CELS191 Lecture 10: DNA Structure Notes

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Introduction

Dr. Annika Bokor from the Department of Biochemistry.

Lecture 10: DNA Structure

CELS191 2025 Molecular Biology & Genetics. This lecture will outline DNA components, the Watson-Crick model, semiconservative DNA replication, and how genetic information is passed to the next generation.

Lecture Objectives

By the end of this lecture, you should be able to:

  • Outline the components of DNA.

  • Describe the main features of the Watson-Crick model of DNA.

  • Explain semiconservative DNA replication.

  • Understand how genetic information is passed to the next generation.

DNA Function

DNA serves as the genetic material within the cell.

The Search for Genetic Material

By the late 18th to early 19th century, it was established that:

  • Nucleic acids and their components had been identified.

  • There were two types of nucleic acids: DNA and RNA.

  • Chromosomes were identified as the carriers of heritable factors.

  • Chromosomes consist of both proteins and nucleic acid (deoxyribonucleic acid = DNA).

Phoebus Levene (1869 – 1940)

  • Identified DNA and RNA and their components.

  • Nucleotide units are linked together through phosphate groups, forming the backbone of the molecule.

  • Claimed nucleotides are present as tetranucleotide repeats, with a 1:1:1:1 ratio of A, T, C, and G nucleotides.

  • Because DNA (and RNA) contain only four bases, Levene concluded that such a "simple" molecule could not encode the complexity of life.

Levene's Conclusion: Proteins, and not DNA, on the chromosomes are the bearers of genetic information.

Chargaff's Discovery

There IS significant DNA variation between species, thus DNA could be the genetic material.

Chargaff’s Rules

First Rule: [A]=[T][A] = [T] and [G]=[C][G] = [C]
Second Rule: The composition of DNA varies between species

The Discovery of DNA

The discovery of DNA as the genetic material involved many experiments and scientists.

DNA Structure

A double-stranded helical molecule with particular features.

Key Figures in DNA Structure Discovery

  • Maurice Wilkins

  • Francis Crick

  • James Watson

  • Rosalind Franklin

Franklin's Photo 51

X-ray Diffraction Pattern of DNA:

  • Helical structure

  • Bases perpendicular to the length of the DNA molecule

  • Double-stranded

Watson and Crick's Synthesis

Watson used Chargaff’s findings and Franklin’s photo to deduce the structure of DNA.

DNA Components

The single unit building blocks of the macromolecule DNA are nucleotides.

Formation of a Polynucleotide

Nucleotide monomers are joined together with phosphodiester bonds to form a polynucleotide = nucleic acid (deoxyribonucleic acid for DNA).

Formation of the Phosphodiester Bond

The hydroxyl group (OH)(OH) on the 3rd carbon of one nucleotide reacts with the phosphate group attached to the 5th carbon on another nucleotide. DNA (and RNA) strands are synthesised in the 5´ → 3´ direction.

DNA Strand Directionality

Each DNA strand has a direction. The two strands are antiparallel, i.e., 5´ → 3´ and 3´ → 5´.

Double-Stranded Helix

The two antiparallel DNA strands form a double-stranded helix.

DNA Structure Summary

A diagram summarizing the structure of a DNA molecule.

The Watson-Crick Model of DNA Structure

  • DNA has a double-stranded helical structure.

  • The sugar-phosphate backbone is on the outside.

  • The bases are on the inside.

  • Stabilized by hydrogen bonds.

  • The two polynucleotide strands are oriented in opposite directions.

Implications of the Watson-Crick Model

  • Provided a stimulus for deciphering the genetic code.

  • Suggested a possible mechanism for the replication of DNA.

Semiconservative Replication

Each DNA strand of the double helix is used as a template strand for the synthesis of two new strands.

Why Replicate DNA?

  • Because your body needs to make more cells!

  • Why does your body need to make more cells?

    • Different growth stages throughout life

    • Pregnancy

    • Repair of injury

    • Mitosis: 1 cell → 2 (daughter) cells.

    • DNA synthesis/replication: 1 dsDNA molecule → 2 ds DNA molecules

Lecture Summary

  • Nucleotide monomers are joined together with phosphodiester bonds to form a DNA strand (a polynucleotide).

  • The DNA molecule is a double-stranded helical structure, composed of two linear DNA strands that are in anti-parallel orientation to each other.

  • The nucleotide bases are found on the inside of the DNA helix, where two and three hydrogen bonds between A=TA=T and G≡CG≡C nucleotide bases respectively, stabilise the structure.

  • The sugar-phosphate backbone is on the outside of the DNA helix.

  • Semiconservative DNA replication allows genetic information to be passed to the next generation.

Objective-Based Questions

  • How many polynucleotide chains make up a DNA molecule and how are these organised in relation to each other?

  • In which ‘direction’ does a DNA chain grow when being synthesised?

  • How is the DNA helix stabilised? (be specific; which component of the nucleotide is involved and include the type and number of bonds between these components).

  • Which nucleotide component(s) are located on the inside of the DNA helix and which are located on the outside of the DNA helix?

  • Does semi-conservative DNA replication allow genetic information to be passed onto newly synthesised cells and the next generation? Explain.

  • Outline the components of DNA.

    • Nucleotides (sugar, phosphate, and nitrogenous base)

    • Nucleotides are the fundamental building blocks of DNA. Each nucleotide consists of three components:

      • A deoxyribose sugar molecule

      • A phosphate group

      • A nitrogenous base: adenine (A), guanine (G), cytosine (C), or thymine (T)

    • Sugar-phosphate backbone

    • The sugar and phosphate groups of adjacent nucleotides are linked together through phosphodiester bonds, creating a chain. This chain forms the backbone of the DNA molecule.

    • Nitrogenous bases (A, T, C, G)

    • The nitrogenous bases are attached to the sugar molecules in the nucleotides. These bases are the information-carrying components of DNA.

  • Describe the main features of the Watson-Crick model of DNA.

    • Double-stranded helix

    • DNA consists of two strands of nucleotides that are wound around each other to form a double helix.

    • Sugar-phosphate backbone on the outside

    • The sugar-phosphate backbones of the two strands are located on the exterior of the helix, providing structural support.

    • Bases on the inside

    • The nitrogenous bases are located on the interior of the helix, where they pair with bases on the opposite strand.

    • Stabilized by hydrogen bonds (A=T, G≡C)

    • The two strands are held together by hydrogen bonds between the bases. Adenine (A) pairs with thymine (T) via two hydrogen bonds, and guanine (G) pairs with cytosine (C) via three hydrogen bonds.

    • Antiparallel strands (5' to 3' and 3' to 5')

    • The two DNA strands run in opposite directions. One strand runs from the 5' (phosphate) end to the 3' (hydroxyl) end, while the other strand runs from the 3' end to the 5' end.

  • Explain semiconservative DNA replication.

    • Each strand serves as a template

    • During DNA replication, the two strands of the double helix separate, and each strand serves as a template for the synthesis of a new complementary strand.

    • Results in two new DNA molecules, each with one original and one new strand

    • The result of DNA replication is two DNA molecules, each consisting of one original (template) strand and one newly synthesized strand. This is why DNA replication is described as semiconservative.

  • Understand how genetic information is passed to the next generation.

    • Through semiconservative DNA replication

    • Semiconservative DNA replication ensures that genetic information is accurately copied and passed on to daughter cells during cell division.

    • Ensures accurate transmission of genetic information from parent to daughter cells

    • Because each new DNA molecule contains one original strand, the genetic information is preserved and transmitted faithfully from parent to daughter cells, ensuring the continuity of genetic information across generations.