Molecular and Cellular Biology: Nucleic Acids, Proteins, Membranes, and Cellular Organization

Comprehensive vocabulary flashcards covering the molecular and cellular biology lectures: nucleic acids, genetic discovery experiments, protein structure and folding, cell membrane composition, transport kinetics, and eukaryotic vs. prokaryotic cellular architecture.

Transformation

The process demonstrated by Frederick Griffith in 1928 wherein genetic material from heat-killed virulent bacteria ($S$ strain) is transferred to living non-virulent bacteria ($R$ strain), converting them into virulent cells.

Avery, MacLeod, and McCarty Experiment

A 1944 experiment demonstrating that DNA, rather than RNA or protein, is the transforming principle; treating heat-killed $S$ cell extract with deoxyribonucleases abolished its ability to transform $R$ cells into virulent $S$ cells.

Hershey and Chase Experiment

A 1952 study using bacteriophage T2 labeled with $^{32}\text{P}$ (DNA) and $^{35}\text{S}$ (protein) to infect E. coli, proving that viral DNA enters the host cell while viral protein coats remain outside, establishing DNA as the genetic material.

Bacteriophage

A virus consisting primarily of DNA enclosed in a protein coat that attacks bacteria and replicates by invading host cells and utilizing their molecular machinery.

Nucleotide

The monomer subunit of nucleic acids, consisting of a phosphate group, a five-carbon pentose sugar (deoxyribose in DNA or ribose in RNA), and a nitrogenous base.

Purines

Double-ring nitrogenous bases found in nucleic acids; specifically adenine (A) and guanine (G).

Pyrimidines

Single-ring nitrogenous bases found in nucleic acids; includes cytosine (C), thymine (T) in DNA, and uracil (U) in RNA.

Phosphodiester Bond

A covalent linkage between the $3'$ hydroxyl group of one sugar molecule and the $5'$ phosphate group of an adjacent sugar molecule, forming the structural backbone of nucleic acids.

Antiparallel Strands

The parallel orientation of the two strands of a DNA double helix running in opposite directions, with one strand oriented $5' \rightarrow 3'$ and the opposing strand oriented $3' \rightarrow 5'$.

Central Dogma of Molecular Biology

A biological framework coined by Francis Crick in 1958 stating that genetic information flows from nucleic acid to nucleic acid and from nucleic acid to protein ($\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$).

Transcription

The initial step of gene expression where RNA polymerase reads one strand of a gene's DNA sequence and synthesizes a complementary messenger RNA ($\text{mRNA}$) transcript.

Translation

The stage of gene expression where ribosomes read the nucleotide sequence of an $\text{mRNA}$ molecule to link amino acids together and assemble a polypeptide chain.

Genome

The complete set of genetic instructions in an organism, stored within long molecules of DNA containing all necessary information to develop and function.

Nucleosome

The basic repeating packaging unit of eukaryotic chromatin, consisting of an octamer of core histones wrapped twice with approximately $146$ nucleotide pairs of DNA.

Core Histones

Four small, evolutionarily conserved basic proteins—H2A, H2B, H3, and H4—two copies of each assemble into an octameric core around which DNA winds.

Histone H1

A linker histone protein that binds and seals the DNA entry and exit points on the nucleosome, allowing nucleosomes to condense into thicker, $30\,\text{nm}$ chromatin fibers.

Euchromatin

The relatively uncoiled, relaxed, transcriptionally active form of chromatin that allows transcription machinery access to express genes.

Heterochromatin

The densely condensed, highly compacted region of chromatin that is inaccessible to transcription proteins, resulting in silenced genes.

DNA Origami

A method in DNA nanotechnology that exploits base-pairing properties to fold long DNA strands into nanoscale two- and three-dimensional shapes for biomedical and robotic applications.

Structural Proteins

Proteins that provide mechanical support to cells and tissues; extracellular examples include collagen and elastin, while intracellular examples include tubulin, actin, and keratin.

Motor Proteins

Proteins that generate cellular movement; examples include kinesin and dynein transporting cargo along microtubules, and myosin generating contraction forces in muscle cells.

Storage Proteins

Proteins that store essential ions or amino acids, such as ferritin (storing iron in the liver), casein (providing nutrients in mammalian milk), and ovalbumin (in egg whites).

Peptide Bond

A covalent bond formed via a condensation reaction between the carboxyl group of one amino acid and the amino group of an incoming amino acid, liberating a molecule of $H_2O$.

Primary Protein Structure

The linear sequence of amino acid residues joined by peptide bonds, oriented from the N-terminus to the C-terminus, encoded directly by a gene.

Secondary Protein Structure

Local conformations of the polypeptide backbone stabilized by hydrogen bonds between carbonyl and amino backbone groups; common types are the $\alpha$-helix (hydrogen bonds every roughly 4 residues) and $\beta$-pleated sheets.

Tertiary Protein Structure

The overall three-dimensional conformation of a single polypeptide chain driven by interactions among variable R groups, including ionic bonds, hydrogen bonds, hydrophobic interactions, and covalent disulfide bridges.

Disulfide Bond

A covalent cross-link formed by the oxidation of sulfhydryl ($-\text{SH}$) groups between neighboring cysteine residues in the endoplasmic reticulum, stabilizing tertiary and quaternary structures.

Quaternary Protein Structure

The functional multi-subunit assembly consisting of two or more bonded polypeptide chains, categorized as homodimers, heterodimers, or higher oligomers.

Molecular Chaperones

Specialized helper proteins, such as Hsp70 and the barrel-like GroEL/GroES complex, that facilitate proper protein folding and assembly while preventing nonfunctional aggregation.

Protein Denaturation

The loss of native secondary, tertiary, and quaternary structure triggered by physical or chemical factors like heat, extreme pH, detergents, or high salt concentrations, causing a loss of function.

Prion

An infectious proteinaceous agent caused by an abnormally folded protein ($\text{PrP}^{\text{Sc}}$) that binds normal counterparts ($\text{PrP}^{\text{C}}$) and forces them to fold into the abnormal, disease-causing conformation.

Fluid Mosaic Model

A biological model describing the cell membrane as a dynamic, fluid two-dimensional lipid bilayer sheet embedded with floating integral, peripheral, and lipid-anchored proteins.

Amphipathic

Possessing both hydrophilic (water-loving, polar) and hydrophobic (water-fearing, nonpolar) regions, a property critical to the self-assembly of the phospholipid bilayer.

Phosphoglycerides

The predominant class of membrane phospholipids consisting of a glycerol backbone esterified to two fatty acid tails and a phosphate-linked polar headgroup.

Lipid Rafts

Dynamic, micro-domains in the plasma membrane enriched in cholesterol and sphingolipids that are more ordered and rigid than surrounding lipids, coordinating cell signaling and protein sorting.

Liposomes

Artificially prepared spherical vesicles composed of a lipid bilayer enclosing an aqueous core, used extensively as membrane models and targeted drug delivery vehicles.

Integral Membrane Proteins

Proteins permanently embedded within the lipid bilayer, typically spanning the membrane with hydrophobic transmembrane regions to serve as channels, transporters, or receptors.

Peripheral Membrane Proteins

Proteins non-covalently attached to the external or internal polar surfaces of the lipid bilayer, involved in signaling cascades and cytoskeletal tethering.

Simple Diffusion

Passive, non-energy-requiring passage of small, nonpolar molecules or lipid-soluble solutes across the phospholipid bilayer down their concentration gradient.

Facilitated Diffusion

Passive transport of polar or charged solutes down their electrochemical gradient across the membrane mediated by channel or carrier proteins without expending cellular energy.

Osmosis

The passive net movement of water molecules across a selectively permeable membrane from an area of higher water concentration (lower solute) to an area of lower water concentration (higher solute).

Tonicity

The ability of an extracellular solution to exert an osmotic gradient determining the direction of water flow into or out of a cell; categorized as isotonic, hypotonic, or hypertonic.

Electrochemical Gradient

The combined influence of a concentration gradient (difference in chemical solute concentration) and an electrical gradient (potential charge difference across the membrane) driving ion movement.

Active Transport

Carrier-mediated movement of substances against an electrochemical gradient (low to high concentration), requiring direct or indirect expenditure of cellular metabolic energy (such as ATP).

$\text{Na}^+/\text{K}^+$ Pump

An ATP-driven electrogenic transport pump that hydrolyzes one molecule of ATP to export three $\text{Na}^+$ ions from the cytosol and import two $\text{K}^+$ ions against their concentration gradients.

Coupled Transport

A mechanism in which the energetically favorable movement of one solute down its concentration gradient drives the active transport of another solute against its gradient; includes symport and antiport systems.

Uniport

A carrier-mediated transport system that carries only a single solute across the membrane in one direction at a time.

Symport

A coupled secondary active transport process that co-transports two distinct molecules or ions across the membrane simultaneously in the same direction, such as the sodium-glucose cotransporter (SGLT1/2).

Antiport

A coupled transport process that moves two distinct solutes in opposite directions across the membrane simultaneously, such as the sodium-calcium exchanger.

Group Translocation

An active bacterial transport process requiring phosphoenolpyruvate (PEP) in which the transported solute (e.g., glucose) is chemically modified (phosphorylated) as it crosses the membrane.

Endocytosis

An energy-dependent bulk vesicular transport mechanism by which cells engulf extracellular particles, fluids, or targeted macromolecules into membrane-bound vesicles; includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Exocytosis

The active secretion process wherein intracellular transport vesicles fuse with the plasma membrane to release packaged substances and secretory products outside the cell.

Transmission Electron Microscope (TEM)

A high-resolution microscope that passes an electron beam through ultrathin specimen sections, generating high-magnification detailed images of internal organellar and viral structures.

Endosymbiotic Theory

The evolutionary hypothesis proposed by Lynn Margulis stating that eukaryotic organelles such as mitochondria and chloroplasts originated when ancestral eukaryotic cells engulfed specialized prokaryotes.

Plasmids

Small, autonomous, circular extrachromosomal DNA elements found in bacteria that replicate independently and carry non-essential adaptive genes, such as toxin production or antibiotic resistance.

Nucleolus

A dense non-membrane-bound substructure within the eukaryotic nucleus where ribosomal RNA ($\text{rRNA}$) is transcribed and ribosomal subunits are assembled.

Ciliary Escalator

A primary lung defense mechanism in the respiratory tract where coordinated beating of epithelial cilia sweeps mucus-trapped dust and pathogens upwards toward the throat for expulsion or swallowing.

Plasmodesmata

Microscopic plasma-membrane-lined cytoplasmic channels that perforate plant cell walls, allowing the direct transport of water, nutrients, and signaling molecules between adjacent cells.

Rough Endoplasmic Reticulum (rER)

A convoluted membranous organelle studded with ribosomes on its cytosolic surface, responsible for the synthesis, processing, and folding of membrane-bound, organellar, and secretable proteins.

Smooth Endoplasmic Reticulum (sER)

A non-ribosome-bearing network of tubular membranes that synthesizes lipids and steroids, metabolizes carbohydrates, and detoxifies drugs and poisons.

Golgi Apparatus

A stack of flattened membrane-enclosed cisternae that receives proteins and lipids from the ER, chemically modifies and sorts them, and packages them into vesicles for targeted transport or exocytosis.

Lysosomes

Membrane-bound digestive organelles containing acidic hydrolase enzymes that break down engulfed debris, macromolecules, and worn-out cell parts.

Peroxisomes

Enzyme-containing oxidative organelles that break down fatty acids, neutralize toxic byproducts, and decompose toxic hydrogen peroxide ($H_2O_2$) into water and oxygen.

Endomembrane System

The coordinated network of interacting internal eukaryotic membranes consisting of the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, vacuoles, and plasma membrane.

Tight Junctions

Specialized intercellular junctions in animal epithelial sheets that tightly stitch plasma membranes together, creating a leak-proof barrier against extracellular fluid flow.

Anchoring Junctions

Mechanical cell-junction complexes (adhering junctions/desmosomes) that fasten adjacent animal cells to one another and connect to the cytoskeleton, providing structural tensile strength.

Gap Junctions

Intercellular communication channels between neighboring animal cells that allow the direct diffusion of small ions and signaling molecules directly between cytosols.

Chloroplast

The double-membraned photosynthetic organelle in plants and algae that contains chlorophyll-bearing thylakoid membranes arranged into stacks (grana) surrounded by stroma fluid.

Fimbriae

Hair-like bacterial surface appendages that allow cells to adhere to biological tissues, external surfaces, and each other to facilitate the formation of biofilms.

Mycoplasmas

A group of bacteria naturally devoid of a peptidoglycan cell wall whose fragile plasma membranes are stabilized against osmotic lysis by incorporated sterols.