Glossary: Key Terms in Cell Cycle, Mitosis, and Meiosis

Term	Definition
Cell cycle	Ordered sequence of events (G1-S-G2-M) by which a cell grows, replicates DNA, and divides
Interphase	G1 + S + G2 phases; the "working" phase (~95% of cycle); NOT a resting phase
G1 phase	First gap phase; active growth (RNA + protein synthesis); DNA = 2C
S phase	Synthesis phase; DNA replication (2C → 4C); chromosome number unchanged
G2 phase	Second gap phase; tubulin synthesis; preparation for M phase; DNA = 4C
G0 phase	Quiescent (resting) state; cell has exited cycle; examples: neurons, RBCs
Mitosis	Equational nuclear division; produces 2 identical diploid cells; stages: PMAT
Karyokinesis	Nuclear division (PMAT stages) — separate from cytokinesis
Cytokinesis	Cytoplasmic division after karyokinesis; cleavage furrow (animal) or cell plate (plant)
Meiosis	Two successive divisions (I + II) producing 4 haploid cells from 1 diploid; for gamete formation
Meiosis I	Reductional division; homologous chromosomes separate; chromosome number halved
Meiosis II	Equational division (like mitosis); sister chromatids separate
Prophase I	Longest/most complex stage of meiosis; 5 substages (LZPDD)
Leptotene	First substage of prophase I; chromosomes begin condensation (thin threads)
Zygotene	Second substage; synapsis occurs; synaptonemal complex forms; bivalents formed
Pachytene	Third substage; crossing over occurs between non-sister chromatids; recombination nodules
Diplotene	Fourth substage; synaptonemal complex dissolves; chiasmata become visible
Diakinesis	Fifth substage; terminalization of chiasmata; nuclear envelope breaks down
Synapsis	Pairing of homologous chromosomes; occurs in zygotene
Bivalent / Tetrad	Two synapsed homologous chromosomes; each with 2 chromatids = 4 chromatids total
Crossing over	Exchange of segments between non-sister chromatids of homologous chromosomes; in pachytene
Chiasma (pl. chiasmata)	X-shaped cytological structure representing the site of crossing over; visible in diplotene
Synaptonemal complex	Proteinaceous zipper structure holding synapsed homologues together (zygotene to diplotene)
Terminalization	Movement of chiasmata towards chromosome ends during diakinesis
Recombination nodules	Electron-dense protein complexes on synaptonemal complex where crossing over occurs (pachytene)
Kinetochore	Protein complex on centromere; attachment site for spindle microtubules
Metaphase plate	Imaginary equatorial plane where chromosomes align during metaphase
Cleavage furrow	Actin-myosin ring in animal cells; constricts centripetally for cytokinesis
Cell plate	New cell wall formed centrifugally from Golgi vesicles; plant cell cytokinesis
Haploid (n)	One set of chromosomes (gamete level)
Diploid (2n)	Two sets of homologous chromosomes (somatic cell level)
C value	DNA content of a cell; 1C = haploid DNA content
Independent assortment	Random orientation of bivalents at metaphase I → diverse chromosome combinations