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, producing healthy 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 attached at the centromere.

• 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, becoming daughter chromosomes.

• 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.

Control of the Cell Cycle

Cell Signals and Regulation

• External signals: Growth factors (signaling proteins) influence cell growth and division.

• Internal signals: Cyclins (regulatory proteins) fluctuate during the cell cycle; specific cyclins must be present for progression past checkpoints.

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: Verifies chromosomes are properly aligned before division.

Chromosome Structure and Number

Chromatin and Chromosomes

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

• Nucleosome: DNA wound around a core of eight histone molecules; helps compact DNA for storage.

• Chromatin condenses into visible chromosomes during mitosis.

Chromosome Numbers

• Each species has a characteristic chromosome number.

• Examples: Yeast (32), Southern adder's tongue fern (>1000), Fruit fly (8), Human (46), Goldfish (94).

Diploid and Haploid Numbers

• 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).

• For humans: Diploid number = 46, Haploid number = 23.

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.

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 such as skin and gut lining are regularly replaced through mitosis.

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

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 wall grow inward, and cytoplasm is distributed equally.

• Result: Two genetically identical daughter cells.

• Binary fission is rapid; e.g., Escherichia 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: The sequence of events from one cell division to the next.

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

• Mitosis: Division of the nucleus into two genetically 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.

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

• Centromere: The 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: Regulatory proteins controlling cell cycle progression.

• Growth factors: External signaling proteins influencing cell division.

Important Equations and Abbreviations

• Diploid number:

• Haploid number:

• For humans: ,

Summary

The cell cycle is essential for growth, repair, and reproduction in eukaryotic cells, with mitosis ensuring accurate distribution of genetic material. Prokaryotes use binary fission for rapid, asexual reproduction. Understanding the stages, regulation, and functions of cell division is fundamental to biology.

Additional info: Some diagrams and figures referenced (e.g., 9.5, 9.6, 9.13, 9.3) are from standard biology textbooks and can be reviewed for visual understanding. The table above is inferred from the lecture's summary of cell division functions.