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, replication of cellular components, and accurate distribution of genetic material to 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 cycle in interphase.

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

Interphase

Subphases of Interphase

Interphase is divided into three main 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 attached at the centromere.

• G2 Phase (Gap 2): 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 phases 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 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 (now daughter chromosomes) are pulled to opposite poles of the cell.

• 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 in concentration and are required for progression through checkpoints.

• Checkpoints monitor DNA integrity, replication accuracy, and chromosome alignment.

Chromosome Structure and Number

Chromatin and Chromosomes

Chromatin is composed of DNA and structural proteins (histones). DNA wraps around histone cores forming nucleosomes, which further fold and coil to fit within the nucleus.

• 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 somatic (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, distributed to each new cell.

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 mitotic cell division.

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

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 the plasma membrane.

• Cell elongates, membrane and wall grow inward, and cytoplasm divides.

• Result: Two genetically identical daughter cells.

• Example: Escherichia coli divides every 20 minutes.

Comparison of Cell Division: Prokaryotes vs. Eukaryotes

Table: Cell Division Processes and Functions

Additional info: Table adapted from Table 9.3 in the textbook; entries inferred for clarity.

Key Terms and Definitions

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

• Interphase: Period of cell growth and DNA replication.

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

• Cytokinesis: Division of the cytoplasm.

• 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: 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 signals promoting cell division.

Summary

The cell cycle is fundamental to the growth, maintenance, and reproduction of all living organisms. Eukaryotic cells undergo mitosis and cytokinesis, while prokaryotes divide by binary fission. Proper regulation and checkpoints ensure genetic stability and prevent errors that could compromise cell function or organismal health.