Unit 3: DNA & Cell Cycle - DNA Structure & Base Pairing

Introduction to DNA

• Deoxyribonucleic Acid (DNA) is the fundamental biological molecule that carries the essential instructions for life.

• DNA contains all the genetic code necessary for cellular processes, organism development, functioning, and reproduction.

Structure of Nucleotides

• DNA is a polymer composed of repeating monomer subunits called nucleotides.

• Each nucleotide is composed of three distinct chemical components:

  • Phosphate Group: Forms the linking framework along the outer sides of the molecule.

  • Sugar (Deoxyribose): A five-carbon sugar component attached to both the phosphate group and a nitrogenous base.

  • Nitrogenous Base: The variable component of a nucleotide, which can be any of four distinct molecules:

  • Adenine ($A$)

  • Thymine ($T$)

  • Cytosine ($C$)

  • Guanine ($G$)

The Double Helix Model

• The overall structural architecture of DNA is described as a double helix, which has a twisted ladder shape.

• Sides of the Ladder: Composed of repeating sugar and phosphate groups, forming the protective sugar-phosphate backbones.

• Steps (Rungs) of the Ladder: Composed of complementary nitrogenous base pairs ($A, T, C, G$) extending inward from opposite strands.

Organization of Genetic Material

• Gene: A specific segment of DNA that codes for a particular trait or biological function.

• Chromosome: DNA in a tightly wound, highly packaged, and organized structural form.

• Structural breakdown of the DNA strand:

  • Outer structural frame: Phosphate-deoxyribose backbone.

  • Core chemical bases: Guanine ($G$), Adenine ($A$), Thymine ($T$), and Cytosine ($C$).

Complementary Base Pairing and Hydrogen Bonds

• Complementary Base Pairing Rules:

  • Adenine ($A$) always pairs specifically with Thymine ($T$) ($A - T$).

  • Cytosine ($C$) always pairs specifically with Guanine ($G$) ($C - G$).

• Chemical Bonding and Function:

  • Complementary nitrogenous bases across the opposing strands are held together by hydrogen bonds.

  • Functional Dual-Nature of Hydrogen Bonds:

  • They are strong enough to hold the two complementary DNA strands together securely under physiological conditions.

  • They are weak enough to allow the strands to unzip during critical biological processes, such as DNA replication.