Cell Cycle and Mitosis: Structure, Regulation, and Cancer

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Cell Cycle and Mitosis

Introduction to Cell Division

Cell division is a fundamental process in all living organisms, allowing for growth, development, and tissue repair. Eukaryotic cells divide by mitosis, while prokaryotic cells use a process called binary fission. Both processes ensure the transmission of genetic material to daughter cells.

Eukaryotes (multicellular) use mitosis for both growth and repair.
Prokaryotes use binary fission to reproduce.
Both prokaryotic and eukaryotic cells undergo mitosis (in the context of eukaryotes; prokaryotes do not undergo mitosis but binary fission).

Chromosomes in Prokaryotes vs. Eukaryotes

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

	Location (nucleoid / nucleus)	Number (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.

	Function	Examples
Somatic Cells	Body (non-reproductive) cells	Skin, liver, brain, blood, etc.
Gametes	Reproductive cells	Sperm cells, egg cells

Chromosome Structure

Chromosomes are composed of chromatin, a complex of DNA and structural proteins. During cell division, chromatin condenses to form visible chromosomes.

Chromatin: Complex of DNA and proteins, not visible under a light microscope.
Chromosome: Condensed chromatin, visible during cell division.

Chromosome (Non-duplicated vs. Duplicated)

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

Number of DNA Molecules

When a cell prepares to divide, each chromosome replicates into two identical DNA molecules (sister chromatids).
In nondividing cells, each chromosome consists of one DNA molecule.

Phases of the Cell Cycle

The cell cycle is divided into several phases, each with specific functions and events.

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

G1 phase: Cell grows and carries out normal functions.
S phase: DNA is replicated.
G2 phase: Cell prepares for mitosis.
M phase (Mitosis): Division of the nucleus.
Cytokinesis: Division of the cytoplasm.

Mitosis

Mitosis is the process by which a eukaryotic cell divides its nucleus and genetic material to produce two identical daughter cells. It consists of several stages:

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 daughter cells

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

Cell Cycle Regulation

The cell cycle is tightly regulated by checkpoints that ensure proper division and prevent errors.

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 replication completeness and DNA damage before mitosis	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 (G0 phase).
Proteins called cyclins and cyclin-dependent kinases (CDKs) regulate the cell cycle.

Cancer and Loss of Cell Cycle Control

Cancer results from uncontrolled cell division due to the loss of normal cell cycle regulation. Cancer cells do not respond to typical regulatory signals.

Normal cells stop dividing when they form a single layer (anchorage dependence and density-dependent inhibition).
Cancer cells continue dividing, forming tumors.
Benign tumors: Abnormal cells remain at the original site.
Malignant tumors: Cells 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
Malignant	Cells invade surrounding tissues and can spread (metastasize)	Cells moving into blood/lymph

Metastasis: The process by which cancer cells spread to other parts of the body.

Key Terms and Concepts

Chromatin: DNA-protein complex in the nucleus.
Chromosome: Condensed chromatin, visible during cell division.
Sister chromatids: Identical copies of a chromosome, joined at the centromere.
Centromere: Region where sister chromatids are joined.
Mitotic spindle: Structure made of microtubules that separates chromosomes during mitosis.
Cytokinesis: Division of the cytoplasm.
Checkpoints: Control points in the cell cycle that ensure proper division.
Anchorage dependence: Cells must be attached to a surface to divide.
Density-dependent inhibition: Cells stop dividing when crowded.
Metastasis: Spread of cancer cells to distant sites.

Equations and Formulas

For a diploid human cell (2n = 46):

Example: In a human cell, after DNA replication but before mitosis, there are 46 chromosomes, each consisting of two sister chromatids (92 DNA molecules). After mitosis, each daughter cell has 46 chromosomes (46 DNA molecules).

Additional info: Cyclins and CDKs are key regulators of the cell cycle. Mutations in genes controlling these proteins can lead to cancer.