The Cell Cycle & Mitosis: Structure, Function, and Regulation

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The Cell Cycle & Mitosis

Introduction

The cell cycle is a fundamental process in eukaryotic cells, governing cell growth, DNA replication, and division. Mitosis is a key stage in the cell cycle, responsible for producing genetically identical daughter cells, essential for growth, tissue repair, and asexual reproduction in multicellular organisms.

Mitosis: Making Genetically Identical Daughter Cells

Definition and Purpose

Mitosis is a type of cell division resulting in two daughter cells, each with the same number of chromosomes and identical genetic information as the parent cell.
Genome: All the genetic material in a cell.
Mitosis is responsible for growth (addition of somatic cells), tissue repair, and asexual reproduction.

Comparison: Mitosis vs. Meiosis

Mitosis: Produces two genetically identical somatic cells.
Meiosis: Produces gametes (egg and sperm) with half the chromosome number, restoring normal hereditary material upon fertilization.

Eukaryotic Cell Cycle

Overview of Selected Organelles

Nucleus: Contains genetic material (DNA).
Nucleolus: Site of ribosome synthesis.
Nuclear membrane: Encloses the nucleus, separating it from the cytoplasm.
Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.
Centrioles: Microtubule structures involved in spindle formation during mitosis.

Chromatin and Chromosome Structure

During interphase, DNA exists as loosely packed chromatin.
Prior to mitosis, chromatin condenses into visible chromosomes.
Each chromosome consists of DNA tightly wound around proteins (histones).

Karyotype

Definition and Significance

Karyotype: An organized profile of a person's chromosomes, arranged and numbered by size.
Human somatic cells are diploid, containing 23 pairs of chromosomes (one set from each parent).
Karyotyping is used to detect chromosomal abnormalities and study genetic inheritance.

The Cell Cycle: Phases and Regulation

Phases of the Cell Cycle

Interphase: Period of cell growth and DNA replication, divided into three phases:
- G1 (Growth 1): Active growth and synthesis of organelles.
- S (Synthesis): DNA replication; each chromosome is copied to produce sister chromatids.
- G2 (Growth 2): Further growth; cell checks for DNA errors and prepares for mitosis.
Mitotic (M) Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Chromatids & Centromeres

After DNA replication, each chromosome consists of two sister chromatids, joined at the centromere.
Sister chromatids are genetically identical and are separated during mitosis.

Mitosis: Stages and Key Events

Stages of Mitosis

Prophase: Chromatin condenses into chromosomes; nucleolus disappears; spindle fibers form.
Prometaphase: Nuclear envelope breaks down; spindle fibers attach to chromosomes at the centromere (kinetochore).
Metaphase: Chromosomes align at the cell's equatorial plate (metaphase plate).
Anaphase: Sister chromatids separate and move to opposite poles of the cell.
Telophase: Chromosomes decondense; nuclear envelope reforms; nucleolus reappears.

Cytokinesis

Division of the cytoplasm, resulting in two genetically identical daughter cells.
In animal cells, a cleavage furrow forms to separate the cells.

Cell Cycle Regulation

Checkpoints

The cell cycle is tightly regulated by checkpoints at specific stages (G1, G2, M).
Checkpoints assess DNA integrity and proper chromosome replication.
If errors are detected, the cell cycle is paused for repair or the cell undergoes apoptosis (programmed cell death).

Apoptosis

Apoptosis is a controlled process of cell death, preventing damaged cells from dividing and potentially leading to cancer.

Cancer: Cell Cycle Dysregulation

Genetic Mutations and Cancer

Cancer results from genetic changes (mutations) that disrupt normal cell cycle regulation.
Mutations may affect genes promoting cell division or those controlling checkpoints.
Uncontrolled cell division leads to tumor formation and loss of specialized cell function.

Types of Tumors

Type	Description
Benign Tumor	Mass of cells similar to normal cells; does not invade neighboring tissues; usually encapsulated.
Malignant Tumor	Mass of cells that invade neighboring tissues and can metastasize (spread to other parts of the body).

Metastasis

Metastasis: The process by which cancer cells spread from the original site to other tissues via the bloodstream or lymphatic system.

Mutagens

Mutagen: An agent that induces permanent changes in DNA. Types include:
- Radiation: X-rays, UV light
- Chemical: Pollutants, metals, tobacco
- Biological: Viruses (e.g., HPV), certain bacteria

Summary Table: Key Terms

Term	Definition
Somatic Cell	Any cell of the body except gametes; produced by mitosis.
Gamete	Sex cell (egg or sperm); produced by meiosis.
Karyotype	Organized profile of chromosomes.
Chromatin	Loosely packed DNA and proteins in the nucleus.
Chromosome	Condensed form of chromatin during cell division.
Sister Chromatids	Identical copies of a chromosome joined at the centromere.
Centromere	Region where sister chromatids are attached.
Mitotic Spindle	Microtubule structure that separates chromosomes during mitosis.
Cytokinesis	Division of the cytoplasm to form two daughter cells.
Apoptosis	Programmed cell death.

Key Equations

Chromosome Number in Mitosis: (Diploid parent cell produces two diploid daughter cells)
Chromosome Number in Meiosis: (Diploid parent cell produces four haploid gametes)

Example: Human Cell Division

Human somatic cells have 46 chromosomes (23 pairs).
After mitosis, each daughter cell has 46 chromosomes, identical to the parent cell.
After meiosis, each gamete has 23 chromosomes.

Additional info:

Cell cycle duration varies by cell type and organism; rapidly dividing cells (e.g., skin, intestine) may complete the cycle in ~12 hours, while others take longer.
Some cells (e.g., neurons, lens cells) remain in a non-dividing state (G0 phase) permanently.