The Cell Cycle and Mitosis

Overview of the Cell Cycle

The cell cycle is the series of events that cells undergo as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis). Understanding the cell cycle is essential for grasping how cells replicate, repair, and maintain genetic stability.

• G1 phase: Cell grows and performs normal functions.

• S phase: DNA is replicated; chromosomes duplicate to form sister chromatids.

• G2 phase: Cell prepares for division; checks for DNA errors.

• M phase (Mitosis): Division of the nucleus and distribution of chromosomes.

• G0 phase: Non-dividing state; cells exit the cycle and perform specialized functions.

Interphase includes G1, S, and G2 phases, where the cell is not actively dividing but is preparing for mitosis.

Phases of Mitosis

Mitosis is the process by which a eukaryotic cell divides its nucleus and distributes identical chromosomes to two daughter cells. It consists of several distinct phases:

• Prophase: Chromosomes condense, spindle forms, centrosomes move apart.

• Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

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

Cytokinesis is not part of mitosis but follows it, completing cell division. In animal cells, cytokinesis occurs via cleavage furrow; in plant cells, a cell plate forms.

Chromosomes and Chromatids

During the cell cycle, chromosomes undergo structural changes:

• Chromosome: A DNA molecule with associated proteins.

• Sister chromatids: Identical copies of a chromosome, joined at the centromere after DNA replication.

• Centromere: Region where sister chromatids are attached.

In S phase, DNA is in the form of euchromatin (loosely packed for replication). During mitosis, DNA condenses into heterochromatin (tightly packed).

Cell Cycle Checkpoints and Regulation

Cell cycle progression is regulated by checkpoints that ensure proper division and prevent errors:

• G1 checkpoint: Checks for cell size, nutrients, DNA integrity.

• G2 checkpoint: Ensures DNA replication is complete and error-free.

• M checkpoint: Verifies chromosome attachment to spindle before anaphase.

Regulatory proteins such as cyclins and cyclin-dependent kinases (CDKs) control checkpoint transitions. Failure of checkpoints can lead to uncontrolled cell division and cancer.

Binary Fission vs. Mitosis

Prokaryotic cells divide by binary fission, a simpler process than mitosis in eukaryotes:

• Binary fission: DNA replicates at the OriC (origin of replication), cell elongates, and divides.

• Mitosis: Involves complex chromosome condensation, spindle formation, and nuclear division.

Prokaryotes have circular chromosomes and lack a nucleus; eukaryotes have linear chromosomes and a defined nucleus.

Role of the Cytoskeleton and Centrosome

The cytoskeleton is crucial for mitosis, especially microtubules:

• Microtubules: Form the mitotic spindle, separate chromosomes.

• Centrosome: Microtubule organizing center; duplicates before mitosis.

• Centriole: Structure within the centrosome; not essential for spindle formation in all cells.

Motor proteins move chromosomes along microtubules during mitosis.

Somatic vs. Gametic Cells

Somatic cells are all body cells except gametes; they divide by mitosis. Gametic cells (sperm and egg) are produced by meiosis and are haploid.

• Somatic cells: Diploid, produced throughout life.

• Gametic cells: Haploid, produced during reproductive events.

Cancer and Loss of Cell Cycle Control

Loss of cell cycle regulation can result in cancer:

• Benign tumor: Non-invasive, localized growth.

• Malignant tumor: Invasive, can spread (metastasize) to other tissues.

• Metastasis: Movement of cancer cells to distant sites.

Key genes involved:

• Proto-oncogene: Normal gene promoting cell growth; mutation can create an oncogene (cancer-causing).

• Tumor suppressor: Gene inhibiting cell division; loss leads to uncontrolled growth.

Summary Table: Cell Cycle Phases and Events

Key Vocabulary

• Microtubule: Cytoskeletal filament involved in chromosome movement.

• Binary fission: Prokaryotic cell division.

• OriC: Origin of replication in prokaryotes.

• Chromosome: DNA molecule with proteins.

• Sister chromatids: Duplicated chromosomes joined at centromere.

• Centromere: Region joining sister chromatids.

• Mitotic spindle: Structure separating chromosomes.

• Daughter cells: Resulting cells after division.

• Cellular checkpoints: Control points in cell cycle.

• Benign vs. malignant tumors: Non-invasive vs. invasive cancer.

• Metastasis: Spread of cancer cells.

• Proto-oncogene: Normal growth gene.

• Oncogene: Mutated, cancer-causing gene.

• Tumor suppressor: Gene preventing uncontrolled growth.

Important Equations and Concepts

• DNA content after S phase:

• Cell cycle regulation:

Example: Cell Cycle Checkpoint Failure

If a cell fails a checkpoint (e.g., DNA damage not repaired at G2), it may proceed to mitosis with errors, leading to mutations and potential cancer development.

Comparison: Plant vs. Animal Cytokinesis

Additional info:

• Most cells in the human body are in G0 phase, performing specialized functions.

• The G1 checkpoint is often considered the most important, as it determines whether a cell will proceed to division or enter G0.

• Motor proteins (e.g., kinesin, dynein) move chromosomes along microtubules during mitosis.