Chapter 12: The Cell Cycle

Big Idea

The cell cycle is fundamental to understanding how organisms grow, reproduce, and maintain their tissues. Cell division is the basis for human reproduction and the generation of life in all classes of organisms.

Key Roles of Cell Division

Functions of Cell Division

Cell division is essential for reproduction, growth, and repair in living organisms.

• Asexual reproduction: Single-celled organisms reproduce by dividing into new organisms.

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

• Tissue renewal: Cell division replaces damaged or dead cells in mature organisms.

Cellular Organization of Genetic Material

Genome and Chromosomes

All the DNA in a cell constitutes its genome. DNA is organized into chromosomes, which carry genes responsible for hereditary traits.

• Prokaryotic cells: Typically have a single DNA molecule.

• Eukaryotic cells: Have multiple DNA molecules packaged into chromosomes.

• Chromatin: The complex of DNA and protein found in eukaryotic chromosomes, condenses during cell division.

• Somatic cells: Nonreproductive cells with two sets of chromosomes.

• Gametes: Reproductive cells (sperm and eggs) with half the chromosome number of somatic cells.

Distribution of Chromosomes During Eukaryotic Cell Division

Chromosome Replication and Separation

Before cell division, DNA is replicated and chromosomes condense. Each duplicated chromosome consists of two sister chromatids joined at the centromere.

• Sister chromatids: Identical copies of a chromosome, joined together.

• Centromere: The region where sister chromatids are most closely attached.

Phases of the Cell Cycle

Overview of Cell Cycle Phases

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

• Interphase: Divided into G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• M phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis: Stages and Events

Stages of Mitosis

Mitosis is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase.

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

• Prometaphase: Microtubules attach to kinetochores, chromosomes begin moving to the cell center.

• Metaphase: Chromosomes align along the metaphase plate.

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

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

Mitotic Spindle and Chromosome Movement

Mitotic Spindle Structure

The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis. In animal cells, spindle assembly begins in the centrosome.

• Centrosome: Microtubule-organizing center, replicates during interphase.

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

Cytokinesis

Division of Cytoplasm

Cytokinesis is the process by which the cytoplasm divides, resulting in two daughter cells.

• Animal cells: Cytokinesis occurs by cleavage, forming a cleavage furrow.

• Plant cells: Cytokinesis occurs by formation of a cell plate.

Binary Fission in Bacteria

Prokaryotic Cell Division

Prokaryotes reproduce by binary fission, a simpler process than mitosis.

• Chromosome replication: Begins at the origin of replication.

• Cell division: Plasma membrane pinches inward, dividing the cell into two.

Cell Cycle Regulation

Checkpoints and Control System

The cell cycle is regulated by checkpoints and signaling molecules. Cyclins and cyclin-dependent kinases (Cdks) are key regulatory proteins.

• G1 checkpoint: Determines if the cell will proceed to division or enter G0 (nondividing state).

• G2 checkpoint: Ensures DNA is replicated and undamaged before mitosis.

• M checkpoint: Ensures all chromatids are properly attached to spindle microtubules.

• Cyclins: Proteins with cyclically fluctuating concentrations.

• Cdks: Kinases that must bind cyclins to be active.

• MPF: Maturation-promoting factor, a cyclin-Cdk complex that triggers entry into M phase.

Loss of Cell Cycle Controls in Cancer

Cancer Cell Characteristics

Cancer cells escape normal cell cycle controls, leading to uncontrolled division and tumor formation.

• Transformation: Cells acquire the ability to divide indefinitely.

• Benign tumors: Abnormal cells remain at the original site.

• Malignant tumors: Invade surrounding tissues and can metastasize.

• Metastasis: Spread of cancer cells to other parts of the body.

Molecular Biology of Cancer

Genetic Changes Leading to Cancer

Cancer results from mutations in genes that regulate cell growth and division.

• Proto-oncogenes: Normal genes that stimulate cell growth; mutations convert them to oncogenes, causing excessive division.

• Oncogenes: Mutated proto-oncogenes with increased activity or protein production.

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

• Ras proto-oncogene: Mutations lead to hyperactive Ras protein and increased cell division.

• p53 tumor suppressor gene: Mutations prevent suppression of the cell cycle, allowing damaged cells to divide.

Multistep Model of Cancer Development

Multiple mutations are required for cancer to develop, often involving both oncogenes and tumor-suppressor genes.

• Inherited predisposition: Individuals may inherit mutant alleles, increasing cancer risk.

• Environmental factors: Can contribute to mutations and cancer development.

Summary Table: Cell Cycle and Cancer Control

Key Terms and Definitions

• Chromosome: DNA molecule carrying genetic information.

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

• Chromatin: DNA-protein complex in eukaryotic chromosomes.

• Centromere: Region joining sister chromatids.

• Kinetochore: Protein structure at centromere for spindle attachment.

• Mitotic spindle: Microtubule structure for chromosome movement.

• Binary fission: Prokaryotic cell division.

• Oncogene: Mutated gene causing cancer.

• Tumor-suppressor gene: Gene inhibiting cell division.

Important Equations

DNA Replication:

Chromosome Number in Humans:

Cell Cycle Regulation:

Additional info: Academic context was added to clarify the stages of mitosis, cell cycle regulation, and the molecular basis of cancer, ensuring completeness and self-contained study notes.