Lecture 8. DNA replication

Dr Robert Spooner Lecture 8, 2024

Topic: DNA Replication Part 1

Location: LF130 Cellular and Molecular Biology

The Story of DNA

Timeline of DNA Research:

  • 1865: Gregor Mendel conducted foundational experiments in genetics using pea plants, establishing the principles of inheritance that would later form the basis of modern genetics.

  • 1910: Thomas Hunt Morgan discovered sex linkage in fruit flies (Drosophila melanogaster), providing critical evidence for the chromosomal theory of inheritance.

  • 1928: Frederick Griffith conducted transformation studies, demonstrating that non-virulent bacteria could become virulent when exposed to heat-killed virulent bacteria, suggesting the transfer of genetic material.

  • 1941: George Beadle and Edward Tatum proposed the one gene-one enzyme hypothesis, which linked specific genes to the production of specific enzymes, reinforcing the relationship between genotype and phenotype.

  • 1944: Oswald Avery and his colleagues discovered that DNA, not protein, is the substance that causes bacterial transformation, establishing DNA as the carrier of genetic information.

  • 1950: Erwin Chargaff formulated Chargaff's rules, stating that in any given DNA sample, the amount of adenine equals thymine (A=T) and the amount of cytosine equals guanine (C=G), highlighting the base pairing mechanism of DNA.

  • 1952: Martha Chase and Alfred Hershey confirmed that DNA is the genetic material in bacteriophages through their famous experiments, providing strong evidence for DNA's role in heredity.

  • 1953: James Watson and Francis Crick proposed the double-helix structure of DNA, elucidating how genetic information is stored and replicated.

  • 2003-2005: The Human Genome Project successfully completed the sequencing of the human genome, marking a monumental advancement in genomics and biotechnology.

  • 2019: Ongoing advancements in genomics continue, including CRISPR technology for gene editing and insights into personalized medicine and evolutionary biology.

Understanding DNA Organization

Measurement Units:

  • 1 km = 1000 m

  • 1 m = 1000 mm

  • 1 mm = 1000 μm

  • 1 μm = 1000 nm

  • 1 nm = 1000 pm

  • 1 pm = 1000 fm

DNA in Escherichia coli:

  • Genome: The genome consists of a circular duplex molecule containing 4639 kilobase pairs (kb).

  • Base-pair spacing: Approximately 0.34 nm between base pairs, which contributes to the total DNA length of about 1.6 mm when fully extended.

  • Length of an E. coli cell: Typically ranges from 1 to 2 μm, underscoring the compact nature of its DNA.

  • Compaction: Bacterial DNA is highly compacted through structures called loops and supercoils to efficiently fit within the limited space of the bacterial cell.

Eukaryotic DNA Organization

Human DNA:

  • Haploid genome: Contains 3.2 billion base pairs (Gbp), while a typical somatic cell has 6.4 Gbp.

  • Total length of DNA: Approximately 2.2 meters per somatic cell, emphasizing the vast amount of genetic material present.

  • Compaction: The degree of DNA compaction in eukaryotic cells is significantly greater than in prokaryotes, allowing it to fit within a nucleus measuring about 6 μm in diameter.

  • Total length of DNA in the human body: If stretched out, the total length of DNA in all human cells is roughly 2.2 x 10^14 meters, capable of traversing the distance to the sun and back approximately 730 times.

Eukaryotic DNA Structure:

  • DNA duplex: Each DNA strand is approximately 2 nm in diameter.

  • Nucleosome fiber: The basic unit of DNA packaging, which has an 11 nm diameter and consists of DNA wrapped around histone proteins.

  • Chromatin fiber: This complex of DNA and proteins further coils and layers, resulting in structures that reach up to 1400 nm in diameter during the metaphase stage of cell division.

DNA Replication Process

  • Decompaction requirement: DNA must undergo decompaction to allow access for replication enzymes during cell division.

  • Error rate: The fidelity of DNA replication is remarkably high, with an approximate error rate of 1 in 10^9 base pairs, which protects the integrity of genetic information.

  • Mutation frequency: On average, an individual cell experiences around 6 mutations per division, necessitating effective repair mechanisms to maintain genomic stability.

Watson-Crick Model & Predictions

  • Discovery Date: The landmark date for the Watson-Crick model is April 25, 1953.

  • Core Predictions:

    • Each DNA strand serves as a template for replication, leading to a semi-conservative replication mechanism, where each resulting daughter DNA strand contains one strand from the parental template.

    • The antiparallel structure of DNA strands, with orientations of 3'-5' and 5'-3', is fundamental to the replication process.

Testing Models of DNA Replication

Three Hypotheses:

  1. Semi-conservative model: Each daughter molecule contains one parental and one new strand.

  2. Conservative model: The entire parental strand is conserved, and a completely new daughter strand is formed.

  3. Dispersive model: The parental strands are fragmented and reassembled into daughter strands.

Meselson and Stahl Experiment (1958):

  • Developed the method of equilibrium density gradient centrifugation to test models of DNA replication.

  • Employed heavy nitrogen (15N) and light nitrogen (14N) isotopes in the growth of E. coli, observing the density of DNA before and after replication.

  • Results: The experiment conclusively demonstrated only hybrid (14N/15N) and light (14N/14N) DNA forms, thus confirming the semi-conservative replication model.

Discovery of DNA Polymerases

  • Key Figure: Arthur Kornberg discovered DNA polymerase in 1957, fulfilling the role of a critical enzyme in DNA replication processes.

  • In vitro conditions: The enzyme requires specific conditions to function, including template DNA, deoxynucleotide triphosphates (dNTPs), a Mg^2+ cofactor, an energy source (typically ATP), and a primer with a free 3’ hydroxyl group to initiate synthesis.

DNA Polymerase Functions

  • New DNA strands are synthesized exclusively in the 5' to 3' direction, which is essential for the overall integrity of the genetic code.

  • DNA Polymerase I: Identified as the primary enzyme in DNA synthesis, it also possesses proofreading capabilities that correct errors during replication.

  • DNA Polymerase III: The enzyme essential for E. coli DNA replication, it was discovered by Thomas Kornberg, Arthur Kornberg's son, highlighting a significant familial contribution to the field of molecular biology.

Conclusion of Lecture 8 Part 1

Key Concepts:

  • The semi-conservative nature of DNA replication is pivotal for maintaining genetic fidelity across generations.

  • The antiparallel structure of DNA strands plays a vital role in replication mechanics and accurate base pairing.

  • Watson-Crick base pairing is crucial for ensuring the fidelity of DNA synthesis, with complementary bases pairing appropriately during the replication process.

  • The introduction of DNA polymerases and understanding their functionalities is foundational for comprehension of the DNA replication machinery.