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 two 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 main 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. It is subdivided into G1, S, and G2 phases.

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

Interphase

Phases of Interphase

Interphase is the longest phase of the cell cycle, comprising about 90% of the cycle. Most cells spend their time in interphase, performing their usual 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, synthesizes proteins (including microtubules), and checks for DNA replication errors.

• G0 Phase: Some cells (e.g., nerve and muscle cells) exit the cell cycle and remain in interphase, not actively dividing.

Checkpoints and Apoptosis

• Checkpoints exist at G1, G2, and M phases to ensure proper progression.

• Apoptosis: Programmed cell death occurs if errors are detected and cannot be repaired, preventing propagation of damaged cells.

Mitosis and Cytokinesis

Stages of Mitosis

Mitosis is the process by which the nucleus divides, ensuring each daughter cell receives an identical set of chromosomes. It is followed by cytokinesis, which divides the cytoplasm.

• Prophase: Chromatin condenses into visible chromosomes; nucleolus disappears; nuclear envelope fragments; spindle fibers assemble; centrioles move apart (forming asters in animal cells).

• 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: Division of cytoplasm, resulting in two genetically identical daughter cells.

Functions of Mitosis

• Growth and development (e.g., embryonic development, tissue formation).

• Repair (e.g., healing cuts, replacing shed skin cells).

• Asexual reproduction in some eukaryotes (e.g., protists, yeast).

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.

• 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 somatic cells.

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

• Humans: Diploid number = 46, Haploid number = 23.

Cell Cycle Regulation

Cell Signals and Checkpoints

• Cell cycle is controlled by internal and external signals.

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

• Internal signals: Cyclins (proteins whose levels fluctuate during the cell cycle).

• Specific cyclins must be present for progression past G1, G2, and M checkpoints.

Summary of Checkpoints

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

• G2 Checkpoint: Ensures DNA has replicated properly; triggers apoptosis if errors cannot be repaired.

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

Prokaryotic Cell Division: Binary Fission

Binary Fission in Prokaryotes

Prokaryotes (bacteria and archaea) divide by binary fission, a simpler process than mitosis. They have a single, circular chromosome located in the nucleoid region.

• Chromosome replicates into two rings prior to division.

• Replicated rings attach to the plasma membrane.

• Cell wall and membrane grow inward, separating the two chromosomes.

• Produces two genetically identical daughter cells.

• Binary fission is rapid (e.g., E. coli divides every 20 minutes).

Comparison of Cell Division in Prokaryotes and Eukaryotes

Table: Cell Division Processes and Functions

Key Terms and Concepts

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

• Interphase: Period of growth and DNA replication.

• Mitosis: Nuclear division resulting in two identical nuclei.

• Cytokinesis: Division of cytoplasm.

• Chromatin: DNA and associated proteins.

• Chromosome: Condensed chromatin visible during mitosis.

• 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: Prokaryotic cell division.

• Apoptosis: Programmed cell death.

• Cyclins: Proteins regulating cell cycle progression.

Relevant Equations

• Diploid and Haploid Relationship:

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

Examples and Applications

• Growth: Trees increase in height and width by mitosis.

• Repair: Healing of cuts and broken bones involves mitosis.

• Replacement: Skin cells are regularly replaced via mitosis.

• Asexual reproduction: Yeast and protists reproduce by mitosis.

• Binary fission: E. coli divides rapidly by binary fission.

Additional info: Chromosome numbers do not correlate with organism complexity. The cell cycle is tightly regulated to prevent propagation of errors, and apoptosis is a normal, protective mechanism.