Biomolecules (Nucleic Acids And Hormones)

Nucleic Acids

Nucleic acids are essential biomolecules that carry genetic information and play a crucial role in protein synthesis. They are polymers made up of monomers called nucleotides.

Chemical Composition Of Nucleic Acids

Each nucleotide consists of three components:

1. A Pentose Sugar:
   • In Deoxyribonucleic Acid (DNA), the sugar is deoxyribose.
   • In Ribonucleic Acid (RNA), the sugar is ribose.
2. A Phosphate Group: This group is derived from phosphoric acid ($H_3PO_4$). It is attached to the 5' carbon of the pentose sugar.
3. A Nitrogenous Base: These are heterocyclic aromatic compounds containing nitrogen. They are of two types:
   • Purines: Have a double-ring structure. The purines found in nucleic acids are Adenine (A) and Guanine (G).
   • Pyrimidines: Have a single-ring structure. The pyrimidines found in nucleic acids are Cytosine (C), Thymine (T) (found only in DNA), and Uracil (U) (found only in RNA).

When a nitrogenous base is attached to a pentose sugar, it forms a nucleoside. When a phosphate group is added to a nucleoside, it forms a nucleotide.

Nucleotides are linked together by phosphodiester bonds to form a nucleic acid chain. The phosphodiester bond forms between the phosphate group attached to the 5' carbon of one sugar and the hydroxyl group attached to the 3' carbon of another sugar.

$5' \text{ phosphate} - \text{Sugar} - 3' \text{ hydroxyl} \rightarrow 5' - \text{Sugar} - 3'$ (after phosphodiester bond formation and water release)

Structure Of Nucleic Acids

Deoxyribonucleic Acid (DNA):

• DNA is typically a double-stranded helix.
• The two strands are antiparallel, meaning they run in opposite directions (one strand runs from 5' to 3', and the other from 3' to 5').
• The strands are held together by hydrogen bonds between complementary nitrogenous bases:
  • Adenine (A) always pairs with Thymine (T) via two hydrogen bonds ($A=T$).
  • Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds ($G \equiv C$).
• The sugar-phosphate backbone is on the outside of the helix, and the bases are stacked on the inside.
• The structure of DNA is often described as a double helix, famously elucidated by Watson and Crick.

Ribonucleic Acid (RNA):

• RNA is typically a single-stranded molecule, although it can fold upon itself to form complex three-dimensional structures with base pairing.
• In RNA, Uracil (U) replaces Thymine (T) and pairs with Adenine (A) ($A=U$). Guanine (G) still pairs with Cytosine (C) ($G \equiv C$).
• There are several types of RNA, each with specific functions:
  • Messenger RNA (mRNA): Carries genetic information from DNA in the nucleus to ribosomes in the cytoplasm for protein synthesis.
  • Transfer RNA (tRNA): Carries specific amino acids to the ribosome and matches them to the mRNA codons during protein synthesis.
  • Ribosomal RNA (rRNA): A structural and catalytic component of ribosomes.
  • Small nuclear RNA (snRNA): Involved in RNA splicing.
  • Small interfering RNA (siRNA) and microRNA (miRNA): Involved in gene regulation.

Biological Functions Of Nucleic Acids

• Genetic Information Storage (DNA): DNA serves as the blueprint for life, storing all the genetic instructions needed for the development, functioning, growth, and reproduction of all known organisms.
• Transmission of Genetic Information: DNA replicates itself, ensuring that genetic information is passed from one generation of cells to the next.
• Protein Synthesis (RNA): RNA plays a central role in translating the genetic code stored in DNA into proteins.
  • Transcription: A segment of DNA is transcribed into a complementary mRNA molecule.
  • Translation: mRNA codons are read by ribosomes, with the help of tRNA, to assemble a specific sequence of amino acids into a polypeptide chain, which then folds into a functional protein.
• Gene Regulation: Certain RNA molecules (like siRNA and miRNA) are involved in controlling gene expression, turning genes on or off.
• Catalysis: Some RNA molecules (ribozymes) can act as enzymes, catalyzing biochemical reactions.

Hormones

Hormones are chemical messengers produced by endocrine glands or specialized cells that travel through the bloodstream to target cells or organs, where they regulate various physiological processes. They act as signaling molecules, coordinating growth, metabolism, reproduction, and response to stimuli.

Types of Hormones:

Hormones can be classified based on their chemical structure:

• Steroid Hormones: Derived from cholesterol. They are lipid-soluble and can easily cross cell membranes, binding to intracellular receptors (e.g., estrogen, testosterone, cortisol, aldosterone).
• Amino Acid Derivatives: Modified amino acids. Some are water-soluble (e.g., epinephrine, thyroxine), while others are lipid-soluble.
• Peptide and Protein Hormones: Composed of amino acids linked by peptide bonds. They are water-soluble and bind to cell surface receptors (e.g., insulin, growth hormone, oxytocin, ADH).

Mechanism of Hormone Action:

The mechanism depends on the type of hormone and its solubility:

• Steroid and Thyroid Hormones (Lipid-Soluble): These hormones diffuse across the plasma membrane and bind to specific intracellular receptors (in the cytoplasm or nucleus). The hormone-receptor complex then acts as a transcription factor, binding to specific DNA sequences and regulating gene expression, thereby affecting protein synthesis.
• Peptide and Amino Acid Derivative Hormones (Water-Soluble): These hormones cannot easily cross the cell membrane. They bind to specific cell surface receptors on the target cell. This binding initiates a cascade of events within the cell, often involving a second messenger system (like cyclic AMP - cAMP or calcium ions), which ultimately leads to a cellular response.

Examples of Hormones and their Functions:

• Insulin: Regulates blood glucose levels by promoting glucose uptake by cells and storage as glycogen.
• Glucagon: Increases blood glucose levels by promoting the breakdown of glycogen into glucose.
• Thyroid Hormones (Thyroxine T4, Triiodothyronine T3): Regulate metabolism, growth, and development.
• Adrenaline (Epinephrine): Prepares the body for "fight or flight" responses (increases heart rate, blood pressure, blood sugar).
• Cortisol: Involved in stress response, metabolism, and immune function.
• Growth Hormone (GH): Stimulates growth and cell reproduction.
• Estrogen and Progesterone: Regulate female reproductive development and cycles.
• Testosterone: Regulates male reproductive development and secondary sexual characteristics.