Science literature summaries

EtOH reduces the solubility of DNA DNA normally stays dissolved in water because DNA is surrounded by a shell of water molecules, because water sticks to its charged phosphate backbone When ethanol is added, it competes with water and reduces the number of water molecules that can surround DNA So the “protective water coat” around DNA becomes weak or incomplete Without that coat, DNA strands are no longer well-separated and stabilized in solution Salt increases the precipitation of DNA A salt (like NaCl or sodium acetate) is usually already added in the extraction mix, and it helps reduce repulsion between DNA strands, making clumping easier Once clumped, DNA becomes visible as a white precipitate, which can be collected easily 

Ethanol is amphipathic, so it can interact with both water and lipid molecules, and thus it can insert into and disrupt the phospholipid membrane, making it more permeable and allowing it to enter the cell. Ethanol also denatures proteins by disrupting hydrogen bonds and hydrophobic interactions, causing proteins to unfold and lose function.  Around 70% works best because water helps ethanol penetrate cells and slows evaporation, giving more time for damage; in contrast, 100% ethanol dehydrates the outside too quickly and is less effective at penetrating and killing cells.

Phospholipids and SDS are both amphipathic molecules.  However, SDS has one hydrophobic tail and one hydrophilic head , while phospholipids have one hydrophilic head and two hydrophobic tails . That difference matters:  Phospholipids pack tightly and form a stable bilayer, while SDS does not fit well into that structure.  Instead, SDS inserts into the membrane and disrupts the interactions between phospholipids, causing the membrane to lose integrity and eventually break apart.