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 proper growth, DNA replication, and division, producing two genetically identical daughter cells. Errors in the cell cycle can be catastrophic for cell health.

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

• The cell cycle consists of two major stages: Interphase and the Mitotic (M) stage.

• Before division, the cell grows larger, doubles its organelles, and replicates its DNA.

Stages of the Cell Cycle

• Interphase: The cell grows, performs normal functions, and prepares for division. Most cells spend about 90% of their time in interphase.

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

Interphase

Substages of Interphase

Interphase is divided into three substages, each with specific functions and checkpoints to ensure proper cell cycle progression.

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

• S (Synthesis) Stage: DNA is synthesized and replicated. Chromosomes enter with one chromatid and leave with two identical sister chromatids.

• G2 (Gap 2) Stage: Cell prepares for mitosis by synthesizing proteins, including microtubules for the mitotic spindle.

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

Checkpoints and Apoptosis

• Checkpoints at G1, S, and G2 ensure the cell is ready to proceed.

• If errors are detected, the cell may undergo apoptosis (programmed cell death) to prevent propagation of damaged cells.

Mitotic Stage (M Stage)

Mitosis

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

• Prophase: Chromatin condenses into visible chromosomes; nucleolus disappears; nuclear envelope fragments; spindle fibers assemble; centrioles move apart (in animal cells, forming asters).

• Metaphase: Chromosomes align at the cell's center (metaphase plate); spindle fibers attach to kinetochores; checkpoint ensures proper alignment.

• Anaphase: Centromeres dissolve; sister chromatids separate and are pulled to opposite poles; cell elongates.

• Telophase: Spindle disappears; new nuclear envelopes form; chromosomes decondense into chromatin; cleavage furrow appears (in animal cells).

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two genetically identical daughter cells.

• In animal cells, a cleavage furrow forms to separate the cells.

• In plant cells, a cell plate forms to divide the cells.

Chromosome Structure and Numbers

Chromatin and Chromosomes

Each eukaryotic chromosome contains one DNA molecule (double helix) and structural proteins called histones. The DNA wraps around histones to form nucleosomes, which further coil and fold to fit inside the nucleus.

• Chromatin: DNA plus histone proteins, diffuse during interphase, condensed during mitosis.

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

Chromosome Numbers

• Each species has a characteristic chromosome number.

• Diploid (2n): Two sets of chromosomes (one from each parent); humans have 46 chromosomes (23 pairs).

• Haploid (n): One set of chromosomes; found in gametes (sperm and egg); humans have 23 chromosomes in gametes.

During fertilization, haploid gametes combine to restore the diploid number.

Sister Chromatids and Centromeres

Formation and Separation

• During the S phase, each chromosome is duplicated, forming two genetically identical sister chromatids joined at the centromere.

• During mitosis, centromeres separate, and sister chromatids become daughter chromosomes distributed to each new cell.

Functions of Mitosis

Growth, Repair, and Development

• Growth: Increases the number of cells in multicellular organisms (e.g., plant stems, tree trunks).

• Repair: Replaces damaged or lost cells (e.g., skin, bone, gut lining).

• Development: From fertilized egg to embryo, fetus, and adult.

Cell Cycle Control

Internal and External Signals

• External signals: Growth factors (signaling proteins) from surrounding cells.

• Internal signals: Cyclins (proteins) that fluctuate in concentration and regulate progression through checkpoints.

• Checkpoints: G1 (DNA damage), G2 (DNA replication accuracy), M (chromosome alignment).

Prokaryotic Cell Division

Binary Fission

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

• Single, circular chromosome replicates.

• Chromosomes attach to the plasma membrane.

• Cell elongates, membrane indents, and cytoplasm divides.

• Produces two genetically identical daughter cells.

• Rapid division; e.g., Escherichia coli can divide every 20 minutes.

Comparison of Cell Division in Prokaryotes and Eukaryotes

Table: Cell Division Processes and Functions

Key Terms and Definitions

• Cell cycle: Series of events from one cell division to the next.

• Interphase: Stage of growth and DNA replication.

• Mitosis: Nuclear division producing identical daughter nuclei.

• Cytokinesis: Division of cytoplasm.

• Chromatin: DNA and associated proteins.

• Chromosome: Condensed chromatin visible during mitosis.

• Sister chromatids: Identical copies of a chromosome joined at the centromere.

• Centromere: Region joining sister chromatids.

• Diploid (2n): Two sets of chromosomes.

• Haploid (n): One set of chromosomes.

• Binary fission: Prokaryotic cell division.

• Apoptosis: Programmed cell death.

• Cyclins: Proteins regulating cell cycle progression.

Equations and Notation

• Diploid number:

• Haploid number:

• Human chromosome numbers: ,

Summary

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

• Mitosis ensures genetic consistency; binary fission is the prokaryotic equivalent.

• Checkpoints and signals regulate the cycle, preventing errors and maintaining health.

Example: Skin cells divide by mitosis to replace lost cells; bacteria divide by binary fission for rapid population growth.

Additional info: Academic context was added to clarify the stages of mitosis, chromosome structure, and cell cycle regulation, as well as to reconstruct the comparison table and provide definitions for key terms.