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 process by which a single cell gives rise to two genetically identical daughter cells, ensuring continuity of life and accurate transmission of genetic material.

Key Roles of Cell Division

Functions of Cell Division

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

Cellular Organization of Genetic Material

Genome and Chromosomes

• Genome: All the DNA in a cell; can be a single DNA molecule (prokaryotes) or multiple DNA molecules (eukaryotes).

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

• Chromatin: Complex of DNA and protein 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 by cohesins.

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

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

Phases of the Cell Cycle

Overview of Cell Cycle Phases

• Interphase: Cell growth and chromosome duplication (G1, S, G2 phases).

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

Interphase

• G1 phase: Major cell growth.

• S phase: DNA replication.

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

Mitosis

• 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 the cell.

Binary Fission in Bacteria

• Prokaryotes reproduce by binary fission: chromosome replicates, daughter chromosomes move apart, cell divides.

• Eukaryotic mitosis may have evolved from binary fission.

Cell Cycle Control System

Checkpoints and Regulation

• Cell cycle is regulated by checkpoints (G1, G2, M) that ensure proper division.

• Cyclins and Cyclin-dependent kinases (Cdks): Regulatory proteins controlling cell cycle progression.

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

• Internal signals (e.g., chromosome attachment) and external signals (e.g., growth factors) influence checkpoints.

Loss of Cell Cycle Controls in Cancer Cells

Cancer Cell Characteristics

• Cancer cells ignore normal cell cycle signals and divide uncontrollably.

• Transformation allows cells to divide indefinitely, forming tumors.

• Benign tumors: Remain at original site; Malignant tumors: Invade tissues and metastasize.

• Cancer treatments target rapidly dividing cells, often affecting normal cells.

Molecular Biology of Cancer

Genetic Changes Leading to Cancer

• Proto-oncogenes: Genes coding for proteins that stimulate cell growth; mutations convert them to oncogenes (cancer-causing).

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

• Ras proto-oncogene: Mutations produce hyperactive Ras protein, increasing cell division.

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

• Multiple mutations are required for cancer development; inherited alleles can predispose individuals to cancer.

Summary Table: Cell Cycle and Cancer Control

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

• Interphase: Cell grows, chromosomes duplicate, DNA copied precisely.

• Mitosis: Chromosome copies separated to opposite ends.

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

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.

• Spindle apparatus: Structure of microtubules guiding chromosome movement.

Relevant Equations

DNA Replication:

Chromosome Number in Humans:

Additional info:

These notes expand on the original content by providing definitions, examples, and a summary table for clarity. The included images directly illustrate key concepts such as cell division, chromosome structure, and the phases of the cell cycle.