Cell Cycle, Mitosis, and Cancer: Study Notes

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Cell Cycle and Cell Division

Introduction to Cell Division

Cell division is a fundamental process by which cells reproduce, enabling growth, development, and tissue repair in multicellular organisms. There are differences in how eukaryotic and prokaryotic cells divide.

Eukaryotes (multicellular organisms) use mitosis for both growth and repair.
Prokaryotes use a process called binary fission, in which the organism grows to twice its size and then splits into two genetically identical cells.
Both prokaryotic and eukaryotic cells undergo mitosis (with differences in mechanism and complexity).

Chromosomes in Prokaryotes vs. Eukaryotes

Chromosomes are structures that contain DNA. Their organization differs between prokaryotes and eukaryotes.

	Location (nucleoid / nucleus)	# (one or multiple)	Shape (circular / linear)
Prokaryotes	nucleoid	one	circular
Eukaryotes	nucleus	multiple	linear

Types of Cells in Multicellular Organisms

Multicellular organisms have specialized cells for different functions, mainly divided into somatic and gamete cells.

	Somatic Cells	Gametes
Function	body (non-reproductive) cells	reproductive cells
Examples	skin, liver, brain, blood, etc.	sperm cells, egg cells

Chromosome Structure

Chromatin vs. Chromosome

DNA in eukaryotic cells exists in two forms depending on the stage of the cell cycle.

Chromatin: Complex of DNA and structural proteins (histones); loosely packed and not visible under a light microscope.
Chromosome: Condensed chromatin visible under a light microscope, especially during cell division.

Chromosome (Non-duplicated vs. Duplicated)

	Non-duplicated	Duplicated
Visualization	Single chromatid with centromere	Two sister chromatids joined at the centromere

When a cell prepares to divide, each chromosome replicates, resulting in two identical DNA molecules (sister chromatids).
In non-dividing cells, each chromosome consists of one DNA molecule.

Cell Cycle Phases

Overview of the Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic (M) phase.

G1 phase: Cell grows and performs normal metabolic activities.
S phase: DNA synthesis/replication occurs.
G2 phase: Cell prepares for division by producing proteins and organelles.
M phase: Includes mitosis (division of genetic material) and cytokinesis (division of cytoplasm).

Label	Phase	Description
A	Preparation for cell division (G1, S, G2)	Cell grows, replicates DNA, and prepares for mitosis
B	Growth	Intense metabolic activities
C	Synthesis	Replication of DNA
D	Cell growth	Cell grows more, makes proteins and organelles
E	Cell division	Division of genetic material (prophase, metaphase, anaphase, telophase)
F	Cytokinesis	Division of cytoplasm resulting in two identical daughter cells

Mitosis

Stages of Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets, resulting in two daughter cells.

Stage	Description	Visualization
Prophase	Chromatin condenses into chromosomes; mitotic spindle forms; nuclear envelope breaks down	Chromosomes visible, spindle forming
Metaphase	Chromosomes align at the metaphase plate (center of the cell)	Chromosomes lined up in the middle
Anaphase	Sister chromatids separate and move toward opposite poles	Chromatids pulled apart
Telophase	Nuclear envelope reforms; chromosomes decondense	Two nuclei forming
Cytokinesis	Division of cytoplasm; animal cells form a cleavage furrow, plant cells form a cell plate	Two separate daughter cells

After mitosis, each daughter cell has the same amount of DNA as the parent cell.

Cell Cycle Regulation

Checkpoints in the Cell Cycle

Checkpoints are control points where stop and go-ahead signals regulate the cell cycle.

Checkpoint	Description	Visualization
G1 checkpoint	Checks for cell size, nutrients, growth factors, and DNA damage before DNA replication	Cell cycle diagram with G1 checkpoint
G2 checkpoint	Checks for DNA damage after replication and ensures all DNA is duplicated	Cell cycle diagram with G2 checkpoint
M checkpoint	Ensures all chromosomes are attached to spindle fibers before anaphase	Cell cycle diagram with M checkpoint

Cells that do not pass checkpoints may enter a non-dividing state called G0.
External signals, such as growth factors, can stimulate cell division.
Cells require attachment to a surface (anchorage dependence) and sufficient space (density-dependent inhibition) to divide.

Cancer and Loss of Cell Cycle Control

Characteristics of Cancer Cells

Cancer results from uncontrolled cell division due to the loss of normal cell cycle regulation.

Cancer cells do not exhibit anchorage dependence or density-dependent inhibition.
They continue to divide even when crowded and can form masses called tumors.
Benign tumors remain at the original site; malignant tumors invade surrounding tissues and can metastasize (spread to other parts of the body).

Tumor Type	Description	Visualization
Benign	Abnormal cells remain at the original site	Cluster of cells in one location
Malignant	Cells invade surrounding tissues and can metastasize	Cells spreading to other tissues/organs

Malignant cells can travel through the blood or lymphatic system to form new tumors elsewhere (metastasis).

Summary Table: Normal vs. Cancer Cell Growth

	Normal Mammalian Cells	Cancer Cells
Growth in culture	Stop dividing when a single layer is formed (contact inhibition)	Continue dividing, pile up, and form multiple layers
Anchorage dependence	Require attachment to a surface	No anchorage dependence; can grow without attachment

Key Terms

Chromatid: One of two identical halves of a duplicated chromosome.
Centromere: Region where sister chromatids are joined.
Spindle fibers: Protein structures that separate chromosomes during mitosis.
Metaphase plate: Imaginary plane where chromosomes align during metaphase.
Cleavage furrow: Indentation that begins the process of cytokinesis in animal cells.
Cell plate: Structure that forms during cytokinesis in plant cells, leading to cell wall formation.

Example: Human Cell Chromosome Number

Human somatic cells have 46 chromosomes (2n = 46).
After DNA replication (S phase), each chromosome consists of two sister chromatids (still counted as 46 chromosomes, but 92 DNA molecules).
After mitosis, each daughter cell receives 46 chromosomes.