The Cell Cycle

Overview of Cell Division

The cell cycle is a fundamental process in biology, enabling growth, development, repair, and reproduction in multicellular organisms. It consists of a series of stages that cells undergo to divide and produce new cells.

• Mitosis: Division of somatic (body) cells, resulting in genetically identical daughter cells. Essential for growth, healing, and asexual reproduction.

• Meiosis: Division of gametes (sex cells), producing cells with genetic diversity. Critical for sexual reproduction.

Example: Skin cells divide by mitosis to repair wounds, while sperm and egg cells are produced by meiosis.

Types of Cellular Reproduction

Cellular reproduction can result in either identical or genetically distinct cells, depending on the process.

• Mitosis: Produces two cells identical to the parent cell.

• Meiosis: Produces cells different from the parent cell, contributing to genetic variation.

Chromosome Structure and Function

Chromosomes and Chromatin

Chromosomes are composed of DNA and proteins, condensing only during cell division to facilitate movement and ensure accurate segregation.

• Chromatin: The DNA-protein complex that makes up chromosomes.

• Sister Chromatids: Duplicated chromosomes sharing a centromere; not considered separate chromosomes until they separate.

• Centromere: The region where sister chromatids attach.

• Kinetochore: Protein structure on the centromere where spindle fibers attach during mitosis.

Somatic Cells vs. Gametes

Multicellular organisms have two main cell types:

• Somatic Cells: Diploid (2N), created via mitosis, mutations not passed to offspring.

• Gametes: Haploid (N), created via meiosis, mutations can be inherited.

The Cell Cycle Phases

Interphase and Mitotic Phase

The cell cycle alternates between interphase (growth and normal function) and the mitotic phase (cell division).

• Interphase: 90% of the cell cycle; includes G1, S, and G2 phases.

• Mitosis: 10% of the cell cycle; includes prophase, prometaphase, metaphase, anaphase, and telophase.

G0 Phase

Some cells enter a resting phase (G0), where they are differentiated and not actively dividing. Examples include mature nerve cells and liver cells.

Stages of Interphase

• G1 Phase: Organelle duplication and preparation for DNA synthesis (5-6 hours).

• S Phase: DNA replication (10-12 hours).

• G2 Phase: Error correction and centrosome formation (4-5 hours).

Mitosis: Steps and Events

Prophase

Chromatin condenses, nucleoli disappear, mitotic spindle forms, and centrosomes move to cell poles.

Prometaphase

Nuclear envelope breaks down, chromosomes fully condense, and microtubules attach to kinetochores.

Metaphase

Chromosomes align at the metaphase plate, with centrosomes at opposite poles.

Anaphase

Proteins holding sister chromatids cleave, chromatids move to opposite poles, and the cell elongates.

• Homologous Chromosomes: Pairs with the same genes but possibly different alleles, one from each parent.

Telophase

Nuclear envelope reforms, nucleoli reappear, and chromosomes decondense.

Cytokinesis

Mechanisms in Animal and Plant Cells

Cytokinesis is the division of the cytoplasm, forming two daughter cells. The process differs between animal and plant cells:

• Animal Cells: Form a cleavage furrow that pinches inward.

• Plant Cells: Form a cell plate that becomes a new cell wall.

Example: In onion root cells, cytokinesis can be observed as the formation of a cell plate in plant cells.

Cell Replacement Time

Cell replacement rates vary by tissue type:

• Stomach: 2-9 days

• Skin Epidermis: 10-30 days

• Red Blood Cells: 4 months

• Fat Cells: 8 years

• CNS, lens, oocytes: Lifetime

Binary Fission in Prokaryotes

Process of Binary Fission

Bacteria reproduce by binary fission, a simpler process than mitosis:

• DNA replicates at the origin of replication.

• Replication proceeds bidirectionally.

• DNA moves to opposite ends, cell elongates.

• Plasma membrane pinches inward, new cell wall forms, cells separate.

Regulation of the Eukaryotic Cell Cycle

Cell Cycle Control System

The cell cycle is tightly regulated by checkpoints to ensure proper division and prevent errors.

• G1 Checkpoint: Commitment to divide; insufficient growth factor leads to exit to G0.

• G2 Checkpoint: Ensures DNA replication is complete and no damage is present.

• M Checkpoint: Ensures all kinetochores are attached to spindle fibers.

Cyclins and CDKs

Cyclins and cyclin-dependent kinases (CDKs) are proteins that drive cell cycle progression:

• Cyclin: Protein whose levels fluctuate during the cell cycle.

• CDK: Kinase that becomes active when bound to cyclin.

• Rising cyclin levels trigger mitosis and other cell cycle events.

Equation:

Cell Signaling and Cancer

Normal Signaling Pathways

Growth factors regulate cell cycle progression, differentiation, and apoptosis. Density-dependent inhibition prevents overcrowding.

Cancer: Unregulated Cell Growth

Cancer arises when cell cycle regulation fails, leading to uncontrolled cell division and loss of normal function.

• Benign Tumors: Slow growing, well differentiated, contained.

• Malignant Tumors: Fast growing, poorly differentiated, capable of metastasis.

Progression and Prevention of Cancer

Cancer cells evade immune removal, attract blood vessels, invade tissues, and cause inflammation. Prevention involves understanding genetic and lifestyle risk factors.

• Genetic Susceptibility: Example: BRCA1 gene increases breast cancer risk.

• Lifestyle Choices: Tobacco, obesity, pathogens, inactivity, diet, UV exposure.

Cancer Treatment Options

Treatment depends on cancer type, location, and risk factors. Options include:

• Physical removal of tumor

• Stem cell replacement after chemo/radiation

• Drugs targeting fast-dividing cells or specific molecules

• Radiation therapy

• Immunotherapy

• Hormone-blocking therapy