Nucleic Acids: Discovery, Structure, and Function
1. Discovery of Nucleic Acids
In 1871, Friedrich Miescher isolated a phosphorus-rich acidic substance from cell nuclei, calling it nuclein. In 1899, Richard Altmann renamed it nucleic acid. Robert Feulgen, in 1914, used a fuchsin dye to identify nucleic acid in plant and animal cell nuclei. Early 1900s research identified two types: “thymus nucleic acid” and “yeast nucleic acid.” Albrecht Kossel and Phoebus Levene (1920) found that nucleic acids contain nitrogenous bases. Levene identified four in “thymus nucleic acid”: thymine (T), cytosine (C), adenine (A), and guanine (G), along with deoxyribose and a phosphate group. “Yeast nucleic acid” differed, having uracil (U) instead of thymine and ribose instead of deoxyribose. “Thymus nucleic acid” became deoxyribonucleic acid (DNA), and “yeast nucleic acid” became ribonucleic acid (RNA).
2. Genetic Material Identification
In 1928, Frederick Griffith studied pneumonia-causing bacteria. He found that heat-killed pathogenic bacteria could transform harmless bacteria into pathogenic ones. Griffith called this the “transforming principle.”
Possible Explanations:
- Pathogenic bacteria were not entirely dead.
- Genetic material from dead pathogenic bacteria transferred to live harmless bacteria.
3. Chemistry of Nucleic Acids
Nucleic acids are polymers of nucleotides. Each nucleotide has three components:
- Nitrogenous Base:
- Purines (double-ringed): adenine (A) and guanine (G)
- Pyrimidines (single-ringed): cytosine (C), thymine (T), and uracil (U)
- Pentose Sugar: deoxyribose (DNA) or ribose (RNA)
- Phosphate Group
4. DNA Structure: Franklin and Wilkins
Franklin and Wilkins used X-ray diffraction to study DNA’s three-dimensional structure. Their findings suggested a double helix shape.
5. DNA Model: Watson and Crick
Watson and Crick proposed that DNA consists of two polynucleotide chains held together by hydrogen bonds: A with T (two bonds) and C with G (three bonds). The chains form a three-dimensional double helix.
6. RNA: Structure and Chemistry
RNA is a single-stranded ribonucleic acid found in the nucleus and cytoplasm. It participates in protein synthesis. RNA has ribose instead of deoxyribose and uracil (U) instead of thymine (T). Chargaff’s rule does not apply to RNA, as it is single-stranded and can adopt various shapes. RNA is more structurally versatile than DNA.
7. Types of RNA
- Messenger RNA (mRNA): Carries genetic information to the cytoplasm.
- Ribosomal RNA (rRNA): Part of ribosomes, found in the cytoplasm.
- Transfer RNA (tRNA): Exists in about 20 forms in the cytoplasm, involved in protein synthesis.
- Small nuclear RNA (snRNA): Found in the nucleus, various functions.