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, doubles organelles, and replicates DNA. Most cells spend about 90% of their time in interphase.

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

Interphase

Subphases of Interphase

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

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

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

• G2 Phase (Gap 2): Cell prepares for mitosis, synthesizes proteins (including microtubules), and checks for DNA replication errors.

• G0 Phase: Some cells (e.g., nerve and muscle cells) exit the cell cycle and remain in interphase, not actively dividing.

Checkpoints and Apoptosis

• Checkpoints occur at G1, G2, and M phases to ensure proper cell function and division.

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

Mitotic (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 stages:

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

• 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.

• Occurs after mitosis.

• In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms.

Control of the Cell Cycle

Cell Signals and Regulation

The cell cycle is regulated by internal and external signals:

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

• Internal signals: Cyclins (proteins) that fluctuate in concentration; specific cyclins must be present for progression through checkpoints.

Checkpoints

• G1 Checkpoint: Checks for DNA damage; commits to division if growth signals and nutrients are present.

• G2 Checkpoint: Ensures DNA has replicated properly; triggers apoptosis if errors cannot be repaired.

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

Chromosome Structure and Numbers

Chromatin and Chromosomes

Chromatin is DNA plus structural proteins (histones) that help package DNA into the nucleus. DNA wraps around histone proteins forming nucleosomes, which further coil and fold to create chromosomes.

• Each chromosome contains one DNA double helix.

• Chromatin condenses into visible chromosomes during mitosis.

Chromosome Numbers

• Each species has a characteristic chromosome number.

• Humans: 46 chromosomes (diploid, 2n); fruit flies: 8; yeast: 32; goldfish: 94; southern adder's tongue fern: >1000.

Diploid and Haploid Cells

• Diploid (2n): Two sets of chromosomes (one from each parent); found in body cells.

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

• Fertilization restores diploid number: in humans.

Sister Chromatids and Chromosome Separation

Sister Chromatids

After DNA replication, each chromosome consists of two genetically identical sister chromatids joined at the centromere. During mitosis, centromeres separate, and sister chromatids become daughter chromosomes.

• Electron micrographs show fluffy, paired chromatids.

• During mitosis, chromatids are separated and distributed to daughter cells.

Functions of Mitosis

Growth, Repair, and Replacement

• Growth: Plants and animals increase in size by adding cells via mitosis.

• Repair: Damaged tissues (e.g., cuts, broken bones) are healed by mitosis.

• Replacement: Cells shed regularly (e.g., skin, gut lining) are replaced by mitosis.

• Development: Fertilized egg divides to form embryo, tissues, organs, and eventually a mature organism.

Cell Division in Prokaryotes

Binary Fission

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

• Single, circular chromosome replicates.

• Replicated chromosomes attach to plasma membrane.

• Cell elongates, membrane indents, and cytoplasm divides.

• Produces two genetically identical daughter cells.

• Rapid division: E. coli can divide every 20 minutes.

Comparison of Cell Division: Prokaryotes vs. 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 cell growth, organelle doubling, and DNA replication.

• Mitosis: Nuclear division producing two identical nuclei.

• Cytokinesis: Division of cytoplasm into two cells.

• Chromatin: DNA and associated proteins (histones).

• Chromosome: Condensed chromatin visible during mitosis.

• Sister chromatids: Two identical strands of a chromosome after DNA replication.

• 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.

• Growth factors: External signals promoting cell growth.

Summary of Mitosis Stages

Important Equations

• Diploid number:

• Haploid number:

• Human chromosome numbers: ,

• Fertilization:

Examples and Applications

• Growth: Trees increase in height and width by mitosis.

• Repair: Healing of cuts and broken bones involves mitosis.

• Replacement: Skin and gut lining cells are regularly replaced by mitosis.

• Binary Fission: E. coli divides rapidly by binary fission, producing identical daughter cells.

Additional info: Chromosome numbers do not correlate with organism complexity. The cell cycle is tightly regulated to prevent errors and maintain organism health.