The Cell Cycle and Cell Division

Overview of the Cell Cycle

The cell cycle is the series of events that cells undergo as they grow and divide. It consists of interphase (where the cell prepares for division) and the mitotic phase (where division occurs).

• Interphase: The longest phase, includes G1, S, and G2 phases.

• Mitotic Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Interphase Subphases

• G1 (Gap 1): Cell grows, synthesizes proteins and organelles.

• S (Synthesis): DNA is replicated; each chromosome forms two sister chromatids.

• G2 (Gap 2): Cell prepares for mitosis; additional growth and protein synthesis.

• G0 (Quiescent phase): Cells exit the cycle and do not divide; common in differentiated cells.

Chromatin, Chromosomes, and Chromatids

• Chromatin: Complex of DNA and proteins (mainly histones) that forms chromosomes.

• Chromosome: Condensed chromatin visible during cell division.

• Sister Chromatids: Identical copies of a chromosome, joined at the centromere after DNA replication.

• Centromere: Region where sister chromatids are attached; important for chromosome movement.

• Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells).

• Haploid (n): Cells with one set of chromosomes (e.g., gametes).

Stages of Mitosis

Mitosis is the process by which a cell divides its nucleus and distributes identical genetic material to two daughter cells.

• Prophase: Chromatin condenses into chromosomes; spindle fibers form; nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equator (metaphase plate).

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Chromosomes decondense; nuclear envelopes reform around each set.

• Cytokinesis: Division of cytoplasm; in animals, a cleavage furrow forms; in plants, a cell plate forms.

Example: In human cells, mitosis produces two diploid daughter cells identical to the parent cell.

Differences in Cytokinesis: Plants vs. Animals

• Animal Cells: Cytokinesis occurs via cleavage furrow.

• Plant Cells: Cytokinesis occurs via cell plate formation.

Meiosis

Overview and Stages

Meiosis is a type of cell division that reduces the chromosome number by half, producing four haploid cells (gametes). It consists of two consecutive divisions: Meiosis I and Meiosis II.

• Meiosis I: Homologous chromosomes separate; cells become haploid after Anaphase I.

• Meiosis II: Sister chromatids separate; similar to mitosis.

Key Events in Meiosis

• Synapsis: Homologous chromosomes pair up during Prophase I.

• Crossing-over: Exchange of genetic material between homologous chromosomes during Prophase I; increases genetic diversity.

• Haploid Cells: Cells become haploid after Meiosis I (specifically after Telophase I).

• Sister Chromatid Separation: Occurs during Anaphase II of Meiosis II.

Gamete Production

• Oogenesis (egg production): One ovum and three polar bodies are produced from one cell.

• Spermatogenesis (sperm production): Four sperm cells are produced from one cell.

Asexual vs. Sexual Reproduction

• Asexual Reproduction: Offspring are genetically identical to the parent; involves mitosis.

• Sexual Reproduction: Offspring inherit a combination of DNA from two parents; involves meiosis and fertilization.

Cellular Respiration

Overview and Chemical Equation

Cellular respiration is the process by which cells extract energy from glucose. It involves oxidation-reduction (redox) reactions and produces ATP.

• General Equation:

Stages of Aerobic Respiration

• Glycolysis: Occurs in cytoplasm; glucose is split into two pyruvate molecules; produces 2 ATP and 2 NADH.

• Pyruvate Oxidation: Pyruvate is converted to acetyl CoA; produces NADH and CO2.

• Citric Acid Cycle (Krebs Cycle): Acetyl CoA enters cycle; produces 2 ATP, 6 NADH, 2 FADH2, and CO2 per glucose.

• Electron Transport Chain (ETC) and Chemiosmosis: NADH and FADH2 donate electrons; ATP is produced via oxidative phosphorylation.

ATP Production

• Glycolysis: Net gain of 2 ATP per glucose via substrate-level phosphorylation.

• Citric Acid Cycle: 2 ATP per glucose via substrate-level phosphorylation.

• Oxidative Phosphorylation: About 26-28 ATP per glucose.

Substrate-level phosphorylation: Direct transfer of phosphate to ADP to form ATP; occurs in glycolysis and citric acid cycle.

Electron Carriers and Redox Reactions

• NAD+: Accepts electrons to become NADH.

• NADH: Donates electrons to the ETC.

• Redox Reaction: Transfer of electrons; oxidation is loss, reduction is gain of electrons.

Example: In glycolysis, NAD+ is reduced to NADH as glucose is oxidized.

Chemiosmosis

• Definition: Movement of H+ ions across a membrane to drive ATP synthesis.

• Direction: In mitochondria, protons move from intermembrane space to matrix via ATP synthase.

Photosynthesis

Overview and Chemical Equation

Photosynthesis is the process by which plants convert light energy into chemical energy, producing glucose and oxygen.

• General Equation:

Structure of Chloroplasts

• Thylakoid Membrane: Site of light reactions; contains chlorophyll.

• Stroma: Fluid surrounding thylakoids; site of Calvin cycle (dark reactions).

Stages of Photosynthesis

• Light Reactions: Occur in thylakoid membrane; convert light energy to ATP and NADPH; water is split, O2 is released.

• Calvin Cycle (Dark Reactions): Occur in stroma; use ATP and NADPH to fix CO2 into glucose.

Redox Reactions in Photosynthesis

• Water is oxidized: Loses electrons to form O2.

• CO2 is reduced: Gains electrons to form glucose.

Chemiosmosis in Photosynthesis

• Direction: Protons move from thylakoid space to stroma via ATP synthase.

• Opposite to mitochondria: In mitochondria, protons move into matrix; in chloroplasts, into stroma.

Reactants and Products

• Light Reactions: Reactants: H2O, light, NADP+, ADP; Products: O2, NADPH, ATP.

• Calvin Cycle: Reactants: CO2, NADPH, ATP; Products: Glucose, NADP+, ADP.

Summary Table: Cellular Respiration vs. Photosynthesis

Additional info:

• Substrate-level phosphorylation accounts for a small percentage of total ATP produced in cellular respiration (about 10%).

• Oxidative phosphorylation (via chemiosmosis) produces the majority of ATP.

• Crossing-over during meiosis increases genetic variation in sexually reproducing organisms.

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