Nucleic Acids as Carriers of Genetic Information

Section 5: Nucleic Acids as Carriers of Genetic Information

"Nucleic acids," as the name suggests, are acidic substances extracted from the cell nucleus. So, what exactly are nucleic acids? Are they only found in the cell nucleus?

Types and Distribution of Nucleic Acids

Nucleic acids include two main types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In eukaryotic cells, DNA is primarily located in the cell nucleus, with small amounts also found in mitochondria and chloroplasts. RNA mainly resides in the cytoplasm.

Nucleic Acids: Long Chains of Nucleotides

Similar to proteins, nucleic acids are large biomolecules. Nucleotides are the basic units of nucleic acids. Each nucleic acid molecule is a long chain composed of tens to billions of nucleotides linked together (Figure 2-14). A nucleotide consists of a nitrogen-containing base, a five-carbon sugar molecule, and a phosphate group. Based on the type of sugar, nucleotides can be classified as deoxyribonucleotides (for DNA) and ribonucleotides (for RNA).

DNA and RNA both contain four types of nitrogenous bases, but the specific types differ between the two (Figure 2-16). DNA forms a double helix structure composed of two strands of deoxyribonucleotides, whereas RNA consists of a single strand of ribonucleotides.

The use of DNA fingerprinting by forensic investigators, mentioned in the "Discussion of Issues" in this section, relies on the fact that genetic information of organisms is stored in DNA molecules, with each individual's DNA having unique sequences of deoxyribonucleotides. Despite having only four types of deoxyribonucleotides, the vast number of possible sequences when linked into long chains results in a molecule with extremely high information capacity. The sequence of deoxyribonucleotides stores genetic information, making DNA molecules essential for storing and transmitting genetic information in living organisms. Some viruses store their genetic information in RNA, such as HIV (Human Immunodeficiency Virus), SARS (Severe Acute Respiratory Syndrome) virus, and others.

Nucleic acids are substances within cells that carry genetic information, playing extremely important roles in genetic inheritance, variation, and the biosynthesis of proteins.

Biological Macromolecules with Carbon Chains as Frameworks

Cells are composed of various elements and compounds. Among the compounds that constitute cells, polysaccharides, proteins, and nucleic acids are biological macromolecules.

Through study, we know that monosaccharides are the basic units of polysaccharides, amino acids are the basic units of proteins, and nucleotides are the basic units of nucleic acids; these basic units are called monomers. Each monomer has a basic skeleton formed by several connected carbon atoms. Biological macromolecules are polymers formed by linking many monomers together (Figure 2-17), thus biological macromolecules also have carbon chains as their basic framework. Scientists emphasize the importance of carbon atoms in forming biological macromolecules, stating "carbon is the core element of life" and "without carbon, there would be no life."

Polysaccharides, proteins, nucleic acids, and other biological macromolecules with carbon chains as frameworks constitute the basic framework of the cell's life building. Sugars and lipids provide important energy sources for life activities. Water and inorganic salts, along with other substances, jointly contribute to the construction of cells and participate in important cellular activities. The quantities and proportions of these compounds in cells are constantly changing but remain relatively stable to ensure the normal functioning of cellular activities.