How Cells Divide: Bacterial Cell Division, the Eukaryotic Cell Cycle, Mitosis, and Cell Cycle Regulation

Comprehensive practice flashcards detailing the mechanisms of cell division, including bacterial binary fission, eukaryotic chromosome organization, the stages of interphase and mitosis, cytokinesis, cell cycle checkpoints, Cdks, and cancer genetics.

Binary Fission

The asexual, clonal process of cell division in bacteria where a single parent cell divides into two identical cells.

Nucleoid Region

The cellular area in prokaryotes where the single, circular bacterial chromosome is tightly packed, in the absence of a membrane-bound nucleus.

Septum (Bacterial)

The dividing wall formed during binary fission that partitions an elongating bacterial cell into two distinct daughter cells.

FtsZ Protein

A bacterial protein with a structure similar to eukaryotic tubulin that assembles into a ring to facilitate septation and cell division.

Origin of Replication

The specific region on a circular bacterial chromosome where DNA replication initiates during binary fission.

Chromatin

The complex of DNA and protein that forms eukaryotic chromosomes.

Heterochromatin

A structural classification of chromatin that is tightly compacted and not active (genetically silent).

Euchromatin

A structural classification of chromatin that is uncoiled and genetically active (expressed).

Nucleosome

A basic unit of chromatin organization consisting of a DNA duplex wound around a core of histone proteins.

Solenoid

A higher-order level of chromatin packaging composed of coils of nucleosomes.

Karyotype

The particular physical array of chromosomes present in an individual organism.

Diploid ($2n$)

Having two complete sets of chromosomes, which equates to $46$ total chromosomes in human somatic cells.

Haploid ($n$)

Having one single complete set of chromosomes.

Homologous Chromosomes

A pair of nearly identical chromosomes (each called a homologue) found in a diploid cell that correspond to the same chromosome number.

Sister Chromatids

Two identical DNA molecules joined at a centromere that are produced when a eukaryotic chromosome undergoes replication.

Centromere

A point of constriction on a chromosome where sister chromatids are held together by cohesin proteins.

Kinetochore

A specialized protein structure located at the centromere that functions as the attachment site for spindle microtubules.

Cohesin

A protein complex that physically binds replicated sister chromatids together at the centromere until anaphase.

Interphase

The phase of the cell cycle preparatory to mitosis during which the cell grows, replicates its DNA, and performs metabolic functions while the nucleus remains visible.

$G_1$ Phase (Gap 1)

The primary growth phase and longest phase of the eukaryotic cell cycle, responsible for most variation in cell cycle duration among cell types.

$G_0$ Phase (Resting Phase)

A non-dividing resting state entered from $G_1$ where cells may pause division temporarily or permanently, as seen in muscle and nerve cells.

$S$ Phase (Synthesis)

The interphase stage where DNA replicates to produce two identical sister chromatids attached at the centromere.

$G_2$ Phase (Gap 2)

The second growth phase of interphase where chromosomes coil more tightly using motor proteins, centrioles replicate, tubulin is synthesized, and organelles replicate.

Asters

A radial array of microtubules extending from centrioles toward the plasma membrane in animal cells, absent in plant cells.

Mitosis

The process of eukaryotic nuclear division consisting of five sequential phases: prophase, prometaphase, metaphase, anaphase, and telophase.

Prophase

The first phase of mitosis in which chromosomes condense and become visible as sister chromatids, the mitotic spindle begins forming, the nuclear envelope breaks down, and the Golgi and ER disperse.

Prometaphase

The stage of mitosis following nuclear envelope disassembly where chromosomes attach to microtubules at their kinetochores and begin moving toward the cell equator.

Metaphase

The mitotic stage where chromosomes are aligned along the cell equator under tension from microtubules attached to opposite spindle poles.

Metaphase Plate

The imaginary equatorial plane across the cell that represents the future axis of cell division upon which chromosomes align during metaphase.

Anaphase

The stage of mitosis initiated by the splitting of centromeres and degradation of cohesins, resulting in sister chromatids being pulled to opposite poles.

Telophase

The final phase of mitosis where chromosomes cluster at opposite poles and decondense, the spindle apparatus disassembles, nuclear envelopes re-form, and nucleoli reappear.

Cytokinesis

The division of the cytoplasm following mitosis, resulting in the cleavage of the cell into two equal daughter cells.

Cleavage Furrow

The constriction formed in animal cells by actin filaments that pinches the cell into two daughter cells during cytokinesis.

Cell Plate

A membrane-bound partitioning structure formed in plant cytokinesis by the fusion of vesicles containing membrane components between daughter nuclei.

Checkpoints

Specific regulatory points in the cell cycle where progression is temporarily put on hold to assess accuracy and respond to internal and external signals.

$G_1/S$ Checkpoint

The primary control point (restriction point) where a cell decides whether to commit to division based on cell size, nutrition, and growth factors, controlled by Cdk2/Cyclin E.

$G_2/M$ Checkpoint

The cell cycle control point where the cell commits to mitosis by verifying the integrity and completed replication of DNA, controlled by Cdk1/Cyclin B.

Spindle Checkpoint

The checkpoint during metaphase that ensures all chromosomes are properly attached to spindle microtubules at the metaphase plate before anaphase begins.

Cyclin-Dependent Kinases (Cdks)

Protein kinases that phosphorylate target proteins to regulate cell cycle progression; activated by binding to specific cyclins and modulated by phosphorylation.

Mitosis-Promoting Factor (MPF)

A Cdk-cyclin regulatory complex whose activation requires specific dephosphorylation and phosphorylation to drive the cell into mitosis.

Anaphase-Promoting Complex (APC/C)

A protein complex (cyclosome) that triggers anaphase by marking securin for destruction, which releases separase to cleave cohesin.

Securin

An inhibitory protein that prevents separase from destroying cohesin; its targeted destruction by APC/C permits the separation of sister chromatids.

Separase

An enzyme that cleaves cohesin proteins holding sister chromatids together once relieved of inhibition by securin.

Cancer

Unrestrained, uncontrolled cellular growth and proliferation caused by a failure of cell cycle control mechanisms.

Tumor-Suppressor Genes

Genes that normally function to prevent tumor formation and uncontrolled cell division; both alleles must lose function for a cancerous phenotype to develop.

Retinoblastoma Susceptibility Gene ($Rb$)

The first identified tumor-suppressor gene, whose protein product is found mutated in $40\%$ of all human cancers.

$p53$ Gene

A key tumor-suppressor gene operating at the $G_1$ checkpoint to monitor DNA integrity, which is absent or mutated in $50\%$ of human cancers.

Proto-Oncogenes

Normal cellular genes that stimulate division and can mutate into oncogenes; a gain-of-function mutation in only one copy is sufficient to promote uncontrolled division.

Oncogenes

Mutated forms of proto-oncogenes that cause a cell to become cancerous.

Ras Protein

A proto-oncogene signaling protein that is activated by mutations in $20\text{--}30\%$ of all human cancers.

Src Kinase

A proto-oncogene kinase that is activated by mutations in $2\text{--}5\%$ of all human cancers.