The Cell Cycle and Cell Division

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G0, G1, S, G2) and the mitotic phase (mitosis and cytokinesis).

• Interphase: The period of cell growth and DNA replication. Divided into G1, S, and G2 phases.

• G0 phase: A resting phase where cells exit the cycle and do not divide.

• G1 phase: Cell grows and prepares for DNA replication.

• S phase: DNA is replicated; each chromosome now consists of two sister chromatids.

• G2 phase: Cell prepares for mitosis; checks for DNA errors.

Chromatin is the complex of DNA and proteins (mainly histones) that forms chromosomes within the nucleus.

Stages of Mitosis

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

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

• Metaphase: Chromosomes align at the cell's equator (metaphase plate); spindle fibers attach to centromeres.

• Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell.

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

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

Centromere: The region of a chromosome where sister chromatids are joined and where spindle fibers attach during mitosis.

Haploid (n): Cells with one set of chromosomes (e.g., gametes). Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells).

Meiosis

Meiosis is the process by which gametes (egg and sperm) are produced, reducing the chromosome number by half and introducing genetic diversity.

• Meiosis I: Homologous chromosomes separate, resulting in haploid cells. Synapsis occurs, where homologous chromosomes pair up.

• Crossing-over: Exchange of genetic material between homologous chromosomes during prophase I, increasing genetic variation.

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

• Haploid cells: Cells become haploid after Meiosis I.

• Separation of sister chromatids: Occurs during Meiosis II.

Asexual reproduction: Offspring are genetically identical to the parent (no mixing of DNA). Sexual reproduction: Offspring inherit a combination of DNA from two parents.

Cytokinesis in plants: Formation of a cell plate. Cytokinesis in animals: Formation of a cleavage furrow.

Cellular Respiration and Photosynthesis

Oxidation-Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons and changes in potential energy. Oxidation is the loss of electrons; reduction is the gain of electrons.

• NAD+: Electron carrier; accepts electrons to become NADH.

• NADH: Reduced form; carries electrons to the electron transport chain.

Equation:

Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It breaks down glucose into pyruvate, producing ATP and NADH.

• Reactants: Glucose, NAD+, ADP, Pi

• Products: Pyruvate, NADH, ATP

• ATP Production: 2 ATP molecules (net gain) per glucose via substrate-level phosphorylation.

Equation:

Stages of Aerobic Respiration

Aerobic respiration consists of several stages that produce ATP by oxidizing glucose.

1. Glycolysis

2. Pyruvate oxidation (to acetyl CoA)

3. Krebs cycle (Citric Acid Cycle)

4. Electron Transport Chain and Chemiosmosis

Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP. Accounts for a small percentage of total ATP produced.

Electron Carriers in Redox Reactions

Electron carriers such as NAD+ and FAD accept electrons during metabolic reactions and transfer them to the electron transport chain.

• NADH: Delivers electrons to the electron transport chain.

• FADH2: Another electron carrier.

Chemiosmosis

Chemiosmosis refers to the movement of H+ ions across a membrane, generating ATP via ATP synthase.

• Cellular respiration: Protons flow from the intermembrane space back into the mitochondrial matrix through ATP synthase.

• Photosynthesis: Protons flow from the thylakoid space into the stroma through ATP synthase.

Equation for ATP synthesis:

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle occurs in the mitochondrial matrix and completes the oxidation of glucose derivatives, producing NADH, FADH2, and ATP.

• Reactants: Acetyl CoA, NAD+, FAD, ADP, Pi

• Products: CO2, NADH, FADH2, ATP

Cellular Respiration Overall Equation

Photosynthesis

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

• Light reactions: Occur in the thylakoid membrane; produce ATP and NADPH.

• Calvin cycle (dark reactions): Occur in the stroma; use ATP and NADPH to fix CO2 into glucose.

• Structure of chloroplast: Contains thylakoid membranes (site of light reactions) and stroma (site of Calvin cycle).

Photosynthesis Overall Equation:

Redox Reactions in Photosynthesis

• Water is oxidized: Loses electrons to form O2.

• CO2 is reduced: Gains electrons to form glucose.

Example: During photosynthesis, the light reactions generate ATP and NADPH, which are then used in the Calvin cycle to synthesize glucose from CO2.

Additional info: Substrate-level phosphorylation produces a small fraction of ATP in cellular respiration; most ATP is generated by chemiosmosis in the mitochondria.