Chapter 12: The Cell Cycle

Big Idea

Understanding cell division is fundamental for grasping the mechanisms of human reproduction, growth, repair, and the generation of life in all organisms. The cell cycle encompasses the processes by which cells duplicate their genetic material and divide, ensuring continuity of life.

Key Roles of Cell Division

Functions of Cell Division

• Asexual reproduction: Single-celled organisms reproduce by cell division.

• Growth and development: Multicellular organisms develop from a single cell through repeated cell divisions.

• Tissue renewal and repair: Cell division replaces damaged or dead cells in multicellular organisms.

Cellular Organization of Genetic Material

Genome and Chromosomes

• Genome: All DNA in a cell; prokaryotes typically have a single DNA molecule, eukaryotes have multiple.

• Chromosomes: DNA molecules packaged with proteins; carry genes.

• Chromatin: Complex of DNA and protein; condenses during cell division.

Somatic vs. Gamete Cells

• Somatic cells: Non-reproductive, two sets of chromosomes.

• Gametes: Reproductive cells (sperm, eggs), one set of chromosomes.

Distribution of Chromosomes During Eukaryotic Cell Division

Chromosome Replication and Structure

• Before division, DNA is replicated and chromosomes condense.

• Each duplicated chromosome consists of two sister chromatids joined by cohesins.

• The centromere is the region where chromatids are most closely attached.

Separation of Chromatids

• During cell division, sister chromatids separate and become individual chromosomes.

Phases of the Cell Cycle

Overview

• The cell cycle consists of interphase (cell growth and DNA replication) and mitotic (M) phase (mitosis and cytokinesis).

Interphase: G1, S, and G2 Phases

• G1 phase: Cell growth.

• S phase: DNA replication.

• G2 phase: Preparation for mitosis; chromosomes coil, centrioles replicate, tubulin synthesis.

Mitosis: Five Stages

• Prophase: Chromosomes condense, spindle apparatus assembles, nuclear envelope breaks down.

• Prometaphase: Nuclear envelope disassembles, microtubules attach to kinetochores, chromosomes move toward cell center.

• Metaphase: Chromosomes align at metaphase plate.

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

• Telophase: Spindle disassembles, nuclear envelope reforms, chromosomes uncoil.

Cytokinesis

• Animal cells: Cleavage furrow forms, cell splits.

• Plant cells: Cell plate forms, dividing cell.

How One Parent Cell Gives Rise to Two Genetically Identical Daughter Cells

Summary of Process

• Interphase: Cell grows, chromosomes duplicate.

• Mitosis: Chromosome copies separated to opposite ends.

• Cytokinesis: Cell divides into two genetically identical daughter cells.

Binary Fission in Bacteria

Prokaryotic Cell Division

• Chromosome replicates, daughter chromosomes move apart.

• Plasma membrane pinches inward, dividing cell.

Cell Cycle Regulation

Checkpoints and Control System

• Cell cycle is regulated by checkpoints (G1, G2, M).

• Cyclins and Cyclin-dependent kinases (Cdks) control progression.

• MPF (Maturation-promoting factor): Cyclin-Cdk complex triggers passage past G2 checkpoint.

Internal and External Signals

• Internal signals: Ensure chromosomes are properly attached before anaphase.

• External signals: Growth factors stimulate cell division.

Loss of Cell Cycle Controls in Cancer Cells

Cancer Cell Characteristics

• Cancer cells ignore normal cell cycle signals.

• They may produce their own growth factors or have abnormal control systems.

• Transformation leads to indefinite division and tumor formation.

• Benign tumors: Remain at original site.

• Malignant tumors: Invade tissues, metastasize, and may form new tumors elsewhere.

Molecular Biology of Cancer

Genetic Changes Leading to Cancer

• Proto-oncogenes: Normal genes that stimulate cell growth; mutations convert them to oncogenes (cause excessive growth).

• Tumor-suppressor genes: Inhibit cell division; mutations reduce their function, contributing to cancer.

• Ras proto-oncogene: Mutations cause hyperactive Ras protein, stimulating cell division.

• p53 tumor suppressor gene: Mutations prevent suppression of cell cycle, allow damaged DNA to be passed on.

Multistep Model of Cancer Development

• Multiple mutations are required for cancer; incidence increases with age.

• Inherited mutations (e.g., BRCA1/BRCA2) can predispose individuals to certain cancers.

Summary Table: Cell Cycle and Cancer Regulation

Key Terms and Definitions

• Chromosome: DNA molecule with associated proteins, carrying genetic information.

• Chromatid: One of two identical halves of a duplicated chromosome.

• Chromatin: DNA-protein complex forming chromosomes.

• Centromere: Region joining sister chromatids.

• Kinetochore: Protein complex at centromere, attaches to spindle microtubules.

• Cytokinesis: Division of cytoplasm to form two cells.

• Binary fission: Prokaryotic cell division.

• Oncogene: Mutated gene causing uncontrolled cell growth.

• Tumor-suppressor gene: Gene inhibiting cell division, repairs DNA.

Equations and Formulas

Chromosome Number Changes

Somatic cells: chromosomes Gametes: chromosomes

Cell Cycle Regulation

MPF activity:

Cancer Development

Multiple mutations required:

Example: Human Life Cycle

• Somatic cells (body cells) have 46 chromosomes (2n).

• Gametes (sperm/egg) have 23 chromosomes (n).

• Fertilization restores diploid number.

Additional info:

• Cell cycle checkpoints are critical for preventing cancer and ensuring genetic stability.

• Modern cancer therapies increasingly target specific molecular pathways involved in cell cycle regulation.