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.

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.

Asexual reproduction, growth and development, tissue renewal

Cellular Organization of Genetic Material

Genome and Chromosomes

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 chromosome contains hundreds to thousands of genes.
Chromatin: The complex of DNA and protein in eukaryotic chromosomes, which condenses during cell division.
Somatic cells: Non-reproductive cells with two sets of chromosomes.
Gametes: Reproductive cells (sperm and eggs) with half the chromosome number of somatic cells.

Eukaryotic chromatin in interphase

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.

Highly condensed, duplicated human chromosome Chromosome replication and separation Homologous chromosomes and sister chromatids

Phases of the Cell Cycle

Interphase and M Phase

Interphase: Consists of G1 (growth), S (DNA replication), and G2 (preparation for division).
M phase: Includes mitosis (division of nucleus) and cytokinesis (division of cytoplasm).

Interphase cell with centrioles, nucleus, and chromatin

Sequence of Events in the Cell Cycle

G1 phase: Cell grows and prepares for DNA replication.
S phase: DNA is replicated.
G2 phase: Cell prepares for mitosis; centrioles replicate.
Mitosis: Chromosomes are separated into two nuclei.
Cytokinesis: Cytoplasm divides, forming two daughter cells.

How one parent cell gives rise to two genetically identical daughter cells

Mitosis: Stages and Key Events

Stages of Mitosis

Prophase: Chromosomes condense, spindle apparatus forms, nuclear envelope breaks down.
Prometaphase: Microtubules attach to kinetochores, chromosomes begin moving to cell center.
Metaphase: Chromosomes align at the metaphase plate.
Anaphase: Sister chromatids are pulled apart to opposite poles.
Telophase: Spindle disassembles, nuclear envelope reforms, chromosomes uncoil.

Stages of mitosis with spindle apparatus

Cell Cycle Control and Checkpoints

Regulation of the Cell Cycle

The cell cycle is regulated by checkpoints (G1, G2, M) that ensure proper division.
Cyclins and Cyclin-dependent kinases (Cdks) are key regulatory proteins.
MPF (Maturation-promoting factor): Cyclin-Cdk complex that triggers entry into M phase.
Internal signals (e.g., chromosome attachment) and external signals (e.g., growth factors) influence progression.

Cell cycle control and genetics of cancer

Loss of Cell Cycle Controls in Cancer

Cancer and Cell Cycle Regulation

Cancer cells escape normal cell cycle controls and divide uncontrollably.
Benign tumors: Abnormal cells remain at original site.
Malignant tumors: Invade tissues and can metastasize to other parts of the body.
Cancer treatments target rapidly dividing cells, but can affect normal dividing cells as well.

Molecular Biology of Cancer

Genetic Changes Leading to Cancer

Proto-oncogenes: Genes that normally stimulate cell growth; mutations convert them to oncogenes that promote excessive division.
Tumor-suppressor genes: Inhibit cell division; mutations can lead to loss of control and cancer.
Mutations in ras proto-oncogene and p53 tumor-suppressor gene are common in cancers.
Multiple mutations are usually required for cancer development; 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 and division	Becomes oncogene, causing excessive cell division
Tumor-suppressor gene	Inhibits cell division, repairs DNA	Loss of function leads to uncontrolled division
ras gene	Cell signaling for growth	Hyperactive Ras protein increases division
p53 gene	Suppresses cell cycle, repairs DNA, induces apoptosis	Loss leads to failure to suppress mutations

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 in eukaryotic cells.
Centromere: Region joining sister chromatids.
Kinetochore: Protein complex at centromere for microtubule attachment.
Cytokinesis: Division of cytoplasm after mitosis.
Binary fission: Prokaryotic cell division.
Checkpoints: Regulatory points in cell cycle.
Cyclin/Cdk: Proteins regulating cell cycle progression.
Oncogene: Mutated gene promoting cancer.
Tumor-suppressor gene: Gene inhibiting cell division.

Key Equations and Concepts

Chromosome number in humans: Somatic cells have 2n = 46 chromosomes; gametes have n = 23.
Cell cycle phases:
MPF activity:

Additional info: Academic context was added to clarify the functions of cell cycle regulators, the stages of mitosis, and the genetic basis of cancer. The summary table was inferred from textbook knowledge to provide a concise comparison of gene types and their roles in cancer.