Cell Biology: Cell Cycle, Signaling, and Membranes — Study Pack Summary & Study Notes

🧬 Cell Cycle Overview

Cell cycle describes the sequence of stages a cell goes through to grow and divide. Major stages include Interphase, Mitosis, and Cytokinesis. Interphase itself has distinct parts (G1, S, G2) and some cells exit into G0 where they are metabolically active but no longer dividing.

⚙️ Chromosomes and Chromatin

Chromosomes are structures made of sister chromatids, which are composed of chromatin (DNA wound around histones). Humans have 22 pairs of autosomes and 1 pair of sex chromosomes; chromosome number is species-specific but consistent within a species.

🔁 Mitosis Stages (Short summary)

Prophase: spindle formation and chromosome condensation. Prometaphase: kinetochores form at centromeres and microtubules attach. Metaphase: chromosomes line up at the metaphase plate. Anaphase: cohesin is degraded and sister chromatids separate. Telophase: nuclear envelope reforms and chromosomes decondense.

🧭 Centrosomes and Spindle

Centrosomes organize microtubules and contain two centrioles; they move to opposite poles to form the mitotic spindle which segregates chromosomes.

✂️ Cytokinesis — Animal vs Plant

In animal cells, a cleavage furrow forms using actin filaments. In plant cells, a cell plate forms from vesicles that build new cell wall material.

🔬 Cell Division in Prokaryotes

Prokaryotes use binary fission: the origin (ori) replicates, origins move to opposite ends, the cell elongates and divides. Eukaryotes typically undergo mitosis instead.

⏳ Senescence, Telomeres, and Telomerase

Telomeres are chromosome ends that shorten with each division, leading to cell senescence. Telomerase can restore telomeres and is active in germ cells, stem cells, and many cancers (e.g., HeLa cells derived from Henrietta Lacks), contributing to cellular immortality.

✅ Checkpoints and Cell Cycle Control

Cell cycle checkpoints (G1, G2, M) ensure DNA integrity and correct progression. Cyclins and cyclin-dependent kinases (CDKs) drive transitions; for example, Cyclin B with CDK1 forms MPF, promoting G2→M entry. Normal cells show density-dependent inhibition, anchorage dependence, and respond to growth factors, whereas cancer cells often lose these controls.

📣 Cell Communication Fundamentals

Cells communicate using signaling molecules (cytokines, growth factors, hormones). The receptor senses the signal, intracellular signal transduction transmits it, and the nucleus may change gene expression as a response. The effect of a signal depends on cellular context (cell type, receptor presence, downstream machinery).

🧭 Modes of Signaling

• Endocrine: long-range via blood (hormones).
• Paracrine and Synaptic: local signaling to nearby cells.
• Juxtacrine: membrane-bound signals given by touching neighbors.
• Autocrine: cells signal to themselves (important in development and cancer).

🧩 Receptor Types

G protein-coupled receptors (GPCRs) span the membrane seven times and activate G proteins; tyrosine kinase receptors (RTKs) autophosphorylate on tyrosines to recruit cytosolic signaling proteins. Ion channel receptors open to permit ion flow, and steroid receptors are intracellular and act as transcription factors.

🧪 Second Messengers and Cascades

Small water-soluble molecules like cAMP (from ATP via adenylyl cyclase) and products of PLC cleavage — DAG and IP3 — act as second messengers. IP3 triggers Ca2+ release from the ER, and DAG helps activate kinases. Kinases and phosphatases regulate phosphorylation states critical for signal propagation.

🧬 Clinical Connections

Pathogens and cancers exploit signaling: cholera toxin locks G-proteins active causing fluid loss, and overexpression of HER2 (an RTK) drives aggressive breast cancer — targeted therapy (e.g., Herceptin) blocks HER2 signaling.

🔁 Outcome Depends on Context

Signaling outcomes vary by receptor subtype, second messengers, and cell-specific transcriptional programs, so the same ligand can produce different responses in different cells.

🧱 Membrane Structure — Composition and Roles

The plasma membrane isolates the cytoplasm and controls interactions. It is built from lipids (~50%), proteins (~50%), and carbohydrates (~5–10%) (glycoproteins and glycolipids) on the external face that mediate cell–cell recognition.

🧩 Membrane Proteins and Mobility

Integral proteins span the bilayer and mediate transport and signaling; peripheral proteins associate via other proteins. Proteins diffuse laterally within the leaflet and can intermingle over time (demonstrated by cell-fusion experiments).

🌡 Membrane Fluidity Factors

Unsaturated fatty acids (with kinks) increase fluidity, while cholesterol modulates fluidity depending on temperature. Fluidity affects protein mobility and membrane function.

🚪 Transport Across Membranes

Two main transport classes: passive transport (no energy, down gradient) including simple diffusion, facilitated diffusion, and osmosis; and active transport (energy-dependent, against gradient).

💧 Osmosis and Tonicity

Osmosis is water diffusion across membranes. In animal cells: hypertonic (shrivel), isotonic (stable), hypotonic (lyse). Plant cells rely on turgor and have different responses due to the cell wall.

📦 Bulk Transport

Endocytosis (pinocytosis, phagocytosis, receptor-mediated) and exocytosis move large molecules or volumes across membranes. Facilitated diffusion employs channels or carriers (e.g., aquaporins for water), while active transporters use ATP to pump solutes.