Cell Cycle and Cell Division

Story: Imagine school timetable.

G1 first gap: Cell grows, makes proteins organelles, checks if everything okay. DNA 2C (C = content). Like morning assembly growth.

S synthesis: DNA replication 2C to 4C doubles like photocopying whole book, centrosome centriole duplication 2 to 4 like making extra pullers for division. Most important checkpoint - if DNA not copied properly, cell stops.

G2 second gap: Growth again, makes proteins for spindle like preparing for exam final touch, checks DNA replication error.

M phase: Mitosis karyokinesis nuclear division + Cytokinesis cytoplasm division like exam + result distribution.

G0 quiescent: Some cells like neurons, muscle heart cells exit cycle and never divide like retired teacher never comes back. Liver can re-enter if needed like substitute teacher.

Checkpoints: G1/S - can I start copying? G2/M - is copying done? Spindle assembly - are all chromosomes attached? Like principal checks entry. Cyclins and Cdks are regulators like timetable incharge - Cyclin increases, activates Cdk, MPF Maturation Promoting Factor pushes cell to M.

Prophase longest phase: Chromatin condenses thick threads like wool becomes visible chromosomes with two sister chromatids joined at centromere. Centrosome moves to opposite poles forms asters like star with microtubules spindle starts forming like ropes. Late prophase nucleolus disappears like library closes, nuclear envelope breaks like wall breaks.

Metaphase: Chromosomes highly condensed best stage to see morphology count karyotype like you arrange books on shelf best to count. All chromosomes align at equatorial plate metaphase plate middle line. Spindle fibres from poles attach to kinetochore disc at centromere like ropes attach to handle. Each chromosome at equator with arms facing poles.

Anaphase shortest most dramatic: Centromere splits sister chromatids separate become daughter chromosomes pulled to opposite poles by spindle shortening like pulling ropes. Anaphasic movement V J L shapes depending on centromere position metacentric V sub-metacentric L etc. Like tug of war.

Telophase reverse of prophase: Chromosomes reach poles decondense thin, nuclear envelope reforms like wall rebuilt, nucleolus reappears, spindle disappears.

Cytokinesis: Animal furrow cleavage actin myosin ring contracts centripetal from outside to inside like tightening purse string. Plant cell plate phragmoplast Golgi vesicles with pectin fuse at centre centrifugal inside to outside like building wall from inside to outside because cell wall present. Result 2 identical 2n cells. Significance growth of body, repair wound, replacement old cells, no variation clones.

Meiosis I Prophase I longest most important 5 sub-stages like 5 episodes:

Leptotene: Thin threads wool-like condensation starts chromomeres beaded like necklace.

Zygotene: Synapsis pairing homologous chromosomes maternal red paternal blue come together like zipping. Synaptonemal complex ladder structure with lateral elements central element transverse filaments like zip. Bivalent = pair of homologous chromosomes 2 chromosomes, Tetrad = 4 chromatids.

Pachytene: Thick threads tetrad 4 chromatids clearly visible, Crossing over exchange of genetic material between non-sister chromatids of homologous at recombination nodules with enzyme recombinase like exchange of notes between friends. Chiasmata X shape visible site of crossing over like X marks exchange.

Diplotene: Repulsion desynapsis homologous start separating but held at chiasmata, Chiasmata terminalization starts moving to ends like zip opening from centre.

Diakinesis: Terminalization completes chiasmata at ends, most condensed bivalents, nuclear envelope breaks, nucleolus disappears, spindle forms like prophase end.

Metaphase I: Bivalents homologous pairs at equator double row like pairs standing together not single like mitosis, kinetochore attachment both sisters same side.

Anaphase I: Homologous chromosomes separate sister chromatids stay together reductional 2n to n centromere does NOT split unlike mitosis like family separation parents separate sisters stay together. That is why reductional.

Telophase I: Two haploid cells n chromosomes each with 2 chromatids (2C content).

Meiosis II: Like mitosis but haploid. Prophase II, Metaphase II haploid at equator single row, Anaphase II sister chromatids separate centromere splits like mitosis, Telophase II 4 haploid cells n with C content.

Significance: Conservation of chromosome number - meiosis halves to n, fertilization n+n restores 2n like you halve to make gametes then combine to restore. Variation due to crossing over in pachytene exchange + independent assortment homologous random separation like shuffling cards creates new combinations evolution raw material.

Mitosis vs Meiosis: Mitosis 1 division equational 2 identical 2n somatic growth repair no variation. Meiosis 2 divisions reductional 4 haploid n germ cells gametes variation crossing over independent assortment evolution. Remember: Mitosis photocopy, Meiosis shuffle.