Chapter 12: The Cell Cycle

Big Idea and Overview

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, Chromosomes, and Chromatin

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

• Chromosomes: DNA molecules packaged with proteins; each carries hundreds to thousands of genes.

• Chromatin: The complex of DNA and protein in eukaryotic chromosomes, which 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 at the centromere.

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

Phases of the Cell Cycle

Interphase and M Phase

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

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

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

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

Stages of Mitosis

Prophase, Prometaphase, Metaphase, Anaphase, Telophase

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

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

• Metaphase: Chromosomes align at the metaphase plate.

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

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

The Mitotic Spindle and Chromosome Movement

Spindle Apparatus and Kinetochore Function

• Mitotic spindle: Structure made of microtubules that controls chromosome movement.

• Centrosomes: Microtubule-organizing centers that replicate and migrate to opposite poles.

• Kinetochore: Protein complex at the centromere where spindle microtubules attach.

• Microtubules shorten by depolymerizing at kinetochore ends, moving chromosomes during anaphase.

Cytokinesis

Differences in Animals and Plants

• 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

• Bacteria reproduce by binary fission, where the chromosome replicates and the cell divides.

• Binary fission is simpler than mitosis and may represent an evolutionary precursor to eukaryotic cell division.

Cell Cycle Regulation

Checkpoints, Cyclins, and Cdks

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

• G1 = cell growing G2=chromosomes have been replicated/DNA not damaged M=sister chromatids are correctly attached to the spindle microtubules

• Cyclins: Proteins with cyclically fluctuating concentrations.

• Cyclin-dependent kinases (Cdks): Enzymes that must bind cyclins to be active; increased = division; rises and falls

• MPF (Maturation-promoting factor): Cyclin-Cdk complex that triggers entry into M phase.

Internal and External Signals

Cell Cycle Control Mechanisms

• Internal signals ensure chromosomes are properly attached before anaphase.

• External signals include growth factors, such as PDGF, which stimulate cell division.

Loss of Cell Cycle Controls in Cancer

Cancer Cell Characteristics

• Cancer cells ignore normal cell cycle controls and continue dividing without growth factors.

• Transformation allows cells to divide indefinitely, forming tumors.

• Benign tumors: Remain at the original site.

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

Molecular Biology of Cancer

Genetic Changes Leading to Cancer

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

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

• tumor supressor proteins

• 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 proliferate.

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

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

Summary Table: Cell Cycle and Cancer Genes

Additional info: The cell cycle is tightly regulated to ensure genetic stability. Disruption of these controls is a hallmark of cancer, and understanding these mechanisms is crucial for developing targeted therapies.