Chapter 10: How Cells Divide

10.1 Bacterial Cell Division

• Bacteria reproduce through binary fission

  • No sexual life cycle involved.

  • Reproduction is clonal, producing genetically identical offspring.

  • Features of division:

    • Single, circular bacterial chromosome is replicated.

    • Replication starts at the origin of replication and proceeds bidirectionally to the site of termination.

    • New chromosomes are divided into opposite ends of the cell.

    • A septum forms, completing cell division.

10.2 Eukaryotic Chromosomes

• Eukaryotes generally have 10 to 50 chromosomes in somatic cells.

• Humans have 46 chromosomes arranged in 23 pairs, with additional or missing chromosomes often leading to lethal conditions.

• Karyotype is the arranged collection of an organism's chromosomes:

  • Humans are diploid (2n) with 46 total chromosomes and haploid (n) with 23 chromosomes per set.

  • Chromosomes in homologous pairs: each is a homologue.

10.3 Overview of the Eukaryotic Cell Cycle

• The cell cycle includes the following phases:

  1. G1 (Gap Phase 1): Primary growth phase, longest phase.

  2. S (Synthesis): DNA replication occurs.

  3. G2 (Gap Phase 2): Organelles replicate, microtubules organize.

  4. M (Mitosis): Subdivided into five phases:

  • Prophase, Prometaphase, Metaphase, Anaphase, Telophase

  5. C (Cytokinesis): Division of cytoplasm to form two new cells.

10.4 Interphase: Preparation for Mitosis

• Interphase contains G1, S, and G2 phases:

  • G1: Major portion of growth occurs.

  • S: DNA is replicated.

  • G2: Chromosomes further condense; centrioles replicate; tubulin synthesis occurs.

10.5 M Phase: Chromosome Segregation and Cytoplasmic Division

• Mitosis divided into five phases:

  • Prophase: Chromosomes become visible, spindle apparatus assembles, and the nuclear envelope breaks down.

  • Prometaphase: Chromosomes attach to spindle microtubules.

  • Metaphase: Chromosomes align along the metaphase plate.

  • Anaphase: Centromeres split and sister chromatids are pulled to opposite poles.

  • Telophase: Nuclear envelopes reform, and chromosomes begin to uncoil.

10.6 Control of the Cell Cycle

• The cell cycle contains irreversible points:

  1. DNA replication.

  2. Separation of sister chromatids.

• Checkpoints exist to halt the process to check for errors, allowing cells to respond to internal and external signals (G1/S, G2/M, Late Metaphase).

• Main regulators include:

  • Maturation-promoting factor (MPF): induces cell division; varies throughout the cycle.

  • Cyclins: proteins produced in synchrony with the cell cycle.

  • Cyclin-dependent kinases (Cdks): phosphorylate proteins and regulate the cell cycle.

10.7 Genetics of Cancer

• Cancer results from uncontrolled cell growth due to faults in cell cycle regulation:

  • Tumor-suppressor genes: genes like p53 and Rb must both lose function for cancer to develop.

  • Proto-oncogenes: normal genes that can become oncogenic when mutated, prompting uncontrolled division.

• Exploration of gene mutations reveals different spectra across tumors, influencing diagnosis and treatment strategies.