Chapter 12: The Cell Cycle

Big Idea

Understanding cell division is fundamental to comprehending human reproduction, organismal development, and the generation of life in all classes of organisms. The cell cycle is a highly regulated process that ensures the accurate transmission of genetic material from one generation to the next.

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

The genome is the complete set of DNA in a cell. DNA is organized into chromosomes, which carry genes responsible for hereditary traits.

• Prokaryotic cells: Typically have a single circular DNA molecule.

• Eukaryotic cells: Have multiple linear chromosomes composed of chromatin (DNA and protein).

• Somatic cells: Contain two sets of chromosomes (diploid).

• Gametes: Contain one set of chromosomes (haploid).

Distribution of Chromosomes During Eukaryotic Cell Division

Chromosome Replication and Separation

Before cell division, chromosomes are duplicated, resulting in two sister chromatids joined at the centromere. During division, sister chromatids separate and are distributed to daughter cells.

• Chromatid: Each of the two identical halves of a duplicated chromosome.

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

Phases of the Cell Cycle

Interphase and M Phase

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).

• Mitosis: Division of the nucleus, conventionally broken down into prophase, prometaphase, metaphase, anaphase, and telophase.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Stages of Mitosis

Overview of Mitotic Stages

Mitosis ensures the equal distribution of chromosomes to daughter cells. The stages are:

• Prophase: Chromosomes condense, spindle apparatus forms.

• Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelope reforms, chromosomes decondense.

Mitotic Spindle and Chromosome Movement

Spindle Apparatus and Kinetochores

The mitotic spindle is composed of microtubules and controls chromosome movement. Kinetochores are protein complexes at centromeres where spindle fibers attach.

• Centrosomes: Microtubule-organizing centers that form spindle poles.

• Kinetochore microtubules: Attach to kinetochores and move chromosomes.

Cytokinesis

Animal vs. Plant Cells

Cytokinesis differs between animal and plant cells:

• Animal cells: Cleavage furrow forms and pinches the cell in two.

• Plant cells: Cell plate forms, dividing the cell.

Binary Fission in Prokaryotes

Prokaryotic Cell Division

Prokaryotes reproduce by binary fission, a simpler process than mitosis. The chromosome replicates, and the cell divides into two.

Cell Cycle Regulation

Checkpoints and Control System

The cell cycle is regulated by checkpoints, cyclins, and cyclin-dependent kinases (Cdks). These ensure proper cell division and prevent errors.

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

• G2 checkpoint: Ensures DNA replication is complete and undamaged.

• M checkpoint: Ensures all chromosomes are properly attached to the spindle.

• Cyclins: Proteins with fluctuating concentrations that regulate Cdks.

• 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 and Cell Cycle Regulation

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

• Benign tumors: Remain at the original site.

• Malignant tumors: Invade tissues and can metastasize to other parts of the body.

• Metastasis: Spread of cancer cells to distant sites.

Molecular Biology of Cancer

Genetic Changes Leading to Cancer

Cancer results from mutations in genes that regulate the cell cycle.

• Proto-oncogenes: Normal genes that promote cell growth and division.

• Oncogenes: Mutated proto-oncogenes that cause excessive cell division.

• Tumor-suppressor genes: Inhibit cell division; mutations can lead to cancer.

• Ras gene: Mutations cause hyperactive Ras protein and increased cell division.

• p53 gene: Mutations prevent suppression of the cell cycle and DNA repair.

Multistep Model of Cancer Development

Multiple mutations are required for cancer to develop, often involving both oncogenes and tumor-suppressor genes. Inherited mutations can predispose individuals to certain cancers.

Summary Table: Cell Cycle and Cancer Genes

Key Equations and Concepts

Chromosome Number Changes

During mitosis, chromosome number remains constant in somatic cells. During meiosis, chromosome number is halved in gametes.

Equation for DNA Replication:

Equation for Cell Division:

Equation for Chromosome Number in Gametes:

Additional info: These notes expand on the provided material with definitions, examples, and equations to ensure completeness and academic quality for exam preparation.