U3L1 - Molecular Genetics

Unit 3: Molecular Genetics

Purpose of DNA

  • Definition: DNA encodes the hereditary (genetic) information of an organism in its sequence of nucleotides.

Organization of DNA

  • Strands of Chromatin: DNA is organized into strands of chromatin, which is DNA wrapped around histone proteins, located in the nucleus of the cell.

  • Chromosomes: During prophase of mitosis and meiosis, chromatin coils into chromosomes.

    • Gene: A gene is a segment of DNA that codes for the primary structure of a protein.

  • Human Genome: The human genome consists of 23 pairs of chromosomes and is estimated to contain about 30,000 genes.

Historical Background

  • Discovery of Nucleic Acid:

    • In 1869, Friedrich Miescher extracted a strange acidic substance rich in phosphates from the nuclei of white blood cells derived from the pus-soaked bandages of soldiers.

    • He called this substance nuclein, which is now known as nucleic acid, specifically DNA.

Classification of Macromolecules

  • Nucleic Acids: Nucleic acids are the 4th class of macromolecules.

    • Categories:

    • Nucleic Acid

    • Carbohydrate

    • Lipid

    • Protein

  • Types of Nucleic Acids: DNA and RNA are two types of nucleic acids.

Structure of Nucleotides

  • Nucleotide Composition: Nucleotides are the monomers of nucleic acids and consist of three components:

    1. Nitrogenous Base

    2. Sugar: Either deoxyribose (in DNA) or ribose (in RNA)

    3. Phosphate Group

Formation of Nucleotides

  • Combination Process: The components of a nucleotide come together through dehydration synthesis reactions, resulting in the loss of water (H₂O).

  • Attachment Points: The phosphate group attaches to carbon #5, and the nitrogenous base attaches to carbon #1 on the sugar.

Types of Nitrogenous Bases

  • Classification: There are two types of nitrogenous bases:

    • Purines: Two-ring structures (Adenine (A) and Guanine (G) found in both DNA and RNA)

    • Pyrimidines: Single-ring structures (Thymine (T) in DNA, Uracil (U) in RNA, and Cytosine (C) in both DNA and RNA)

Chargaff's Rule

  • Erwin Chargaff's Research: From his experiments, Chargaff demonstrated a consistent ratio of nitrogen bases across different organisms:

    • The ratio of Adenine (A) to Thymine (T) and Guanine (G) to Cytosine (C) is approximately equal to 1.00, meaning the percentage of adenine is equal to thymine and the percentage of guanine is equal to cytosine.

  • This principle is now known as Chargaff's Rule.

Structure of DNA

  • Discoveries:

    • In 1953, James Watson and Francis Crick published a paper describing the three-dimensional structure of DNA.

    • Their model was influenced and somewhat facilitated by the work of Rosalind Franklin, who provided crucial data on DNA through X-ray diffraction.

  • DNA Ladder Structure:

    • The rails of the DNA ladder are comprised of deoxyribose sugars and phosphates linked together by phosphodiester bonds.

  • Linking of Nucleotides:

    • Nucleotides are linked through dehydration synthesis reactions to form these rails; the resulting bond is described as a phosphodiester bond or linkage.

  • Directional Ends:

    • One end of the DNA strand is termed the 5’ phosphate end, while the other is called the 3’ hydroxyl end.

  • Antiparallel Orientation:

    • The sugar-phosphate rails of the DNA ladder are oriented in opposite directions, referred to as antiparallel structure.

Base Pairing in DNA

  • Rungs of the Ladder: The rings of the DNA ladder are made up of pairs of nitrogenous bases linked by hydrogen bonds.

  • Base Pairing Rules:

    • Following Chargaff’s rule:

    • Thymine (T) only bonds with Adenine (A)

    • Guanine (G) only bonds with Cytosine (C)

    • Reason for Pairing: A purine must bond with a pyrimidine to maintain a consistent rung length; two purines would be too wide, while two pyrimidines would be too narrow, thus the purine-pyrimidine pairing results in an optimal structure.

DNA Replication

  • Key Experiment: The experiment conducted by Matthew Meselson and Franklin Stahl in 1958 proved that DNA replication is semi-conservative.

    • This means that each strand of DNA serves as a template for the production of a new complementary strand, thereby conserving one original strand in each new DNA molecule.

Cultural References and Humor

  • Genetic Puns and References:

    • Humor about DNA and genes is commonly explored in cultural references and is exemplified by jokes, such as:

    • "I'm sorry G, but we're base-ically not a good match…".

Summary

  • Molecular genetics is a vast field that incorporates elements of biology, history, and the fundamental operations that govern heredity and biological processes. Understanding the structure and function of DNA is crucial in this study as it forms the basis of genetic information transmission in living organisms.