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DNA Structure & Replication

Overview

  • DNA (Deoxyribonucleic Acid) provides the genetic blueprint for all living organisms.

  • RNA (Ribonucleic Acid) plays a role in various biological processes, including protein synthesis.

Learning Objectives

  • Structural and Functional Differences: Distinguish between DNA and RNA.

  • DNA Replication Steps: Describe the process of DNA replication.

  • Building Blocks: Identify the basic units of DNA, RNA, and proteins.

Types of Nucleic Acids

  • Two main types:

    • DNA: It contains deoxyribose sugar and the nitrogen bases adenine (A), thymine (T), cytosine (C), and guanine (G).

    • RNA: It contains ribose sugar and substitutes uracil (U) for thymine.

  • Function of DNA: Directs the synthesis of mRNA, thus controlling protein synthesis.

The Minds Behind DNA Discovery

  • Francis Crick: Co-discoverer of the DNA double-helix structure.

  • James Watson: Co-discoverer alongside Crick.

  • Maurice Wilkins: His work contributed to DNA structure understanding.

  • Rosalind Franklin: Pioneered X-ray diffraction techniques—Photo 51 was crucial in identifying DNA's structure.

Structure of DNA and RNA

  • Basic Composition: Nucleic acids are polymers consisting of monomers called nucleotides.

    • Each nucleotide includes:

      • A nitrogenous base (A, T, C, G for DNA; A, U, C, G for RNA)

      • A pentose sugar (deoxyribose in DNA, ribose in RNA)

      • Phosphate groups

  • Sugar-Phosphate Backbone: Repeating pattern of sugar and phosphate units with nitrogenous bases attached.

Nitrogenous Bases

  • Pyrimidines:

    • Cytosine (C)

    • Thymine (T, in DNA)

    • Uracil (U, in RNA)

  • Purines:

    • Adenine (A)

    • Guanine (G)

DNA vs RNA

  • DNA vs RNA Comparison:

    • DNA discovered as a double helix; RNA generally single-stranded.

    • Base pairing mechanism varies (Thymine/Uracil difference).

DNA Replication Process

Basic Principles

  1. Base Pairing to Template Strand: Each strand serves as a template for replication.

  2. Unwinding the Double Helix: The parent DNA unwinds, allowing new strands to be synthesized.

Semiconservative Model of Replication

  • Each daughter molecule consists of one old strand and one new strand.

  • Discovery based on Watson and Crick's model.

Steps of DNA Replication

  1. Initiation:

    • Begins at specific sites known as origins of replication.

    • Initiator proteins bind to the origin and separate the strands.

  2. Elongation:

    • DNA polymerase synthesizes new DNA strands by adding nucleotides in a 5' to 3' direction.

    • Leading strand synthesized continuously, while lagging strand synthesized in Okazaki fragments.

  3. Termination:

    • New double helices formed, primers removed, DNA polymerases proofread and correct errors.

Key Enzymes in DNA Replication

  • Helicase: Unwinds DNA strands.

  • Primase: Synthesizes RNA primers for initiation.

  • DNA Polymerase III: Main enzyme for elongation of new DNA strands.

  • DNA Polymerase I: Replaces RNA primers with DNA.

  • DNA Ligase: Joins Okazaki fragments and seals nicks in the backbone.

Summary of Key Concepts

  • DNA Replication Steps:

    1. Initiator proteins bind and unwinds DNA.

    2. RNA primers initiated by primase.

    3. Continuous synthesis of leading strand and discontinuous synthesis of lagging strand.

    4. Proofreading by DNA polymerases to correct errors.

  • The replication process is essential for cell division and maintaining genetic continuity.