NUCLEIC ACIDS: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future pharmacists and healthcare professionals!
Nucleic acids (DNA and RNA) are formed by the polymerisation of nucleotide subunits. Nucleotides are made up of a ribose sugar, a nitrogenous base, and phosphate group(s). Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA) are the two types of nucleic acids that act as sources and carriers of genetic information. Johann Friedrich Miescher (a Swiss researcher) discovered DNA in 1869. In the year 1944, three scholars Avery, Macleod, and MacCarty first demonstrated that DNA carries genetic information.
Dpharmguru’s exam insights:
In my years of teaching biochemistry, I have observed that students often get confused between DNA and RNA. Remember: DNA has deoxyribose sugar and thymine base, while RNA has ribose sugar and uracil base. This is a fundamental concept that appears in almost every biochemistry exam!
NUCLEOTIDES
Nucleotides are the structural units of nucleic acids, and also perform various cellular functions. These functions include acting as structural components for some B-complex vitamin coenzymes (e.g., FAD, NAD+), involved in the energy reactions of cells, and controlling the metabolic reactions. The phosphoric acid esters of nucleosides make up the nucleotides. These nucleotides contain nitrogenous bases, sugars, and phosphate group in an ester linkage.
Components of Nucleotides
- Nitrogenous bases: Purines (Adenine, Guanine) and Pyrimidines (Cytosine, Thymine, Uracil).
- Pentose sugar: Ribose (in RNA) or Deoxyribose (in DNA).
- Phosphate groups: One or more phosphate groups attached to the sugar.
Nitrogenous Bases – Purines and Pyrimidines
- Purines: Two carbon-nitrogen rings. Examples: Adenine (A) and Guanine (G).
- Pyrimidines: Single carbon-nitrogen ring. Examples: Cytosine (C), Thymine (T), and Uracil (U).
Dpharmguru’s exam insights:
Remember the purines: “Pure As Gold” (Purines = Adenine and Guanine). Pyrimidines are Cytosine, Thymine, and Uracil. This mnemonic helps students recall which bases are purines and which are pyrimidines!
Bases and Nucleotides Found in RNA and DNA
| Base | RNA Nucleotide | DNA Nucleotide |
|---|---|---|
| Adenine (A) | Adenylic acid (AMP) | Deoxyadenylic acid (dAMP) |
| Guanine (G) | Guanylic acid (GMP) | Deoxyguanylic acid (dGMP) |
| Cytosine (C) | Cytidylic acid (CMP) | Deoxycytidylic acid (dCMP) |
| Thymine (T) | – | Thymidylic acid (dTMP) |
| Uracil (U) | Uridylic acid (UMP) | – |
Pentose Sugar
It is the sugar part of a nucleotide. In RNA, the pentose sugar is ribose; while in DNA, deoxyribose sugar is present. Only two sugars D-Ribose and D-2-Deoxyribose are isolated from the hydrolysis products of nucleic acids.
Components of Nucleosides
Nucleoside is the combination of a base and a pentose sugar. The 1’C of pentose attaches to the 3-position of a pyrimidine or at the 9-position of a purine.
| Base | Ribonucleoside | Deoxyribonucleoside |
|---|---|---|
| Adenine | Adenosine | Deoxyadenosine |
| Guanine | Guanosine | Deoxyguanosine |
| Cytosine | Cytidine | Deoxycytidine |
| Uracil | Uridine | – |
| Thymine | – | Thymidine |
DNA (DEOXYRIBONUCLEIC ACID)
DNA (a polymer molecule) is made up of monomeric units called nucleotides, thus, the polymer is known as a polynucleotide. A 5-carbon sugar (deoxyribose), a nitrogen containing base attached to the sugar, and a phosphate group constitute a nucleotide.
Structure (Watson and Crick Model)
- Double-Stranded Helix: The two strands of DNA double helix coil around each other forming a helical structure. The base sequences of both the strands are complementary to each other due to the presence of A—T, G—C base pairings.
- Antiparallel Strands: Both the strands run in antiparallel direction and are linked via hydrogen bonds present between the bases.
- Complementary Base Pairing: A of one strand pairs with T of the other; C of one strand pairs with G of the other. This plays a major role in DNA replication.
Dpharmguru’s exam insights:
Remember the base pairing rules: “A-T, G-C” in DNA. Also, in RNA, “A-U, G-C”. This is a very common exam question! Also, Watson and Crick proposed the double helix model of DNA in 1953.
Functions of DNA
- Genetic Information: DNA carries all the hereditary information. The segment of DNA carrying protein information is known as a gene.
- Replication: DNA can synthesise another copy of DNA, essential for transferring genetic information.
- Mutations: Changes in sequence of nitrogen bases give rise to mutations, the fountain head of all variations and evolution.
- Transcription: DNA can generate RNA through the transcription process.
- Cellular Metabolism: DNA regulates the metabolic reactions of the cells through certain RNAs and protein synthesis.
- DNA Finger Printing: DNA sequences of each individual are different, used in identification of individuals.
- Gene Therapy: Defective heredity can be corrected by integrating actual genes in place of defective ones.
RNA (RIBONUCLEIC ACID)
RNA in a cell is present in amount 10 times more than that of DNA, because it performs a large number of cellular functions. Ribosomal RNA (r-RNA, the most abundant), transfer RNA (tRNA), and messenger RNA (mRNA) are the three RNA types.
Structure of RNA
- Primary structure: Sequencing of bases within RNA molecules.
- Secondary structure: Helical regions and various kinds of bulges, loops, and junctions. Stabilised by Watson-Crick base pairing.
- Tertiary structure: Arrangement of secondary structures into a 3D structure, often compact and stabilised by metal cations.
- Quaternary structure: Arrangement of RNA strand with respect to other RNA strands or with protein molecules.
Functions of RNA
- RNA molecules are less stable than DNA molecules. In some organisms, they also carry the genetic information (e.g., viruses).
- tRNA (transfer RNA): Has an amino acid and an anticodon which involves the recognition of the information on mRNA template during protein synthesis.
- rRNA (ribosomal RNA): Provides the structural framework to the ribosomes.
- mRNA (messenger RNA): Carries genetic information from the nucleus to the ribosomes present in the cell cytoplasm.
Dpharmguru’s exam insights:
Remember the three types of RNA and their functions: mRNA carries the message, tRNA transfers amino acids, and rRNA makes up the ribosomes. This is a frequently tested concept!
Differences between DNA and RNA
| Feature | DNA | RNA |
|---|---|---|
| Full Form | Deoxyribonucleic Acid | Ribonucleic Acid |
| Sugar | Deoxyribose | Ribose |
| Bases | A, G, C, T | A, G, C, U |
| Structure | Double-stranded helix | Single-stranded |
| Stability | Very stable | Less stable |
| Location | Nucleus, mitochondria | Cytoplasm, ribosomes, nucleus |
| Function | Genetic material, transfers information | Protein synthesis |
| Types | B-DNA, A-DNA, Z-DNA | mRNA, tRNA, rRNA |
Dpharmguru’s exam insights:
Nucleic acids are the blueprint of life. In exams, pay special attention to: (1) The differences between DNA and RNA, (2) The Watson-Crick model of DNA, (3) The base pairing rules (A-T, G-C and A-U, G-C), and (4) The functions of mRNA, tRNA, and rRNA. Remember: “DNA is the genetic blueprint, RNA is the messenger!”
REFERENCES AND FURTHER READING
- Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry (7th ed.). W. H. Freeman.
- Berg, J. M., Tymoczko, J. L., & Stryer, L. (2019). Biochemistry (9th ed.). W. H. Freeman.
- Satyanarayana, U., & Chakrapani, U. (2021). Biochemistry (5th ed.). Elsevier.
- Vasudevan, D. M., Sreekumari, S., & Vaidyanathan, K. (2022). Textbook of Biochemistry for Medical Students (9th ed.). Jaypee Brothers.
- National Institutes of Health (NIH). (2022). Nucleic Acid Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
