Proteins, Nucleic Acids, and Biological Dogma Explained

Posted on Apr 6, 2025 in Biology

Protein Functions

  • Structural: Proteins are essential components of cells and tissues, supporting growth, repair, and maintenance.
  • Contractile: Proteins like actin and myosin are crucial for muscle cell contraction.
  • Transportation: Proteins transport substances throughout the body. For example, hemoglobin carries oxygen (O2) and carbon dioxide (CO2).
  • Reception: Proteins on the cell surface detect and respond to stimuli.
  • Catalytic: Enzymes, which are mostly proteins, regulate the speed of chemical reactions, including synthesis of new molecules, breakdown of molecules during digestion, and energy production.
  • Defense: Antibodies are proteins that identify and neutralize foreign substances in the body.
  • Hormonal: Many hormones are proteins. They are produced by cells to influence the function of other cells, such as thyroid hormones (regulating metabolism, growth, and development) and adrenaline.

Protein Structure

Amino acids join together to form peptides. The amino group reacts with the carboxyl group. The union of two amino acids forms a dipeptide, three a tripeptide, and so on. Chains with a molecular weight greater than 10,000 atomic mass units are considered proteins. Proteins have different levels of structure depending on the polypeptide.

Nucleic Acids: DNA and RNA

Nucleic acids are macromolecules with acidic properties. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). They are composed of nucleotides, each containing three fundamental units:

  • Pentose: Ribose (for RNA) and deoxyribose (for DNA).
  • Phosphate Group: Provides acidity to the molecule and links nucleotides together.
  • Nitrogenous Bases: Cyclic compounds containing nitrogen (N), hydrogen (H), oxygen (O), and carbon (C). Adenine lacks oxygen. They are divided into two groups:
    • Purines: Adenine and guanine.
    • Pyrimidines: Thymine, cytosine, and uracil.

DNA (Deoxyribonucleic Acid)

DNA contains adenine, cytosine, guanine, and thymine. It consists of two chains joined in a double helix. The bases pair complementarily (adenine-thymine with 2 hydrogen bonds, cytosine-guanine with 3 hydrogen bonds). Its primary function is to store genetic information. During replication, the double helix unwinds via an enzyme complex, and each strand serves as a template for creating new strands.

RNA (Ribonucleic Acid)

RNA contains adenine, cytosine, guanine, and uracil. It is a single linear chain of nucleotides linked by phosphodiester bonds (phosphate and sugar). Its main function is to act as an intermediary between DNA and protein synthesis, carrying the message from DNA to encode and synthesize required proteins.

Viruses and Genes

Viral DNA molecules are arranged in nucleotide sequences called genes. According to Watson and Crick, the sequence of nucleotides in a gene directs polypeptide (protein) synthesis.

The Central Dogma of Biology

The central dogma describes the flow of hereditary information and genetics from DNA to RNA to protein. This is generally a unidirectional process.

Exceptions to the Central Dogma

Viruses are not cells, and their classification as living beings is debated because they cannot develop independently and require a host. Some viruses can transfer information from RNA to DNA, originating life from RNA to DNA. There are three main exceptions to the central dogma:

  • Synthesis of DNA from RNA: Some viruses synthesize DNA from RNA using the enzyme reverse transcriptase.
  • Prion Proteins: Prions are proteins with abnormal 3D folding. They lack pathogenic action and a genome. When coupled to other proteins, they can cause degenerative diseases.
  • Artificial Protein Synthesis: This occurs through cell culture, where RNA is used without ribosome action.