The Cell Cycle: Replication, Mitosis, and Cancer

Chapter 12: The Cell Cycle

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It ensures that genetic material is accurately copied and distributed to daughter cells. The cell cycle consists of interphase (growth and DNA replication) and the M phase (mitosis and cytokinesis).

• Interphase: Includes G1 (first gap), S (DNA synthesis), and G2 (second gap) phases.

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

12.1 How Do Cells Replicate?

Cellular replication involves three key steps to ensure the faithful transmission of genetic material:

1. Copying the DNA: Each chromosome is duplicated.

2. Separating the copies: Sister chromatids are separated into two nuclei.

3. Dividing the cytoplasm: Cytokinesis produces two complete daughter cells.

Key Definitions

• DNA: Encodes the cell’s genetic information.

• Gene: A segment of DNA containing hereditary information.

• Chromosome: A single long double helix of DNA wrapped around proteins.

• Ploidy: Number of copies of each type of chromosome.

  • Haploid (N): One copy of each chromosome.

  • Diploid (2N): Two copies of each chromosome.

• Chromatin: DNA and associated proteins.

• Genome: All genetic material in an organism.

Chromosome Morphology

Chromosomes exist in different forms during the cell cycle:

• Unreplicated chromosome: Single DNA double helix.

• Replicated chromosome: Two identical DNA molecules (sister chromatids) joined at the centromere.

• Condensed replicated chromosome: Highly compacted for cell division.

The Eukaryotic Cell Cycle

New cells arise from the division of preexisting cells. The cell cycle is divided into:

• Interphase: Cell grows, replicates DNA, and prepares for division.

• M phase: Mitosis or meiosis occurs, followed by cytokinesis.

Overview of Cell Cycle Phases

• G1 phase: Cell growth and normal function.

• S phase: DNA replication.

• G2 phase: Preparation for mitosis.

• M phase: Division of nucleus and cytoplasm.

M Phase: Nuclear Division

The M phase is characterized by the division of the nucleus (karyokinesis) and is followed by cytokinesis.

• Mitosis: Occurs in both haploid and diploid cells; produces genetically identical daughter cells.

• Meiosis: Produces gametes with half the chromosome number (not detailed here).

• Cytokinesis: Division of the cytoplasm into two daughter cells.

Stages of Mitosis

Mitosis is divided into four main stages:

1. Prophase: Chromosomes condense; mitotic spindle forms; nuclear envelope breaks down; chromosomes attach to spindle via kinetochores.

2. Metaphase: Nuclear envelope gone; spindle fully formed; chromosomes align at the metaphase plate.

3. Anaphase: Centromeres separate; sister chromatids (now daughter chromosomes) move to opposite poles; kinetochore microtubules shrink and motor proteins push poles apart.

4. Telophase: Chromosome migration complete; chromosomes decondense; new nuclear envelopes form; spindle breaks down; two independent nuclei form.

Structures Involved in Mitosis

Structure	Definition
Chromosome	Structure containing genetic information in the form of genes.
Chromatin	Material that makes up eukaryotic chromosomes; DNA molecule complexed with histone proteins.
Sister chromatids	Two identical, double-stranded DNA copies of a replicated chromosome.
Centromere	Specialized region where sister chromatids are most closely joined.
Kinetochore	Structure on sister chromatids where microtubules attach.
Centrosome	Microtubule organizing center in animals and certain plants and fungi.
Microtubule	Cytoskeletal filament that forms the spindle apparatus.
Microtubule motor protein	Proteins that move chromosomes and poles of the spindle apparatus.

Bacterial Cell Division

Bacterial cells divide by binary fission, not mitosis. The process involves:

1. DNA is copied, and protein filaments attach.

2. Copies separate, and ring of protein forms.

3. Membrane is pulled inward by protein ring.

4. Fission is complete, producing two cells.

Cell-Cycle Checkpoints

Cell-cycle checkpoints are regulatory mechanisms that ensure proper division:

• G1 checkpoint: Cell size, nutrients, growth factors, and DNA damage are assessed.

• G2 checkpoint: DNA replication and damage are checked.

• M checkpoint: Chromosome attachment to spindle and proper alignment are verified.

Cancer: Out-of-Control Cell Division

Cancer results from uncontrolled cell division due to defects in cell-cycle regulation:

• Defects activate proteins required for cell growth when they should not be active.

• Defects prevent tumor suppressor genes from shutting down the cell cycle.

Properties of Cancer Cells

• Malignant tumors: Cancerous and invasive; can spread via blood or lymph (metastasis).

• Benign tumors: Noncancerous and noninvasive.

Summary Table: Tumor Types

Tumor Type	Characteristics
Malignant	Cancerous, invasive, can metastasize
Benign	Noncancerous, noninvasive

Example: Cell-Cycle Regulation and Cancer

Mutations in genes that regulate the cell cycle, such as proto-oncogenes and tumor suppressor genes, can lead to cancer. For example, a mutation in the p53 tumor suppressor gene can prevent cells from undergoing apoptosis in response to DNA damage, allowing abnormal cells to proliferate.

Additional info: The cell cycle is tightly regulated to prevent errors in DNA replication and division. Disruption of these controls is a hallmark of cancer biology.