The Cell Cycle and Molecular Biology of Cancer: Study Notes

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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.

Diagram showing how one parent cell gives rise to two genetically identical daughter cells

Key Roles 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.

Images showing asexual reproduction, growth and development, and tissue renewal

Concept 12.1: Genetically Identical Daughter Cells

Cellular Organization of Genetic Material

All the DNA in a cell constitutes its genome. Prokaryotic cells typically have a single DNA molecule, while eukaryotic cells have multiple DNA molecules organized into chromosomes. Chromosomes are composed of chromatin, a complex of DNA and protein that condenses during cell division.

Image of chromatin in a cell

Somatic cells: Nonreproductive cells with two sets of chromosomes.
Gametes: Reproductive cells (sperm and eggs) with half as many chromosomes as somatic cells.

Chromosome Structure and Replication

Before cell division, chromosomes are duplicated. Each duplicated chromosome consists of two sister chromatids joined at a centromere. During cell division, sister chromatids separate and become individual chromosomes.

Electron micrograph of a duplicated chromosome with sister chromatids and centromeres Diagram showing homologous chromosomes, sister chromatids, centromeres, and kinetochores Diagram showing chromosome replication and formation of sister chromatids

Concept 12.2: The Cell Cycle Phases

Phases of the Cell Cycle

The cell cycle consists of two main stages: interphase and the mitotic (M) phase. Interphase is subdivided into three phases:

G1 phase: Cell growth and preparation for DNA replication.
S phase: DNA replication occurs.
G2 phase: Further growth, preparation for mitosis, and synthesis of proteins needed for cell division.

Diagram of a cell in interphase showing centrioles, nucleus, and chromatin

Mitosis: Stages and Events

Mitosis is conventionally divided into five stages:

Prophase: Chromosomes condense, spindle apparatus forms, nuclear envelope breaks down.
Prometaphase: Microtubules attach to kinetochores, chromosomes begin moving toward the cell center.
Metaphase: Chromosomes align at the metaphase plate.
Anaphase: Sister chromatids separate and move to opposite poles.
Telophase: Nuclear envelope reforms, chromosomes decondense, nucleolus reappears.

Series of images showing mitosis stages: prophase, metaphase, anaphase, telophase, and cytokinesis

Cytokinesis

Cytokinesis is the division of the cytoplasm. In animal cells, it occurs by cleavage, forming a cleavage furrow. In plant cells, a cell plate forms.

Binary Fission in Bacteria

Prokaryotes reproduce by binary fission, where the chromosome replicates and the cell divides into two.

Concept 12.3: Cell Cycle Regulation

Cell Cycle Control System

The cell cycle is regulated by a molecular control system involving checkpoints, cyclins, cyclin-dependent kinases (Cdks), and maturation-promoting factor (MPF). Checkpoints ensure that critical events are completed before the cell proceeds to the next phase.

G1 checkpoint: Determines if the cell will proceed to division or enter a nondividing state (G0).
G2 checkpoint: Ensures DNA replication is complete and undamaged.
M checkpoint: Ensures all chromosomes are properly attached to the spindle before anaphase.

Flowchart summarizing cell cycle control, checkpoints, and genetics of cancer

Concept 18.5: Molecular Biology of Cancer

Genetic Changes Leading to Cancer

Cancer results from mutations that disrupt normal cell cycle control. Key genes involved include:

Proto-oncogenes: Genes that stimulate normal cell growth and division. Mutations convert them to oncogenes, which can cause excessive cell division.
Tumor-suppressor genes: Genes that inhibit cell division. Mutations can lead to loss of function, contributing to cancer.

Mutations in genes such as ras (proto-oncogene) and p53 (tumor-suppressor gene) are common in human cancers. Multiple mutations are usually required for cancer to develop, and inherited mutations can predispose individuals to certain cancers.

Summary Table: Cell Cycle and Cancer Genes

Gene Type	Normal Function	Effect of Mutation
Proto-oncogene	Stimulates cell growth/division	Oncogene causes excessive division
Tumor-suppressor	Inhibits cell division, repairs DNA	Loss of function leads to uncontrolled division
ras	Signal transduction for growth	Hyperactive Ras protein increases division
p53	DNA repair, cell cycle arrest, apoptosis	Loss leads to mutation accumulation and cancer

Additional info:

Cell cycle regulation is crucial for preventing uncontrolled cell growth and cancer.
Understanding the molecular basis of cell cycle control and cancer genetics is essential for developing targeted therapies.