General Biology: Cell Structure, Metabolism, DNA Replication, and Cell Division

Comprehensive practice flashcards defining core biological terms from lecture units covering cell biology, organelle function, the endomembrane system, the cytoskeleton, enzymatic regulation and bioenergetics, DNA structure and replication, mitosis, cell cycle checkpoints, and meiosis.

Cell Theory

The foundational biological principle stating that all organisms are composed of one or more cells, the cell is the fundamental unit of life, and all cells arise from preexisting cells.

Prokaryotic Cell

A cell lacking a membrane-bound nucleus and membrane-bound organelles (domains Bacteria and Archaea), whose genetic material is located in an open nucleoid region and which is on average about $10$ times smaller in diameter and $1000$ times smaller in volume than eukaryotic cells.

Eukaryotic Cell

A complex cell (domain Eukarya) possessing a membrane-bound nucleus enclosing chromosomes and compartmentalized cytoplasmic organelles.

Nucleolus

A dense subregion of the eukaryotic nucleus dedicated to the manufacturing and processing of RNA molecules and the assembly of ribosome subunits.

Ribosome

A non-membranous macromolecular machine composed of RNA and protein complexes that synthesizes proteins in all cells.

Rough Endoplasmic Reticulum (Rough ER)

A network of branching membranous sacs studded with ribosomes on its outer surface that functions as a site of protein synthesis and processing.

Smooth Endoplasmic Reticulum (Smooth ER)

A network of branching membranous sacs lacking ribosomes that contains enzymes specialized for synthesizing and processing lipids and phospholipids.

Golgi Apparatus

An organelle consisting of stacks of flattened membranous cisternae with distinct polarity (a cis face receiving goods from the rough ER and a trans face shipping them onward) that processes and sorts proteins, lipids, and carbohydrates.

Lysosome

A single-membrane recycling organelle found in animal cells containing proton pumps and acid hydrolases that hydrolyze and digest macromolecules.

Peroxisome

A globular organelle originating from empty vesicles of the ER that contains enzymes, such as catalase, which catalyze oxidation reactions of fatty acids, ethanol, and hydrogen peroxide.

Mitochondrion

A double-membraned organelle whose inner membrane forms sac-like cristae surrounding the matrix; contains enzymes that harvest energy from molecules to generate $ATP$.

Chloroplast

A double-membraned photosynthetic organelle found in plants and algae that contains chlorophyll pigments and membrane-bound thylakoids arranged into grana within the stroma to convert light energy into sugars.

Exocytosis

A transport process in which intracellular materials packaged inside transport vesicles fuse with the plasma membrane to secrete cargo into the extracellular environment.

Endocytosis

A bulk transport process where the plasma membrane folds inward or invaginates to form an intracellular vesicle bringing extracellular substances into the cell.

Autophagy

A lysosomal pathway where a damaged organelle is enclosed within a double membrane to form an autophagosome, which then fuses with a lysosome for digestion and recycling into small cytosol-released molecules.

Actin Filaments (Microfilaments)

Cytoskeletal elements measuring approximately $7\,\text{nm}$ in diameter formed from two coiled strands of actin monomers that resist tension, facilitate cell crawling and muscle contraction, and divide animal cells via cytokinesis.

Myosin

A motor protein that hydrolyzes $ATP$ to move along actin filaments, generating kinetic energy for muscle contraction, animal cytokinesis, and cytoplasmic streaming in plant cells.

Intermediate Filaments

Tough, fibrous cytoskeletal cables measuring approximately $10\,\text{nm}$ in diameter (composed of proteins such as keratins or lamins) that act as an internal scaffold to maintain cell shape by resisting tension and anchoring organelles.

Microtubules

Hollow cytoskeletal cylinders measuring $25\,\text{nm}$ in diameter formed from $\alpha$- and $\beta$-tubulin dimers that resist compression, move chromosomes during cell division, provide transport tracks, and compose cilia and flagella.

Kinesin

A motor protein composed of head, stalk, and tail regions that uses $ATP$ hydrolysis at every step to walk transport vesicles along microtubule tracks toward the plus ($$+$) end.

Activation Energy ($E_a$)

The kinetic energy required to strain chemical bonds in reactant molecules to achieve the unstable transition state and initiate a chemical reaction.

Transition State

The high-free-energy intermediate state in a chemical reaction during which old chemical bonds are being broken and new bonds are being formed.

Induced Fit

The conformational change that occurs in an enzyme when substrates bind to its active site, reorienting the substrates and binding them more tightly.

Competitive Inhibitor

A regulatory molecule that mimics the substrate and binds directly to the enzyme's active site, competing with the substrate for access.

Noncompetitive Inhibitor

An inhibitor that binds to a regulatory site distinct from the active site, altering the enzyme's three-dimensional shape and slowing its catalytic rate.

Monoamine Oxidase Inhibitors (MAOIs)

Antidepressant drugs that inhibit monoamine oxidase, preventing the breakdown and removal of neurotransmitters such as norepinephrine, serotonin, and dopamine from brain circuits.

$\beta$-Lactam Antibiotics

A class of antibiotics, including ampicillin produced by organisms such as Penicillium chrysogenum, that work by inhibiting cell wall biosynthesis in bacteria.

Chargaff's Rule

The principle stating that DNA across all organisms has a $1:1$ ratio of pyrimidines to purines, specifically where the percentage of adenine equals thymine ($\%A = \%T$) and guanine equals cytosine ($\%G = \%C$).

Phosphodiester Linkage

The covalent bond joining the $5'$ phosphate group of one nucleotide to the $3'$ hydroxyl group of the adjacent sugar, building the primary sugar-phosphate backbone of a nucleic acid chain.

Semiconservative Replication

The mechanism of DNA synthesis in which each parental DNA strand acts as a template for a new strand, yielding two daughter double helices each composed of one conserved parental strand and one newly formed strand.

DNA Helicase

An enzyme that breaks hydrogen bonds between nitrogenous base pairs to separate the two strands of the DNA double helix at the replication fork.

Single-Strand DNA-Binding Proteins (SSBPs)

Proteins that attach to separated single DNA strands during replication to prevent them from snapping back together into a double helix.

Topoisomerase

An enzyme that cuts and rejoins DNA strands ahead of the advancing replication fork to relieve excessive twisting forces and torsional strain caused by unwinding.

RNA Primase

An enzyme that synthesizes a short complementary sequence of RNA primer, providing the free $3'\text{-OH}$ group essential for DNA polymerase to initiate elongation.

Okazaki Fragments

Short discontinuous segments of newly synthesized DNA produced on the lagging strand that polymerize in the $5' \rightarrow 3'$ direction away from the replication fork.

DNA Ligase

An enzyme that catalyzes the covalent joining of Okazaki fragments along the lagging strand by sealing nicks in the sugar-phosphate backbone.

Cell Proliferation

The process by which cells increase in number as a consequence of cell growth, genetic material duplication, and regulated cell division governed by the cell cycle.

Apoptosis

Programmed, controlled cell death that occurs as a normal part of an organism's development, growth, and tissue maintenance.

Chromatin

The eukaryotic chromosomal complex consisting of linear DNA molecules wrapped around histone proteins.

Sister Chromatids

Two identical double-stranded DNA copies of a replicated chromosome that remain physically attached to each other at the centromere until anaphase.

Centromere

The specialized, constricted chromosomal region where sister chromatids are most closely held together and where kinetochore complexes assemble.

Kinetochore

A protein structure assembled on the centromere of each chromatid that serves as the attachment site for spindle microtubules.

Mitotic Spindle

A microtubule-based apparatus originating from centrosomes that coordinates the alignment and segregation of chromosomes during mitosis.

Cleavage Furrow

A pinch or indentation that forms on the plasma membrane of dividing animal cells during cytokinesis, driven by a contracting ring of actin and myosin.

Cell Plate

A flattened, membrane-bound structure assembled from Golgi-derived vesicles along the equatorial plane of a dividing plant cell that eventually fuses to become the new cell wall.

$G_1$ Checkpoint

The cell cycle control point passed if cell size is adequate, nutrients are sufficient, social signals are present, and DNA is undamaged; mature non-dividing cells exit here to enter $G_0$.

$G_0$ Phase

A quiescent, non-dividing exit state of the cell cycle occupied by mature, differentiated cell types such as neurons.

Homologous Chromosomes

A pair of chromosomes (one maternal, one paternal) identical in length, centromere location, and gene loci that can carry differing alleles for given traits.

Locus

The specific physical location or address of a gene along the length of a chromosome.

Synapsis

The physical pairing and alignment of homologous duplicated chromosomes along their lengths during prophase I of meiosis, forming a tetrad.

Crossing Over

The reciprocal exchange of corresponding chromosomal segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis.

Chiasma

The visible X-shaped structural site where non-sister chromatids of homologous chromosomes crossover and undergo genetic exchange.

Independent Assortment

The random, independent orientation of maternal and paternal homologous chromosome pairs at the metaphase I plate, generating distinct combinations of alleles in gametes.

Nondisjunction

An error during meiosis where homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II) fail to disjoin and separate, leading to gametes with aneuploidy.

Down Syndrome (Trisomy 21)

An aneuploid human condition caused by meiotic nondisjunction, characterized by the presence of three copies of chromosome 21 in the karyotype.