Cell Division and the Cell Cycle

Why Cells Divide

Cell division is a fundamental process that distinguishes living organisms from nonliving matter. It ensures the continuity of life by producing new cells for reproduction, growth, development, and tissue repair.

• Unicellular organisms (e.g., bacteria, amoeba): Cell division reproduces the entire organism (asexual reproduction, often by binary fission).

• Multicellular eukaryotes: Cell division enables growth, development (e.g., embryogenesis), and tissue renewal (e.g., bone marrow cell replacement).

• Most cell divisions produce two genetically identical daughter cells, maintaining genetic fidelity.

• Proliferation = cell division + growth.

Organization of Genetic Material

The genome is the complete set of DNA in a cell. Its organization differs between prokaryotes and eukaryotes:

• Prokaryotes: Single circular DNA molecule (plus plasmids).

• Eukaryotes: Multiple linear DNA molecules, each packaged into a chromosome.

• Chromatin: DNA-protein complex (mainly histones) that forms chromosomes.

• Karyotype: The complete set of chromosomes in an individual, often visualized for diagnostic purposes.

• Somatic cells (non-reproductive): 46 chromosomes in humans (2 sets of 23).

• Gametes (sperm, egg): 23 chromosomes in humans.

Chromosome Packaging (smallest to largest):

• DNA double helix (2 nm)

• Nucleosome ("beads on a string," 10–11 nm)

• 30 nm fiber (tight helical)

• Supercoiled/looped domains (300 nm)

• 700 nm fiber

• Metaphase chromosome (1400 nm)

Chromosome Vocabulary and Counting Rules

• Chromosome: One DNA molecule with associated proteins; counted by centromeres.

• Sister chromatids: Identical copies joined after DNA replication; one duplicated chromosome = 2 sister chromatids.

• Centromere: Constricted region joining sister chromatids; site of kinetochore assembly.

• Kinetochore: Protein complex at the centromere; attachment site for spindle microtubules.

• After separation, each chromatid becomes an independent chromosome.

Counting Rules:

1. Chromosomes = number of centromeres.

2. Each duplicated chromosome (X-shaped) has 2 chromatids; unduplicated (I-shaped) has 1 chromatid.

3. DNA molecules = number of chromatids.

4. Replication (S phase) doubles DNA and chromatids, not chromosome number.

5. Chromosome number doubles only in anaphase, when sisters separate.

The Cell Cycle

The cell cycle consists of interphase (about 90% of the cycle) and the M phase (mitosis + cytokinesis).

• Interphase subphases:

  • G1 (first gap): Cell growth, protein/organelle synthesis.

  • S (synthesis): DNA replication (chromosomes duplicated).

  • G2 (second gap): Further growth, preparation for mitosis.

• Cell grows during all interphase phases; chromosomes duplicate only in S.

• G0 phase: Nondividing state entered from G1 due to damage, maturity, or unfavorable conditions (e.g., nerve cells remain in G0).

Mitosis: Stages and Features

Mitosis divides the nucleus, producing genetically identical daughter nuclei. Cytokinesis divides the cytoplasm, overlapping late mitosis.

• Order: Prophase → Prometaphase → Metaphase → Anaphase → Telophase (P-P-M-A-T).

The Mitotic Spindle

• Composed of microtubules (tubulin polymers) and proteins; controls chromosome movement.

• Centrosome: Microtubule organizing center; duplicates in interphase, migrates to poles in prophase.

• Aster: Radial array of short microtubules from each centrosome.

• Kinetochore microtubules: Attach to kinetochores; nonkinetochore microtubules overlap and elongate the cell.

• During anaphase, microtubules shorten at the kinetochore end, pulling chromatids apart.

Cytokinesis

• Animal cells: Cleavage furrow forms via a contractile ring of actin microfilaments.

• Plant cells: Cell plate forms from vesicles, developing into a new cell wall.

Binary Fission (Prokaryotes)

Prokaryotes divide by binary fission, a simpler process than mitosis:

1. Chromosome replicates from the origin of replication.

2. Origins move to opposite ends as the cell elongates.

3. Replication finishes; plasma membrane pinches inward; new cell wall forms.

4. Two daughter cells result.

Evolution of Mitosis

• Mitosis likely evolved from prokaryotic binary fission.

• Some protists (e.g., dinoflagellates, diatoms) show intermediate forms where the nuclear envelope remains intact during division.

Mitosis vs. Meiosis

• Mitosis: 1 diploid cell → 2 identical diploid cells.

• Meiosis: 1 diploid cell → 4 non-identical haploid gametes (covered in Ch 10).

Cellular Respiration

Overview

Cellular respiration is the exergonic breakdown of organic molecules to produce ATP. It includes aerobic and anaerobic pathways.

• Fermentation: Partial breakdown of sugars without O2.

• Aerobic respiration: O2 is the final electron acceptor.

• Anaerobic respiration: Uses other final electron acceptors (e.g., sulfate, nitrate).

• Overall equation:

• Glucose is oxidized; O2 is reduced.

Redox Reactions

• Oxidation: Loss of electrons (reducing agent).

• Reduction: Gain of electrons (oxidizing agent).

• Mnemonic: LEO (Lose Electrons = Oxidation), GER (Gain Electrons = Reduction).

• In respiration: glucose is the reducing agent; O2 is the oxidizing agent.

• NAD+ and FAD are electron carriers, becoming NADH and FADH2 when reduced.

ATP Production Mechanisms

Stages of Cellular Respiration and Yields

• Order: glucose → glycolysis → pyruvate oxidation → citric acid cycle → ETC.

• Glycolysis + citric acid cycle: 4 ATP (substrate-level); most energy carried by NADH/FADH2.

• Overall yield: ~30–32 ATP per glucose (exact number may vary).

• Alternate fuels (fats, proteins) enter at various points in these pathways.

Electron Transport Chain (ETC) and Chemiosmosis

• ETC: Series of protein complexes (I–IV, mainly cytochromes) in the inner mitochondrial membrane (or plasma membrane in prokaryotes).

• Electrons from NADH enter at complex I (highest energy); FADH2 enters lower.

• O2 is the final electron acceptor, forming water.

• Electron transfer pumps H+ into the intermembrane space, creating a proton-motive force (PMF).

• H+ flows back through ATP synthase, driving ATP production (chemiosmosis).

• Oxidative phosphorylation = ETC + chemiosmosis.

• Immediate energy source for ATP synthesis: the proton-motive force (not NADH or glucose directly).

Anaerobic Processes

• Without O2 (or another acceptor), ETC and oxidative phosphorylation stop.

• Anaerobic respiration: Uses ETC with a non-O2 acceptor (e.g., sulfate, nitrate); less ATP produced due to smaller energy drop.

• Fermentation: No ETC; ATP only from glycolysis; fermentation regenerates NAD+ for glycolysis.

• Types:

  • Alcohol fermentation: Pyruvate → ethanol + CO2 (yeast).

  • Lactic acid fermentation: Pyruvate → lactate (muscle cells, some bacteria).

Cell Cycle Regulation and Cancer (Ch 9.3)

Checkpoints and Control

• Checkpoints: G1 (restriction point), G2, and M (metaphase) checkpoints regulate cycle progression.

• G1: Checks size, nutrients, signals, DNA integrity; G2: Checks DNA replication, damage, MPF presence; M: Checks spindle attachment.

• Cyclins and Cyclin-dependent kinases (Cdks): Cdk levels constant; cyclin levels oscillate; active Cdk-cyclin complex (e.g., MPF) triggers mitosis.

• At M checkpoint, all kinetochores must be attached to activate separase and allow anaphase.

• External signals: growth factors (e.g., PDGF), density-dependent inhibition, anchorage dependence.

• Cancer cells lack normal inhibition, divide uncontrollably, and may metastasize.

Cancer and Treatments

• Transformation: Cells acquire ability to divide indefinitely.

• Benign tumors: Remain localized; malignant tumors: Invade tissues, metastasize.

• HeLa cells: Famous immortal cell line from Henrietta Lacks (1951).

• Treatments: radiation, chemotherapy (targets rapidly dividing cells), personalized therapy, immunotherapy.

Practice Questions and Key Concepts

• Chromosome and chromatid counting: After S phase, chromatids double but chromosome number stays the same until anaphase.

• Stages of mitosis: Prophase, prometaphase, metaphase, anaphase, telophase (cytokinesis overlaps end).

• Key structures: kinetochores (spindle attachment), cleavage furrow (animal cytokinesis), cell plate (plant cytokinesis).

• Binary fission is the division mechanism in prokaryotes.

• G0 phase: Nondividing state, especially in nerve and some muscle cells.

• Cell cycle exit occurs from G1 to avoid incomplete or excess DNA content.

• Cellular respiration order: Glycolysis → pyruvate oxidation → citric acid cycle → ETC.

• ETC location: Plasma membrane (prokaryotes), inner mitochondrial membrane (eukaryotes).

• Anaerobic respiration uses ETC with non-O2 acceptors; fermentation does not use ETC.

• ATP yield is lower in anaerobic respiration due to less electronegative acceptors.

Key Equations

• Cellular respiration overall:

Summary Table: Mitosis vs. Meiosis

Summary Table: ATP Production in Cellular Respiration (per glucose)

Additional info: Some details (e.g., exact ATP yield, specific drug mechanisms, and protist mitosis variations) are expanded for academic completeness and clarity.