CR

Nucleotides & Nucleic Acids

Introduction

  • Course: Biol1004 - Molecular and Cell Biology

  • Lecture: 6: Nucleotides & nucleic acids

  • Instructor: Kiaran Kirk, Dean, College of Science & Medicine, Australian National University

Hereditary Material and Inherited Diseases

  • Cystic Fibrosis: Example of an inherited disease where the primary defect is in the hereditary material, specifically the DNA.

  • DNA Definition:

    • DNA stands for deoxyribonucleic acid.

    • It is a linear polymer of nucleotides (i.e., a ‘polynucleotide’).

    • DNA molecules are responsible for encoding hereditary information, which is passed from generation to generation.

    • This information is essential for protein synthesis, which involves another type of nucleic acid called RNA.

RNA

  • RNA Definition:

    • RNA stands for ribonucleic acid.

    • Like DNA, it is also a polymer of nucleotides.

    • RNA functions by copying information from DNA and serves as a component of the machinery responsible for protein production.

Structure and Function of Nucleotides

Composition of Nucleotides

  • A nucleotide comprises three main components:
    i. Nitrogenous base
    ii. Pentose sugar (5-carbon sugar)
    iii. Phosphate group

  • In DNA: The sugar is deoxyribose.

  • In RNA: The sugar is ribose, which has one more oxygen atom than deoxyribose.

Nitrogenous Bases in DNA & RNA

  • Types of Bases:

    • Pyrimidines: Have a single six-membered ring.

    • Purines: Contain a six-membered ring fused to a five-membered ring.

Nucleosides

  • The portion of the nucleotide without the phosphate group is termed a nucleoside.

Functions of Nucleotides

  • Beyond storing and transmitting hereditary information, nucleotides have other crucial roles:

    • Adenosine Triphosphate (ATP):

    • Acts as the major energy currency in most cells.

    • The energy required for biological processes primarily comes from breaking the phosphate-phosphate bonds in ATP.

    • Cyclic AMP (cAMP):

    • Serves as an intracellular signaling molecule (second messenger).

    • Formed from ATP through the action of the enzyme adenylate cyclase.

Polynucleotide Structure

Formation of Polynucleotides

  • Linking of Nucleotides: Nucleotide polymers are joined together to form a polynucleotide through covalent bonds known as phosphodiester bonds.

  • Bonding Details:

    • Adjacent nucleotides are connected by forming a bond between the -OH group on the 3' carbon of one nucleotide and the phosphate group on the 5' carbon of the next nucleotide.

    • This forms a backbone of sugar-phosphate units, with nitrogenous bases acting as appendages.

  • The base sequences along a DNA or mRNA polymer are unique to each gene.

RNA Structure

  • Most RNA molecules are typically single-stranded, consisting of one polynucleotide chain, utilizing nitrogenous bases A, G, C, and U.

DNA Structure

  • DNA Configuration:

    • Consists of two polynucleotides spiraling around a common axis, forming a double helix.

    • The two strands run in antiparallel directions (5’ to 3’).

    • Each DNA double helix contains numerous genes:

    • Base Pairing: The nitrogenous bases pair in a complementary manner: A always pairs with T, and G always pairs with C.

Visualization of DNA Double Helix

  • Key Features of the Double Helix:

    • Presence of a sugar-phosphate backbone with nitrogenous bases on the inside; bases interact through hydrogen bonding.

Historical Context

Contribution of Watson and Crick

  • 1953: Watson and Crick propose the double helix structure of DNA, suggesting that specific base pairing is essential for genetic material replication.

  • Watson and Crick's model utilizes data from X-ray diffraction and insights from Wilkins and Franklin concerning the structure of DNA.

  • Their discovery contributed significantly to molecular biology, earning them and James Watson the Nobel Prize in Physiology or Medicine, 1962.

Chargaff’s Rule

  • Chargaff's Rule: Establishes that in DNA:

    • The quantity of guanine (G) equals the quantity of cytosine (C), and the quantity of adenine (A) equals the quantity of thymine (T).

Conclusion

  • Nucleotides and nucleic acids underpin much of our understanding of molecular biology, genetics, and heredity, showcasing the fundamental role they play in all living organisms.