Adenine, guanine, cytosine, and uracil
Because of the presence of the 2′-OH group, RNA is less stable than DNA.
Types of RNA
Messenger RNA (mRNA)
Carries genetic information from DNA to ribosomes
Serves as the template for protein synthesis
Transfer RNA (tRNA)
Transfers amino acids to ribosomes
Contains an anticodon that recognizes mRNA codons
Ribosomal RNA (rRNA)
Forms the structural and catalytic core of ribosomes
Catalyzes peptide bond formation
Other RNAs
Small nuclear RNA (snRNA)
Small nucleolar RNA (snoRNA)
MicroRNA (miRNA)
Small interfering RNA (siRNA)
Long non-coding RNA (lncRNA)
DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Strands | Double | Usually single |
| Stability | More stable | Less stable |
| Main function | Genetic storage | Gene expression and protein synthesis |
Biological functions of nucleic acids
Storage of genetic information
DNA stores hereditary information that is transmitted from parents to offspring.
DNA replication
DNA duplicates itself before cell division, ensuring genetic continuity.
Protein synthesis
RNA molecules participate in transcription and translation to produce proteins.
Regulation of gene expression
Non-coding RNAs regulate gene activity, development, and cellular differentiation.
Evolution
Mutations in nucleic acids generate genetic variation, which drives evolution.
Central dogma of molecular biology
The flow of genetic information follows the pathway:
DNA → RNA → Protein
This process involves:
Replication
Transcription
Translation
DNA replication
DNA replication is semi-conservative, meaning each daughter DNA molecule contains one parental strand and one newly synthesized strand.
Major enzymes involved include:
DNA helicase
DNA polymerase
Primase
DNA ligase
Topoisomerase
Replication proceeds in the 5′ → 3′ direction.
Transcription
Transcription is the synthesis of RNA from a DNA template.
Steps:
Initiation
Elongation
Termination
In eukaryotes, mRNA undergoes:
5′ capping
Splicing
3′ polyadenylation
Translation
Translation occurs on ribosomes.
Major stages:
Initiation
Elongation
Termination
During translation, codons on mRNA specify the sequence of amino acids in proteins.
Importance in biotechnology
Nucleic acids have numerous applications.
Polymerase chain reaction (PCR)
Amplifies DNA fragments.
DNA fingerprinting
Used in forensic science and paternity testing.
Recombinant DNA technology
Produces genetically modified organisms and therapeutic proteins.
Gene therapy
Treats genetic diseases by introducing functional genes.
CRISPR-Cas genome editing
Allows precise modification of DNA sequences.
Nucleic acid metabolism
Purine metabolism
Purines are synthesized and degraded to uric acid.
Pyrimidine metabolism
Pyrimidines are degraded to beta-alanine and beta-aminoisobutyrate.
Disorders of nucleotide metabolism include:
Gout
Lesch-Nyhan syndrome
Severe combined immunodeficiency (SCID)
Key points
DNA contains deoxyribose; RNA contains ribose.
Thymine is present in DNA; uracil is present in RNA.
DNA strands are antiparallel.
A pairs with T; G pairs with C.
DNA replication is semi-conservative.
mRNA carries genetic information.
tRNA transports amino acids.
rRNA forms ribosomes.
The central dogma is DNA → RNA → Protein.
Conclusion
Nucleic acids are the molecular basis of life. DNA stores hereditary information, while RNA mediates the expression of that information through protein synthesis. Their structure, replication, transcription, and translation are central to genetics, molecular biology, biotechnology, and medicine.
References
Nelson DL, Cox MM. Lehninger Principles of Biochemistry. 8th ed. W.H. Freeman; 2021.
Alberts B, et al. Molecular Biology of the Cell. 7th ed. Garland Science; 2022.
Watson JD, et al. Molecular Biology of the Gene. 7th ed. Pearson; 2014.
Lodish H, et al. Molecular Cell Biology. 9th ed. W.H. Freeman; 2021.
NCERT. Biology Class XI. National Council of Educational Research and Training; Latest Edition.
Watson JD, Crick FHC. Molecular structure of nucleic acids. Nature. 1953;171:737–738.
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