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 cycle 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, synthesizes proteins (including microtubules), and checks for DNA replication errors.

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 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, distributing chromosomes equally to two daughter nuclei. 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: >1000.

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

• Haploid (n): One set of chromosomes (found in gametes).

Chromatid and Chromosome Replication

• During S phase, each chromosome is replicated, forming two genetically identical sister chromatids joined at the centromere.

• During mitosis, sister chromatids separate, becoming 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, organs, and eventually adult organisms.

Cell Cycle Control

Cell Signals and Regulation

• Cell signals are agents (often proteins) that influence cell activities.

• 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 monitor DNA integrity, replication, and chromosome alignment.

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, separating chromosomes.

• 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 processes and functions in prokaryotes and eukaryotes:

Key Terms and Definitions

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

• Interphase: The stage where the cell grows, doubles organelles, and replicates DNA.

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

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

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

• Chromosome: Condensed chromatin visible during cell division.

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

• Centromere: The region where sister chromatids are joined.

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

• Haploid (n): One set of chromosomes (found in gametes).

• Binary fission: Asexual cell division in prokaryotes.

• Apoptosis: Programmed cell death.

• Cyclins: Proteins regulating cell cycle progression.

Relevant Equations

• Diploid and Haploid Numbers:

\text{For humans: } 2n = 46, n = 23$

Example: In humans, body cells are diploid (2n = 46), while gametes (sperm and egg) are haploid (n = 23).

Additional info: Chromosome numbers do not correlate with organism complexity; each species has a unique characteristic number.