Gene Concept, DNA, and Its Replication

I. Introduction to the Gene Concept

1. Definition of a Gene:
   • A gene is the basic physical and functional unit of heredity. It is a segment of DNA that contains the instructions for the synthesis of specific proteins or functional RNA molecules.
2. Historical Milestones in Gene Concept:
   • Gregor Mendel (1865): Father of genetics; first described the concept of inheritance through “factors” (now known as genes).
   • Watson and Crick (1953): Discovered the double-helical structure of DNA.
   • Central Dogma of Molecular Biology (Crick, 1958): Describes the flow of genetic information:
     DNA → RNA → Protein.
3. Functional Aspects of Genes:
   • Coding Regions (Exons): Encode the amino acid sequence of proteins.
   • Non-coding Regions: Include introns, promoters, enhancers, and regulatory sequences involved in gene expression.
   • Types of Genes:
     • Structural Genes: Code for proteins.
     • Regulatory Genes: Control gene expression.
     • Housekeeping Genes: Involved in essential cellular processes and expressed in all cells.

II. Structure of DNA

1. Definition and Overview:
   • DNA (Deoxyribonucleic Acid) is the hereditary material in most organisms, located in the nucleus and mitochondria.
   • Serves as the blueprint for all cellular activities.
2. Chemical Composition:
   • Nucleotides: The building blocks of DNA, each consisting of:
     • A nitrogenous base (adenine, guanine, cytosine, or thymine).
     • A deoxyribose sugar.
     • A phosphate group.
   • Bases and Pairing:
     • Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds.
     • Cytosine (C) pairs with Guanine (G) via 3 hydrogen bonds.
3. Double Helix Model:
   • Two complementary strands wound around each other in a right-handed helix.
   • Strands are antiparallel (one runs 5’→3’, the other 3’→5’).
   • Stabilized by hydrogen bonds and base stacking interactions.
4. Levels of Organization:
   • DNA is packaged into chromosomes in eukaryotes, with the help of histones.
   • Chromatin exists in two forms:
     • Euchromatin: Loosely packed, transcriptionally active.
     • Heterochromatin: Densely packed, transcriptionally inactive.

III. DNA Replication

1. Definition:
   • DNA replication is the process by which DNA makes an exact copy of itself, ensuring genetic information is passed to daughter cells.
   • Occurs during the S-phase of the cell cycle.
2. Mechanism of Replication:
   • DNA replication is semiconservative: Each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.
3. Key Steps in DNA Replication:
   • Initiation:
     • Replication begins at specific sequences called origins of replication.
     • Helicase unwinds the double helix by breaking hydrogen bonds, creating a replication fork.
     • Single-strand binding proteins (SSBs) stabilize the unwound strands.
     • Topoisomerase relieves supercoiling stress ahead of the fork.
   • Priming:
     • Primase synthesizes short RNA primers complementary to the DNA template.
   • Elongation:
     • DNA Polymerase III adds nucleotides to the 3’ end of the primer in the 5’→3’ direction.
     • Leading Strand: Synthesized continuously in the direction of the replication fork.
     • Lagging Strand: Synthesized discontinuously as Okazaki fragments, which are later joined by DNA Ligase.
   • Termination:
     • Replication stops when the forks meet or when specific termination sequences are reached.
     • RNA primers are replaced by DNA (via DNA Polymerase I), and gaps are sealed.
4. Enzymes Involved:
   • Helicase: Unwinds DNA strands.
   • Primase: Synthesizes RNA primers.
   • DNA Polymerase: Adds nucleotides and proofreads.
   • DNA Ligase: Joins Okazaki fragments on the lagging strand.
   • Topoisomerase: Prevents supercoiling.
5. Accuracy and Proofreading:
   • DNA replication is highly accurate, with an error rate of ~1 in 10⁹ nucleotides.
   • Proofreading by DNA Polymerase: Removes mismatched nucleotides via 3’→5’ exonuclease activity.

IV. Applications in Veterinary Science

1. Understanding Genetic Diseases:
   • Mutations in specific genes can lead to hereditary diseases in animals (e.g., progressive retinal atrophy in dogs).
2. Selective Breeding:
   • Identifying beneficial genetic traits for improved breeding programs.
3. Molecular Diagnostics:
   • Techniques like PCR and DNA sequencing are used for disease detection.
4. Gene Therapy:
   • Emerging treatments targeting genetic defects in animals.