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, duplicate their contents, and divide accurately to produce healthy daughter cells. Errors in the cell cycle can lead to catastrophic consequences for the cell.

• 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 the G0 phase.

Checkpoints and Apoptosis

• Checkpoints occur at G1, G2, and M stages to ensure proper progression.

• 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 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.

Chromosome Structure and Numbers

Chromatin and Chromosomes

• Chromatin: DNA double helix plus structural proteins (histones).

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

• 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: over 1,000.

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

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

Chromatid and Chromosome Separation

• During S phase, chromosomes are duplicated, forming sister chromatids joined at the centromere.

• During mitosis, sister chromatids separate to become daughter chromosomes.

Cell Cycle Regulation

Cell Signals and Checkpoints

• Cell cycle is controlled by internal and external signals.

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

• Internal signals: Cyclins, proteins whose levels fluctuate during the cell cycle; specific cyclins must be present for progression.

• Checkpoints:

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

  • G2 checkpoint: Ensures DNA replication is accurate; triggers apoptosis if errors cannot be repaired.

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

Functions of Mitosis

Growth, Repair, and Replacement

• Growth: Plants and animals grow by adding new cells via mitosis.

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

• Replacement: Cells such as skin and gut lining are regularly replaced through mitosis.

• Development: Fertilized eggs divide to form embryos, tissues, organs, and eventually adult organisms.

Cell Division in Prokaryotes

Binary Fission

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

• Single, circular chromosome is replicated.

• Replicated chromosomes attach to the plasma membrane.

• Cell elongates, membrane and cell wall grow inward, and cytoplasm is distributed equally.

• Result: Two genetically identical daughter cells.

• Binary fission is rapid; Escherichia coli can divide every 20 minutes.

Comparison of Cell Division: Prokaryotes vs. Eukaryotes

Summary Table

The following table compares cell division in prokaryotes and eukaryotes, including function and process:

Key Terms and Definitions

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

• Interphase: The stage where the cell grows, performs normal functions, and prepares for division.

• Mitosis: Division of the nucleus into two identical nuclei.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

• Chromatin: DNA and associated proteins in a non-condensed state.

• Chromosome: Condensed chromatin visible during cell division.

• Chromatid: One of two identical strands of a duplicated chromosome.

• Centromere: Region where sister chromatids are joined.

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

• Haploid (n): One set of chromosomes.

• Binary fission: Asexual cell division in prokaryotes.

• Apoptosis: Programmed cell death.

• Cyclins: Proteins regulating cell cycle progression.

Relevant Equations and Abbreviations

• Diploid number:

• Haploid number:

• Human chromosome numbers: ,

Summary

The cell cycle is essential for growth, repair, and reproduction in eukaryotic cells, with mitosis ensuring accurate distribution of genetic material. Prokaryotes divide by binary fission, a simpler process. Regulation of the cell cycle through checkpoints and signals maintains cellular health and prevents propagation of errors.