The Cell Cycle

Overview of the Cell Cycle

The cell cycle is a series of stages that a eukaryotic cell undergoes from one division to the next. It ensures that cells grow, replicate their DNA, and divide properly to produce two genetically identical daughter cells. Errors in the cell cycle can have catastrophic consequences for cellular health.

• Eukaryotic cells have a true, membrane-bound nucleus containing chromosomes.

• The cell cycle is divided into two major stages: Interphase and the Mitotic (M) stage.

• Before division, the cell grows, doubles its organelles, and replicates its DNA to ensure each daughter cell receives the necessary components.

Main Stages of the Cell Cycle

• Interphase: The cell grows, performs its normal functions, and replicates its DNA. This phase comprises about 90% of the cell cycle.

• Mitotic (M) Stage: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Interphase

Subphases of Interphase

Interphase is subdivided into three stages, each with distinct roles:

• G1 Phase (Gap 1): The cell recovers from previous division, grows in size, increases its organelles, and accumulates materials for DNA synthesis.

• S Phase (Synthesis): DNA replication occurs, producing two identical sister chromatids for each chromosome. Proteins associated with DNA are also synthesized.

• G2 Phase (Gap 2): The cell prepares for mitosis by synthesizing proteins, including microtubules necessary for chromosome separation.

Some cells, such as nerve and muscle cells, exit the cell cycle and remain in a non-dividing state called the G0 phase.

Checkpoints and Cell Cycle Control

The cell cycle is regulated by checkpoints that ensure proper progression and prevent the division of damaged cells.

• G1 Checkpoint: Checks for DNA damage and ensures growth signals and nutrients are present. If errors are detected, the cell may undergo apoptosis (programmed cell death).

• G2 Checkpoint: Verifies that DNA replication in the S phase was successful and error-free.

• M Checkpoint: Ensures chromosomes are properly aligned before division proceeds.

Cell cycle progression is controlled by internal signals (such as cyclins) and external signals (such as growth factors).

Chromosomes and Chromatin Structure

Organization of Genetic Material

Each eukaryotic chromosome contains a single DNA double helix molecule, packaged with proteins to form chromatin. Chromatin is further organized into structures called nucleosomes, where DNA is wound around histone proteins.

• Nucleosome: DNA wrapped around a core of eight histone molecules.

• Chromatin condenses into visible chromosomes during mitosis.

Chromosome Numbers and Types

• Each species has a characteristic chromosome number.

• Diploid (2n): Two sets of chromosomes (one from each parent). In humans, 2n = 46.

• Haploid (n): One set of chromosomes, found in gametes (sperm and egg). In humans, n = 23.

During DNA replication, each chromosome forms two identical sister chromatids joined at the centromere.

Mitosis and Cytokinesis

Stages of Mitosis

Mitosis is the process by which the nucleus divides, ensuring each daughter cell receives an identical set of chromosomes. It is divided into several stages:

1. Prophase: Chromatin condenses into visible chromosomes; the nucleolus disappears; the nuclear envelope fragments; spindle fibers begin to form; centrioles move apart (in animal cells, forming asters).

2. Metaphase: Chromosomes align at the cell's equator (metaphase plate); spindle fibers attach to kinetochores on chromosomes; a checkpoint ensures proper alignment.

3. Anaphase: Centromeres dissolve; sister chromatids separate and are pulled to opposite poles of the cell by spindle fibers.

4. Telophase: Spindle fibers disappear; new nuclear envelopes form around the daughter chromosomes; chromosomes decondense back into chromatin; cleavage furrow begins to form (in animal cells).

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two genetically identical daughter cells. In animal cells, this occurs via the formation of a cleavage furrow; in plant cells, a cell plate forms.

Functions of Mitosis

• Growth and development (e.g., from fertilized egg to embryo to adult)

• Tissue repair (e.g., healing cuts, replacing skin cells)

• Asexual reproduction in some organisms

Comparison: Eukaryotic Mitosis vs. Prokaryotic Binary Fission

Prokaryotic Cell Division: Binary Fission

Prokaryotes (bacteria and archaea) lack a nucleus and divide by binary fission, a simpler process than mitosis.

• Single, circular chromosome replicates.

• Replicated chromosomes attach to the plasma membrane.

• Cell elongates, and the plasma membrane and cell wall grow inward, dividing the cell into two identical daughter cells.

• Binary fission is a form of asexual reproduction and can occur rapidly (e.g., Escherichia coli divides every ~20 minutes).

Comparison Table: Eukaryotic Mitosis vs. Prokaryotic Binary Fission

Key Terms and Definitions

• Apoptosis: Programmed cell death, a normal process to eliminate damaged or unnecessary cells.

• Cyclins: Internal regulatory proteins that control progression through the cell cycle.

• Growth Factors: External signaling proteins that stimulate cell growth and division.

• Centromere: Region where sister chromatids are joined together.

• Mitotic Spindle: Structure made of microtubules that separates chromosomes during mitosis.

• Cleavage Furrow: Indentation that begins the process of cytokinesis in animal cells.

• Metaphase Plate: Imaginary plane where chromosomes align during metaphase.

Formulas and Equations

• Diploid Number (2n):

• Haploid Number (n):

Summary

• The cell cycle is essential for growth, repair, and reproduction in eukaryotic organisms.

• It is tightly regulated by checkpoints and signaling molecules to ensure genetic stability.

• Mitosis produces two genetically identical daughter cells, while prokaryotes use binary fission for asexual reproduction.

Additional info: Some figures and tables referenced (e.g., 9.5, 9.6, 9.13) are from a standard biology textbook and can be reviewed for visual understanding. The summary table and some definitions were expanded for clarity and completeness.