7.1 DNA Structure and Function

DNA, the blueprint of life, holds our genetic information. Its structure, a double helix of nucleotides, is crucial for storing and transmitting this data. Understanding DNA's components and configuration is key to grasping how genes work.

The nucleotides in DNA are like puzzle pieces that fit together perfectly. They're made up of bases, sugars, and phosphates. This unique structure allows DNA to replicate accurately and pass on genetic info to new cells.

Nucleotide Components

Nitrogenous Bases

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• Adenine (A) and guanine (G) are purines contain two fused rings
• Thymine (T) and cytosine (C) are pyrimidines contain a single ring
• Bases form hydrogen bonds with their complementary base on the opposite strand (A with T, C with G)
• Specific base pairing allows for accurate DNA replication and transcription

Sugar-Phosphate Backbone

• Deoxyribose sugar is a pentose sugar lacks a hydroxyl group at the 2' carbon position compared to ribose
• Phosphate groups connect the 5' carbon of one sugar to the 3' carbon of the next sugar
• Alternating sugar-phosphate backbone runs in opposite directions on each strand (5' to 3' and 3' to 5')
• Negatively charged phosphate groups contribute to the overall negative charge of DNA

DNA Structure

Double Helix Configuration

• DNA exists as a double helix consists of two complementary strands wound around each other
• Strands are antiparallel run in opposite directions (one 5' to 3', the other 3' to 5')
• Sugar-phosphate backbones are on the outside of the helix, while bases face inward
• Hydrogen bonding between complementary bases (A-T and C-G) stabilizes the double helix structure

Grooves and Base Accessibility

• Major groove is wider and deeper allows for easier access to the bases by proteins (transcription factors, DNA-binding proteins)
• Minor groove is narrower and shallower plays a role in DNA recognition and binding by certain proteins (histones, DNA repair enzymes)
• Spacing between the grooves is determined by the specific base pair sequence
• Grooves provide unique structural features for protein-DNA interactions and recognition

Base Stacking and Stability

• Base pairs are stacked on top of each other like rungs on a ladder
• Hydrophobic interactions between the stacked bases contribute to the stability of the DNA double helix
• π-π stacking interactions occur between the aromatic rings of adjacent base pairs
• Stacking interactions minimize the exposure of the hydrophobic bases to the aqueous environment