DNA Structure and Nucleosomes

DNA-Histone Association

• The initial process involves DNA associating with small basic proteins called histones.
• The structural unit formed by the association of DNA and histones is called the nucleosome.
• A nucleosome has a core consisting of eight histones.
• Around 150 base pairs (bp) of DNA are wrapped around the core of histones in a nucleosome.

The Plasma Membrane Structure

• The plasma membrane ranges from 7.5 to 10 nm in thickness and is only visible through an electron microscope.
• Membrane phospholipids are amphipathic, consisting of nonpolar (hydrophobic) long-chain fatty acids linked to a charged polar (hydrophilic) head that bears a phosphate group.
• Phospholipids are most stable when organized into a double layer (bilayer) with the hydrophobic fatty acid chains located in a middle region away from water and the hydrophilic polar head groups contacting water.
• Molecules of cholesterol, a sterol lipid, insert at varying densities among the closely-packed phospholipid fatty acids, restricting their movements and modulating the fluidity of all membrane components.

Transmembrane Proteins and Membrane Transport

• Proteins are major constituents of membranes (~50% by weight in the plasma membrane).
• Integral proteins are incorporated directly within the lipid bilayer, whereas peripheral proteins are bound to one of the two membrane surfaces, particularly on the cytoplasmic side.
• Channels are multipass proteins forming transmembrane pores through which ions or small molecules pass selectively.
• Cells open and close specific channels for Na+, K+, Ca2+, and other ions in response to various physiological stimuli.
• Carriers are transmembrane proteins that bind small molecules and translocate them across the membrane via conformational changes.
• Membrane pumps are enzymes engaged in active transport, utilizing energy from the hydrolysis of adenosine triphosphate (ATP) to move ions and other solutes across membranes, against often steep concentration gradients.

Signal Reception and Transduction

• Cells in a multicellular organism communicate with one another to regulate tissue and organ development, control growth and division, and coordinate their functions.
• Adjacent cells form communicating gap junctions that couple the cells and allow exchange of ions and small molecules.
• Cells use different types of receptors to detect and respond to various extracellular molecules and physical stimuli.
• Membrane proteins perform several specific recognition and signaling functions, playing a key role in the interactions of the cell with its environment.
• Each cell type in the body contains a distinctive set of cell surface and cytoplasmic receptor proteins that enable it to respond to a complementary set of signaling molecules in a specific, programmed way.
• Receptors for hydrophilic signaling molecules, including polypeptide hormones and neurotransmitters, are usually transmembrane proteins in the plasmalemma of target cells.
• Three important functional classes of receptors are: • Channel-linked receptors that open associated channels upon ligand binding to promote transfer of molecules or ions across the membrane. • Enzymatic receptors, in which ligand binding induces catalytic activity in associated peripheral proteins. • G-protein–coupled receptors that, upon ligand binding, stimulate associated G-proteins which then bind the guanine nucleotide GTP and are released to activate other cytoplasmic proteins.