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 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, 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

Subphases of Interphase

Interphase is divided into three subphases, each with distinct functions:

• G1 (Gap 1) Phase: Cell recovers from previous division, grows, increases organelles, and accumulates materials for DNA synthesis.

• S (Synthesis) Phase: DNA is synthesized and replicated. Chromosomes enter with one chromatid and leave with two identical sister chromatids attached at the centromere.

• G2 (Gap 2) Phase: 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 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 (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 four main 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); kinetochore fibers attach to chromosomes; checkpoint ensures proper alignment.

• Anaphase: Centromeres dissolve; sister chromatids separate and are pulled to opposite poles, becoming daughter chromosomes; 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.

• Occurs after mitosis.

• In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms.

Regulation of the Cell Cycle

Cell Signals and Checkpoints

• 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.

• 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: >1000.

• No direct correlation between organism size and chromosome number.

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).

• Fertilization restores diploid number in zygote.

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 egg divides to form embryo, tissues, organs, and eventually an adult organism.

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 plasma membrane.

• Cell elongates, membrane and wall grow inward, separating chromosomes.

• Produces two genetically identical daughter cells.

• Rapid division: 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 after DNA replication.

• 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.

Important Equations

• Diploid and Haploid Relationship:

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

Examples and Applications

• Example: Human somatic cells (skin, muscle, etc.) are diploid (2n = 46). Sperm and egg cells are haploid (n = 23).

• Application: Mitosis is essential for healing wounds, growing organisms, and replacing lost cells.

• Example: Binary fission in Escherichia coli allows rapid population growth in favorable conditions.

Additional info: Figures referenced (9.5, 9.6, 9.13, 9.3) are from standard biology textbooks and illustrate chromosome structure, mitosis stages, and binary fission. The table is inferred from Table 9.3 as described in the lecture.