Ch.10 Cell division, cell cycle and cancer

Binary fission

A process of asexual reproduction in which a bacterial cell divides into two nearly equal daughter cells.

Bacterial chromosome organization

Typically consists of a single, circular double-stranded DNA molecule localized in the nucleoid.

Septation

The process of forming a dividing cross-wall or partition that pinches one bacterial cell into two.

FtsZ

A bacterial cytoskeletal protein homologous to tubulin that forms a contractile ring at the division site during septation.

Eukaryotic chromosomes

Linear structures composed of DNA complexed with packaging proteins located within the nucleus.

Chromatin

The nuclear complex composed of DNA and associated packaging proteins such as histones.

Heterochromatin

Densely compacted, transcriptionally inactive regions of chromatin that are largely inaccessible for gene expression.

Euchromatin

Loosely packaged, transcriptionally active regions of chromatin that are readily accessible for gene expression.

Nucleosome

The fundamental repeating unit of chromatin, consisting of DNA coiled around an octamer of histone proteins.

Histones

Basic, positively charged packaging proteins around which negatively charged DNA coils to form nucleosomes.

Solenoid (30-nm fiber)

A higher-order level of chromatin compaction produced by the tight coiling and helical stacking of nucleosomes.

Karyotype

A standardized visual display of an individual's complete chromosome complement arranged in matching pairs by size and shape.

Diploid ($2n$)

Having two complete sets of homologous chromosomes, with one set inherited from each parent.

Haploid ($n$)

Having only a single complete set of unpaired chromosomes.

Gamete

A specialized haploid reproductive cell (such as sperm or egg) that fuses during fertilization.

Homologous chromosomes (homologues)

A pair of matching chromosomes containing genes for the same traits at corresponding loci, one derived from each biological parent.

DNA replication

The biological process of producing two identical replicas of DNA from one original DNA molecule prior to cell division.

Sister chromatids

Two identical copies of a replicated chromosome joined together at the centromere by cohesin proteins.

Centromere

The constricted chromosomal region composed of repeated DNA sequences where kinetochore proteins assemble and sister chromatids link.

Kinetochore

A specialized multiprotein complex assembled onto the centromere to which spindle microtubules attach during mitosis.

Cohesin

A multi-subunit protein complex that physically tethers sister chromatids together from replication until anaphase.

Condensin

A ring-shaped protein complex that drives the structural condensation and loop organization of chromosomes during cell division.

Interphase

The extended preparatory phase of the cell cycle comprising $G_1$, $S$, and $G_2$ phases.

$G_1$ phase

The initial cell-cycle gap phase focused on cytoplasmic growth, metabolic activity, and biosynthesis.

$S$ phase

The synthesis phase of interphase during which genomic DNA and centrosomes are duplicated.

$G_2$ phase

The second gap phase during which the cell continues growing, synthesizes mitotic spindle machinery, and prepares for division.

$G_0$ phase

A non-dividing, quiescent resting state in which a cell withdraws from the active cycle prior to entering $S$ phase.

$M$ phase (Mitosis)

The division phase of the cell cycle where replicated chromosomes are partitioned into two separate nuclei.

Cytokinesis (C)

The division of the cytoplasm and associated organelles following karyokinesis, yielding two separate daughter cells.

Chromatid count in $G_2$ for a $2n = 8$ cell

$16$ chromatids, because all $8$ chromosomes have duplicated into pairs of sister chromatids during $S$ phase.

Chromosome count per daughter cell after mitosis ($2n = 8$)

$8$ chromosomes, preserving the original diploid chromosome number.

Five stages of mitosis in sequential order

Prophase, prometaphase, metaphase, anaphase, and telophase.

Prophase

The initial mitotic stage characterized by chromosome condensation, nucleolus disappearance, and mitotic spindle assembly.

Prometaphase

The stage of mitosis where the nuclear envelope disintegrates and spindle microtubules capture chromosomes at their kinetochores.

Metaphase

The mitotic stage where paired sister chromatids align along the central equatorial plane of the cell.

Anaphase

The mitotic phase where cohesin proteins are cleaved, allowing sister chromatids to separate and migrate toward opposite poles.

Telophase

The final stage of mitosis wherein new nuclear envelopes reconstruct around separated chromosomes as they decondense.

Centrosome

A cytosolic microtubule-organizing center (MTOC) that establishes and organizes the bipolar mitotic spindle during division.

Centrioles

Cylindrical microtubule triplet structures located within animal centrosomes that assist in organizing spindle fibers.

Asters

Radial, star-like arrays of short microtubules extending outward from each centrosome toward the plasma membrane.

Kinetochore microtubules

Spindle fibers that attach directly to chromosome kinetochores to align and segregate sister chromatids.

Polar microtubules

Non-kinetochore spindle fibers that project from opposite poles and interdigitate to push cell poles apart.

Cleavage furrow

A shallow groove formed in animal plasma membranes during cytokinesis by the constriction of an underlying actin-myosin ring.

Cell plate

A partition structure formed in dividing plant cells via the fusion of Golgi-derived vesicles along the division plane.

Cell-cycle checkpoints

Molecular surveillance mechanisms that verify whether previous biochemical steps are completed before allowing cycle progression.

$G_1/S$ checkpoint (Restriction point)

The primary control transition assessing cell size, nutrients, growth signals, and DNA integrity before initiating replication.

$G_2/M$ checkpoint

The regulatory transition that verifies complete and accurate DNA replication before committing the cell to mitosis.

Spindle checkpoint (Late metaphase checkpoint)

A control mechanism that ensures all kinetochores are properly attached to spindle microtubules prior to anaphase onset.

Cyclin-dependent kinases (Cdks)

Enzymes that regulate progression through cell-cycle stages by phosphorylating target proteins when bound to cyclins.

Cyclins

Regulatory proteins whose cyclical concentration changes govern the activation and specificity of partner kinases.

Maturation-promoting factor (MPF)

A protein kinase heterodimer consisting of cyclin B and $Cdc2$ (CDK1) that triggers entry into mitosis.

Anaphase-promoting complex / Cyclosome (APC/C)

A multiprotein ubiquitin ligase that degrades securin and mitotic cyclins, triggering chromatid separation and mitotic exit.

Growth factors (e.g., PDGF)

Extracellular signaling proteins that bind cell-surface receptors to stimulate cellular proliferation and survival.

Receptor tyrosine kinase (RTK)

A transmembrane cell-surface receptor that dimerizes and autophosphorylates tyrosine residues upon ligand binding.

MAP kinase cascade

A sequential intracellular protein phosphorylation relay activated downstream of Ras that stimulates cell proliferation.

Proto-oncogenes

Normal cellular genes encoding proteins that positively regulate cell division and survival under strict physiological control.

Oncogenes

Mutated, gain-of-function forms of cellular genes that promote unregulated cell proliferation and neoplastic growth.

Tumor-suppressor genes

Genes encoding proteins that restrain cell division, facilitate DNA repair, or trigger apoptosis when damage is detected.

p53 protein

A key tumor-suppressor transcription factor that halts the cell cycle or triggers apoptosis in response to severe DNA damage.

Retinoblastoma susceptibility gene (Rb)

A tumor-suppressor gene encoding a protein that sequesters E2F transcription factors to prevent premature $G_1/S$ transition.

Ras protein

A small monomeric GTPase switch that transmits mitogenic signals from receptor tyrosine kinases to downstream kinase cascades.

Src kinase

A cytoplasmic non-receptor tyrosine kinase that modulates signaling pathways involved in cell adhesion, growth, and survival.