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 division into 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

Subphases of Interphase

Interphase is divided into three main subphases, each with specific functions:

• 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 by synthesizing proteins, including microtubules for the mitotic spindle.

Some cells, such as nerve and muscle cells, enter a G0 phase and do not actively divide.

Checkpoints and Apoptosis

• Checkpoints exist 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 Stage (M Phase)

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 (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 (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

• 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 and are required for progression through checkpoints.

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

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).

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

• Mitosis enables growth (e.g., plant stems, tree width), development (embryo to fetus to adult), and repair (healing cuts, broken bones).

• Cell replacement: Skin cells, gut lining, and other tissues are regularly replaced via 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 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:

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.

• Cytokinesis: Division of the cytoplasm.

• Chromatin: DNA and associated proteins.

• Chromosome: Condensed chromatin visible during cell division.

• Sister chromatids: Two identical strands 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: Proteins regulating cell cycle progression.

• Growth factors: External signals promoting cell growth.

Relevant Equations

• Diploid and Haploid Numbers:

Summary

The cell cycle is fundamental to the growth, development, and maintenance of all eukaryotic organisms. Mitosis ensures genetic continuity, while binary fission provides rapid reproduction in prokaryotes. Understanding these processes is essential for studying cellular biology and inheritance.

Additional info: Academic context and definitions were expanded for clarity and completeness. Table 9.3 and figures referenced in the lecture were summarized and recreated for study purposes.