Cell Biology, Molecular Genetics, and Cell Division

Comprehensive vocabulary flashcards covering cell anatomy and organelles, the cytoskeleton, the cell cycle and mitosis, meiosis and chromosomal inheritance, DNA replication and PCR, and gene expression through transcription, translation, and mutations.

Cell Theory

The biological principle stating that all living organisms are composed of one or more cells, the cell is the basic structural and functional unit of life, all cells arise from pre-existing cells, cells contain genetic material ($\text{DNA}$) passed on during division, and energy flow (metabolism) occurs within cells.

Surface-to-Volume Ratio

A critical physical constraint on cell size wherein as cell radius increases, surface area increases by a factor of $n^2$ while volume increases by a factor of $n^3$, necessitating structural folding (such as microvilli and cristae) to maintain sufficient exchange surface.

Nucleoid

The non-membrane-enclosed region in a prokaryotic cell where the large circular bacterial chromosome and associated proteins are located.

Plasmid

A small circular extrachromosomal $\text{DNA}$ molecule found in prokaryotes that carries non-essential genes, such as antibiotic resistance, and can be transferred between cells via a tube-like pilus.

Nuclear Envelope

A double lipid bilayer enclosing the eukaryotic nucleus, whose inner bilayer faces the nucleoplasm and chromosomes, while the outer bilayer is continuous with the endoplasmic reticulum and perforated by nuclear pores.

Nucleolus

A dense region within the eukaryotic nucleus dedicated to the synthesis of ribosomal $\text{RNA}$ ($\text{rRNA}$) and the assembly of ribosomal subunits.

Ribosomes

Macromolecular complexes composed of $\text{rRNA}$ and proteins (divided into large and small subunits) that translate messenger $\text{RNA}$ sequences into polypeptide chains; exists as free ribosomes in the cytosol or bound ribosomes on the rough endoplasmic reticulum.

Endoplasmic Reticulum (ER)

A network of membranous cisternae continuous with the nuclear envelope, divided into the rough $\text{ER}$ (studded with ribosomes; synthesizes secretory and membrane proteins) and smooth $\text{ER}$ (lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies poisons, and stores calcium).

Golgi Apparatus

An endomembrane organelle consisting of flattened membranous sacs called cisternae that receives transport vesicles containing lipids and proteins at its cis face, modifies and sorts them, and packages them into vesicles exiting from its trans face.

Lysosome

A membrane-bound sac containing hydrolytic enzymes maintained at an acidic $\text{pH} \thickapprox 5$ via membrane proton pumps; functions in the degradation of macromolecules acquired via phagocytosis and the recycling of damaged organelles through autophagy.

Cristae

The extensive folds of the inner mitochondrial membrane that provide a large surface area for the enzymes and protein complexes responsible for synthesizing $\text{ATP}$ during cellular respiration.

Thylakoid

Flattened, interconnected internal membranous compartments within chloroplasts containing chlorophyll, a photosynthetic pigment that absorbs red and blue light to convert photon energy into chemical energy.

Endosymbiotic Theory

The evolutionary theory proposed by Dr. Lynn Margulis stating that mitochondria and chloroplasts originated as prokaryotic endosymbionts engulfed by an ancestral eukaryote, supported by their double membranes, independent circular $\text{DNA}$, and autonomous ribosomes.

Peroxisome

A single-membrane metabolic organelle that produces hydrogen peroxide ($\text{H}_2\text{O}_2$) while breaking down molecules like fatty acids and contains enzymes to degrade this toxic reactive oxygen byproduct into water.

Cytoskeleton

A dynamic cytoplasmic network of protein fibers comprising microtubules ($25\text{ nm}$), microfilaments ($7\text{ nm}$), and intermediate filaments that provides structural shape, mechanical support, and routes for internal transport.

Microtubules

Hollow cylinders measuring approximately $25\text{ nm}$ in diameter assembled from $\alpha$- and $\beta$-tubulin dimers; they exhibit dynamic instability, radiate from the centrosome, construct the mitotic spindle, and form the structural core of eukaryotic cilia and flagella.

Dynamic Instability

The rapid switching of individual microtubules between phases of polymerisation (growth) and depolymerisation (shrinkage), allowing rapid cytoskeletal remodelling according to cellular needs.

Centrosome

The primary microtubule-organizing center near the nucleus in animal cells, containing a pair of centrioles, each constructed from nine triplets of microtubules arranged in a ring.

Microfilaments

Solid cytoskeletal rods measuring about $7\text{ nm}$ in diameter composed of a twisted double chain of actin monomers; they bear tension, establish the cellular cortex beneath the plasma membrane, drive cytoplasmic streaming, and mediate contraction in concert with myosin.

Intermediate Filaments

Cable-like cytoskeletal elements constructed from diverse fibrous protein subunits (such as keratins and laminins) that are more permanent than microtubules and actin, providing high mechanical strength to bear tension and stabilize the nucleus.

Motor Proteins

$\text{ATP}$-dependent transport proteins including kinesin and dynein (which travel along microtubules toward the plus and minus ends, respectively) and myosin (which walks along actin microfilaments) to transport vesicles and organelles.

Extracellular Matrix (ECM)

A complex meshwork surrounding animal cells consisting of collagen fibers, elastin, and proteoglycan complexes attached to cell-surface integrin receptors via fibronectin, mediating signaling, structural anchoring, and tissue mechanics.

Cell Wall

A rigid extracellular layer that prevents cell lysis under turgor pressure; constructed primarily of peptidoglycan in bacteria, cellulose in plants, and chitin in fungi.

Archaea

A domain of prokaryotic microorganisms characterized by cell walls lacking peptidoglycan, plasma membranes composed mostly of ether lipids, and unique metabolic pathways distinct from bacteria and eukaryotes.

Capsid

The protective protein shell enclosing the viral genome ($\text{DNA}$ or $\text{RNA}$), constructed from repeating protein subunits known as capsomeres.

Binary Fission

The asexual reproductive division of prokaryotic cells in which the circular chromosome attaches to the inner cell membrane, replicates bidirectionally, and separates as the cell elongates and synthesizes cross-wall material at its midpoint.

Nucleosome

The fundamental repeating structural unit of eukaryotic chromatin, composed of approximately $1.7$ loops of $\text{DNA}$ wound around an octamer core of basic histone proteins.

Sister Chromatids

Two identical copies of a replicated chromosome formed during the S phase of interphase, held together at their centromere until anaphase.

Homologous Chromosomes

A pair of matched chromosomes in a diploid organism that are identical in size and shape and carry the same gene loci, with one copy inherited from the maternal parent and one from the paternal parent.

G0 Phase

A non-dividing, quiescent state entered from the $G_1$ phase by cells that have exited the cell cycle, either temporarily (such as certain muscle cells) or permanently (such as terminally differentiated neurons).

Cyclins

A family of regulatory proteins whose intracellular concentrations fluctuate cyclically throughout the cell cycle, binding to and activating specific cyclin-dependent kinases ($\text{CDKs}$).

Cell Cycle Checkpoints

Regulatory stop points monitored by the cell control system—specifically the $G_1$ checkpoint ($\text{DNA}$ damage), $G_2$ checkpoint ($\text{DNA}$ replication completion), and M checkpoint (spindle assembly/attachment)—that halt progression until prerequisite cellular conditions are fulfilled.

p53 Protein

A tumor suppressor protein that is phosphorylated upon $\text{DNA}$ damage to prevent its nuclear export and degradation, functioning as a transcription factor that activates genes inhibiting the cell cycle for repair; mutated in approximately $50\%$ of all cancers.

Anchorage Dependence

The requirement that animal cells must be physically adhered to a solid substratum or extracellular matrix in order to divide.

Density-Dependent Inhibition

The phenomenon wherein crowded normal cells stop dividing once they form a complete monolayer due to the localized depletion of available growth factors.

Metastasis

The spread of malignant tumor cells from their primary site of origin to distant body locations via the circulatory or lymphatic systems.

Semi-Conservative Replication

The mechanism of $\text{DNA}$ replication in which the parental double helix unzips and each parental strand serves as a template, producing daughter molecules containing one original strand and one newly synthesized strand.

Replication Fork

The Y-shaped region resulting from the unwinding of the parental $\text{DNA}$ duplex at each end of a replication bubble, driven by helicase, stabilized by single-strand binding proteins ($\text{SSBP}$), and relieved of torsional strain by topoisomerase.

Leading Strand

The continuous daughter $\text{DNA}$ strand synthesized in the direction of replication fork unwinding by adding nucleotides continuously to the free $3'\text{-OH}$ end in a $5' \rightarrow 3'$ direction.

Lagging Strand

The discontinuously synthesized daughter $\text{DNA}$ strand produced in short fragments away from the replication fork, each requiring an individual $\text{RNA}$ primer synthesized by primase before ligation.

Proofreading

The enzymatic error-correcting process of $\text{DNA}$ polymerase, which uses $3' \rightarrow 5'$ exonuclease activity to excise mispaired nucleotides, lowering the replication error rate from $1\text{ in }10^5$ to $1\text{ in }10^{10}$.

Telomeres

Repetitive, non-coding tandem sequences ($\text{TTAGGG}$ in humans) capping the ends of eukaryotic linear chromosomes that prevent sequence loss in essential genes caused by the end-replication problem of lagging strand synthesis.

Telomerase

A reverse transcriptase enzyme that replenishes telomeric repeat sequences at the ends of eukaryotic chromosomes, maintaining length and allowing cells (such as cancer or stem cells) to divide indefinitely.

Polymerase Chain Reaction (PCR)

An in vitro technique for exponential amplification of a specific target $\text{DNA}$ region utilizing cycles of high-heat denaturation ($\thickapprox 94^\circ\text{C}$-$95^\circ\text{C}$), oligonucleotide primer annealing ($\thickapprox 45^\circ\text{C}$-$65^\circ\text{C}$), and enzymatic extension ($\thickapprox 72^\circ\text{C}$) with Taq polymerase.

Mitotic Spindle

An apparatus comprising centrosomes, spindle microtubules (kinetochore, polar, and astral), and asters that governs the mechanical movement and segregation of sister chromatids during mitosis.

Kinetochore

A specialized multi-protein complex located at the centromeric region of each sister chromatid that serves as the attachment site for spindle microtubules.

Cytokinesis

The physical division of the cytoplasm following mitosis, occurring via a contractile ring of actin microfilaments forming a cleavage furrow in animal cells, or by Golgi-derived vesicle fusion creating a cell plate in plant cells.

Apoptosis

Programmed, non-inflammatory cellular suicide initiated by intrinsic or extrinsic triggers that activate proteolytic caspases, causing $\text{DNA}$ fragmentation, chromatin condensation, membrane blebbing, and subsequent phagocytic clearance.

Necrosis

Uncontrolled cell death resulting from acute injury, trauma, or pathology, characterized by cellular swelling, membrane rupture, and the release of intracellular contents into the extracellular matrix, provoking an inflammatory response.

Karyotype

An ordered visual display of the complete set of condensed chromosome pairs from an individual eukaryotic cell, arranged systematically by size and structural features.

Autosomes

The $22$ pairs of homologous chromosomes in the human karyotype that do not determine the biological sex of the individual.

Zygote

The diploid cell resulting from the union (fertilization) of two haploid gametes (an ovum and a spermatozoon), possessing two complete sets of chromosomes ($2n = 46$ in humans).

Meiosis I

The initial reductional division of meiosis wherein duplicated homologous chromosomes pair, undergo crossing over, and segregate into two haploid daughter cells that retain replicated sister chromatids.

Synapsis

The intimate gene-for-gene pairing of homologous chromosomes along their lengths during prophase I of meiosis, forming a four-chromatid structure termed a bivalent or tetrad.

Crossing Over

The reciprocal exchange of genetic material between non-sister chromatids of a homologous pair during prophase I, generating recombinant chromatids with novel allele combinations.

Chiasma

The X-shaped physical connection point between non-sister chromatids in a homologous bivalent where crossing over and reciprocal genetic exchange took place.

Independent Assortment

The random orientation and segregation of maternal and paternal homologous chromosome pairs at the metaphase I plate, generating $2^n$ possible gametic chromosome combinations ($> 8.4\times 10^6$ in humans where $n = 23$).

Nondisjunction

An error during meiosis wherein homologous chromosomes fail to separate during meiosis I, or sister chromatids fail to separate during meiosis II, yielding gametes with $n + 1$ or $n - 1$ chromosomes.

Aneuploidy

A condition in which a zygote has an abnormal chromosome count resulting from the fertilization of an aberrant gamete produced by nondisjunction (e.g., monosomy or trisomy).

Down Syndrome

An autosomal aneuploid condition characterized by trisomy of chromosome 21, occurring in roughly $1\text{ in }700$ births in North America, with incidence correlating positively with maternal age.

Turner Syndrome

The only known viable human monosomy ($45,\text{X}$ or $\text{X}0$), producing sterile females typically characterized by short stature, webbed neck, shield chest, and rudimentary ovaries.

Klinefelter Syndrome

A sex chromosome aneuploidy in biological males resulting from an extra X chromosome ($47,\text{XXY}$).

Genome

The entirety of an organism's hereditary genetic information encoded within its complete set of $\text{DNA}$.

Epigenetics

The study of heritable changes in gene expression that occur without altering the underlying nucleotide sequence, mediated through chemical modifications such as $\text{DNA}$ methylation (typically repressing transcription) and histone acetylation (facilitating open, active transcription).

Transcription Initiation Complex

The complete molecular assembly comprising regulatory transcription factors bound to the promoter TATA box, cofactors, and $\text{RNA}$ polymerase II aligned at the start point to begin RNA synthesis.

RNA Polymerase

The enzyme that opens the double-stranded $\text{DNA}$ duplex and catalyzes the polymerization of an $\text{RNA}$ transcript from the template $\text{DNA}$ strand in the $5' \rightarrow 3'$ direction, substituting uracil ($\text{U}$) for thymine ($\text{T}$).

Introns and Exons

Segments of pre-messenger $\text{RNA}$; introns are non-coding intervening sequences removed during $\text{RNA}$ splicing, whereas exons are expressed coding sequences spliced together to yield mature $\text{mRNA}$.

Alternative RNA Splicing

A regulated post-transcriptional mechanism in which different combinations of exons from a single pre-$\text{mRNA}$ primary transcript are included or excluded, yielding multiple protein isoforms such as the low-affinity insulin receptor in liver cells and the high-affinity receptor in skeletal muscle.

Transfer RNA (tRNA)

An adaptor $\text{RNA}$ molecule with an amino acid covalently attached to its $3'\text{-CCA}$ terminus and a specific nucleotide anticodon loop that base-pairs with a complementary mRNA codon.

Codon

A three-nucleotide sequence on messenger $\text{RNA}$ that specifies a single amino acid or signals termination ($\text{UAA}$, $\text{UAG}$, $\text{UGA}$) during translation according to the universal genetic code.

Point Mutation

A chemical alteration involving a change, insertion, or deletion of a single nucleotide pair within a gene.

Silent Mutation

A nucleotide-pair substitution that alters a codon but, because of redundancy in the genetic code, results in the translation of the same amino acid without altering protein function.

Missense Mutation

A base-pair substitution that alters a codon such that it codes for a different amino acid, exemplified by the single nucleotide change yielding sickle-cell hemoglobin.

Nonsense Mutation

A point mutation that converts an amino-acid-specifying codon into a premature stop codon, leading to truncated and almost always non-functional proteins.

Frameshift Mutation

A genetic mutation caused by the insertion or deletion of a number of nucleotide pairs not divisible by three, displacing the triplet reading frame downstream and altering all subsequent amino acids or causing immediate premature termination.

Chromosomal Inversion

A structural chromosomal alteration occurring when a chromosome segment breaks off, rotates $180^\circ$, and reattaches within the same chromosome.

Chromosomal Translocation

A large-scale chromosomal mutation in which a broken fragment from one chromosome attaches to a non-homologous chromosome.