Overview
This lecture covers the structure and functions of proteins and nucleic acids (DNA and RNA) as biological macromolecules, then introduces universal and distinguishing features of prokaryotic and eukaryotic cells.
Protein Functions
- Proteins serve as enzymes, facilitating chemical reactions.
- They defend the body, e.g., antibodies in the immune system.
- Proteins transport substances like oxygen in blood.
- They provide structural integrity (cartilage, nails).
- Responsible for muscle contraction.
- Act as regulatory molecules, such as protein hormones.
Protein Structure
- Proteins are polymers of 20 different amino acids, joined by peptide bonds.
- Amino acids vary in size, polarity, and charge, allowing protein diversity.
- Most proteins are hydrophilic and dissolve in water.
- Four levels of protein structure:
- Primary: linear sequence of amino acids.
- Secondary: regular patterns (alpha helices, beta sheets) formed by hydrogen bonds.
- Tertiary: overall 3D shape stabilized by hydrogen bonds, ionic bonds, disulfide linkages, and hydrophobic interactions.
- Quaternary: association of multiple polypeptide subunits (not present in all proteins).
- Protein function depends on correct 3D structure; loss of structure (denaturation) leads to loss of function.
- Denaturation can be caused by heat, pH changes, or salt concentration and is often irreversible.
Nucleic Acids: DNA and RNA
- DNA and RNA are polymers of nucleotides.
- Each nucleotide includes a five-carbon sugar (ribose for RNA, deoxyribose for DNA), a phosphate group, and a nitrogenous base.
- Nitrogenous bases: adenine (A), guanine (G), cytosine (C), thymine (T - DNA only), uracil (U - RNA only).
- Bases group into purines (A, G, double-ringed) and pyrimidines (C, T, U, single-ringed).
- Nucleotides link via phosphodiester bonds (5' phosphate to 3' hydroxyl).
- DNA is double-stranded (double helix), held by hydrogen bonds between complementary bases (A-T, C-G).
- RNA is usually single-stranded.
- DNA stores genetic information; RNA is a short-term copy (and sometimes has structural/enzymatic roles).
Cell Structure: Prokaryotes vs. Eukaryotes
- All cells contain a plasma membrane, cytoplasm, DNA, and ribosomes.
- Plasma membrane: lipid bilayer boundary.
- Cytoplasm: fluid space inside the cell.
- DNA: genetic blueprint (free in prokaryotes, in nucleus in eukaryotes).
- Ribosomes: protein synthesis machinery, made of RNA and protein.
- Eukaryotic cells possess additional membrane-bound organelles; prokaryotes do not.
Key Terms & Definitions
- Amino Acid — Building block of proteins.
- Peptide Bond — Covalent bond linking amino acids.
- Denaturation — Loss of protein's 3D structure and function.
- Nucleotide — Building block of nucleic acids.
- Phosphodiester Bond — Linkage between nucleotides in nucleic acids.
- Purine — Nitrogenous base with two rings (A, G).
- Pyrimidine — Nitrogenous base with one ring (C, T, U).
- Plasma Membrane — Lipid bilayer enclosing the cell.
- Cytoplasm — Internal fluid of the cell.
- Ribosome — Organelle that synthesizes proteins.
Action Items / Next Steps
- Review the structures and functions of proteins and nucleic acids.
- Memorize base-pairing rules (A-T, C-G) and the distinction between purines and pyrimidines.
- Prepare for next lecture on eukaryotic membrane-bound organelles.