BIOL 2040: DNA Replication and Genome Organization

Learning Objectives for BIOL 2040

  • A. Relate concepts from BIOL 1000 to those in BIOL 2040.

  • C. Describe the molecular anatomy of genes and genomes (how DNA is organized) and how genetic information is duplicated.

    • Describe DNA replication using appropriate terminology and referencing appropriate components, relating DNA replication to the formation of sister chromatids.

    • Apply knowledge of DNA replication (both in vivo & in vitro) to molecular techniques used to study genetics.

Molecular Anatomy: Review of DNA Replication

  • Visual Representation of DNA: While many ways exist to visually represent DNA, the most realistic representation is typically not the most helpful for understanding its processes.

  • Secondary Structure of DNA:

    • DNA exists as a double helix.

    • Composed of two antiparallel DNA strands.

    • These two strands form one molecule of DNA.

DNA Structure and Replication

  • Facilitation of Replication: The unique structure of DNA directly facilitates its replication.

  • Semi-conservative Replication: DNA replication occurs in a semi-conservative manner.

    • During replication, each new daughter DNA molecule consists of one old (template) strand and one newly synthesized strand.

    • This process directly leads to the formation of sister chromatids, which are identical copies of a chromosome joined together.

Evidence for Semi-Conservative Replication

  • Historical Models: Before definitive evidence, three models were proposed for DNA replication:

    • Conservative: The original DNA molecule remains intact, and an entirely new DNA molecule is synthesized.

    • Semi-conservative: Each new DNA molecule consists of one original strand and one newly synthesized strand.

    • Dispersive: Each new DNA molecule contains a mixture of old and newly synthesized DNA interspersed throughout both strands.

  • Meselson-Stahl Experiment: This pivotal experiment provided conclusive evidence that DNA replicates semi-conservatively.

    • It utilized isotopes of nitrogen, specifically ^{14}N (light nitrogen, common) and ^{15}N (heavy nitrogen).

    • The experiment tracked the density of DNA over subsequent rounds of replication in different nitrogen media, allowing differentiation between the proposed replication models.

    • The results supported the semi-conservative model, ruling out conservative and dispersive replication.

Mechanism of DNA Synthesis

  • Localized Unwinding: DNA replication begins with the localized unwinding of the double helix.

    • This unwinding creates replication forks, allowing access to the template strands for synthesis.

    • The purpose of unwinding is to expose the template strands for the production of new complementary DNA strands.

  • Primer Requirement: DNA synthesis requires a primer.

    • A primer provides a free 3'-OH (hydroxyl) group.

    • This free 3'-OH group is essential because DNA polymerase can only add new nucleotides to an existing 3'-OH group.

  • DNA Polymerase Function:

    • DNA polymerase is the enzyme that catalyzes the addition of new nucleotides.

    • Nucleotides are always added to the 3' end of the growing DNA strand.

    • Consequently, DNA synthesis always occurs in the 5' o 3' direction.

    • This directional synthesis means the DNA template strand is read in the 3' o 5' direction.

  • Leading and Lagging Strands: Due to the antiparallel nature of DNA and the 5' o 3' synthesis direction of DNA polymerase:

    • Leading Strand: Synthesized continuously in the 5' o 3' direction, moving towards the replication fork.

    • Lagging Strand: Synthesized discontinuously in short segments, also in the 5' o 3' direction, but moving away from the replication fork.

      • These short segments on the lagging strand are known as Okazaki fragments.

    • After the synthesis of Okazaki fragments, an enzyme called DNA ligase is required to join these fragments together to form a continuous strand.

  • Error Correction: The structure of DNA facilitates not only replication but also error correction mechanisms, ensuring high fidelity of genetic information.

Timing of DNA Replication

  • Cell Cycle: DNA synthesis (replication) is a tightly regulated process that occurs at a specific time during the cell cycle.

    • This particular phase is known as the S phase (synthesis phase), during which the cell duplicates its entire genome.

Practice Questions

  1. Where does DNA replication take place?

  2. If DNA polymerase can add nucleotides only in the 5' o 3' direction, what direction is the DNA template read?

  3. We know that DNA replication occurs semi-conservatively, but what would the results of the Meselson-Stahl experiment look like if it happened conservatively?

  4. Aside from S phase, when else might DNA replication occur?

  5. The diagram from the video:
    a. Label a – f with the following terms: Lagging strand, RNA primer, replication fork, leading strand, template strands, Okazaki fragments.
    b. Where on the diagram would DNA ligase function?