Cell Division and Cycle

Overview

This lecture covers the process of cell division, focusing on mitosis, cytokinesis, the cell cycle, chromosome structure, the relationship between cell division and cancer, and theories of cellular aging. It explains how cells reproduce, how genetic material is managed during division, and how errors in these processes can lead to diseases like cancer.

Cell Division and Cell Theory

• The cell is the fundamental unit of life; nothing smaller than a cell is considered living.
• All living organisms are made up of one or more cells.
• New cells arise only from pre-existing cells through cell division.
• Cell division involves a parent cell dividing to form two daughter cells, which are genetically identical to each other and to the parent cell.
• This process is essential for growth (from a single zygote to trillions of cells), development, tissue repair (such as healing after a sunburn or injury), and, in some organisms, regeneration of lost body parts (e.g., lizard tails, sea star arms).

Chromosomes, Chromatin, and DNA Replication

• Before a cell divides, it must duplicate all its genetic material so each daughter cell receives a complete set of chromosomes.
• Chromosomes exist as homologous pairs—one from each parent. Homologous chromosomes are similar but not identical.
• DNA replication produces identical sister chromatids, which are joined at a region called the centromere.
• Chromatin is the uncondensed form of DNA found in non-dividing cells; it condenses into visible chromosomes during cell division.
• Chromosome structure includes the centromere (the constricted region where sister chromatids are joined) and telomeres (the protective ends of chromosomes).
• Genes are sequences of DNA (A, T, C, G) that code for proteins or functional RNA, and are located on chromosomes.

The Cell Cycle

• Non-dividing cells are in the G₀ (G naught) phase, outside the active cell cycle.
• When signaled to divide, a cell enters the cell cycle, which consists of:
  • G1 phase (Gap 1): cell grows and prepares for DNA replication.
  • S phase (Synthesis): DNA is replicated, creating sister chromatids.
  • G2 phase (Gap 2): cell grows further and prepares for division.
• G1, S, and G2 together are called interphase, making up about 90% of the cell cycle.
• The mitotic phase (about 10% of the cycle) includes mitosis (division of the nucleus and chromosomes) and cytokinesis (division of the cytoplasm).

Mitosis and Cytokinesis

• Prophase: Chromatin condenses into chromosomes; the nuclear envelope disintegrates; mitotic spindles form and centrosomes move to opposite poles; spindle fibers attach to chromosomes.
• Metaphase: Chromosomes (sister chromatids) align along the metaphase plate (center of the cell), guided by spindle fibers.
• Anaphase: Spindle fibers shorten, pulling sister chromatids apart to opposite sides of the cell; the cell elongates.
• Telophase: Chromosomes decondense back into chromatin; nuclear envelopes reform around each set of chromosomes; spindle fibers retract.
• Cytokinesis: The cytoplasm divides, usually overlapping with telophase. A cleavage furrow forms, pinching the cell into two identical daughter cells.

Cancer and Cell Cycle Control

• Cancer is the result of uncontrolled cell division, often due to the failure of cell cycle checkpoints.
• The cell cycle has built-in checkpoints (after G1, S, G2, and during mitosis) to ensure DNA is properly replicated and errors are corrected before division continues.
• Cancer cells bypass these checkpoints, dividing uncontrollably, forming tumors, invading nearby tissues, and potentially spreading (metastasis) through blood or lymphatic vessels.
• Tumors can be benign (non-cancerous) or malignant (cancerous and invasive).
• Early detection is crucial; treatment options include surgical removal, chemotherapy (which targets dividing cells), and radiation.
• Prevention strategies include avoiding carcinogens (e.g., tobacco, excessive UV exposure), eating a healthy diet, regular medical checkups, and self-examinations.

Cell Structure and Function

• The structure of a cell is closely related to its function:
  • Fibroblasts: Found in connective tissue, produce fibers.
  • Red blood cells (erythrocytes): Biconcave shape increases surface area for gas exchange; lack a nucleus.
  • Epithelial cells: Have multiple sides to connect and form protective sheets (e.g., skin, stomach lining).
  • Skeletal muscle fibers: Long, rod-shaped cells that contract to produce movement.
  • Fat cells (adipocytes): Store fat in large lipid droplets.
  • Macrophages: Have pseudopods to engulf and digest foreign material.
  • Neurons: Long processes for transmitting signals throughout the body.
  • Oocytes: Largest cell in the human body.
  • Sperm cells: Only motile human cell, with a flagellum for movement.

Theories of Aging

• Free Radical Theory: Over time, reactive molecules (free radicals) produced during metabolism damage DNA, proteins, and cell membranes, contributing to aging.
• Mitochondrial Theory: Aging is linked to decreased ATP production by mitochondria, leading to reduced cellular energy and function.
• Genetic Theory: Each time a cell divides, telomeres (the ends of chromosomes) shorten. When telomeres become too short, essential genes may be lost, leading to cell aging and malfunction.
  • The enzyme telomerase can lengthen telomeres, helping to maintain chromosome integrity and delay aging.

Key Terms & Definitions

• Mitosis: Division of the nucleus and chromosomes, resulting in two identical daughter cells.
• Cytokinesis: Division of the cytoplasm, completing cell division.
• Homologous chromosomes: Similar chromosome pairs, one from each parent.
• Sister chromatids: Identical copies of a chromosome, joined at the centromere after DNA replication.
• Genome: The complete set of an organism’s genes.
• Gene: A DNA sequence that codes for a protein or functional RNA.
• Centromere: The region where sister chromatids are joined.
• Telomere: The protective DNA at the ends of chromosomes.
• Checkpoint: A control point in the cell cycle that ensures proper division.
• Metastasis: The spread of cancer cells to other parts of the body.
• Telomerase: An enzyme that restores telomere length, helping to maintain chromosome stability.

Action Items / Next Steps

• Review and understand the stages of mitosis and their key events.
• Memorize the main phases of the cell cycle and their sequence.
• Learn how cell cycle checkpoints prevent errors and how their failure can lead to cancer.
• Study the definitions of key terms for exams and assessments.
• Practice identifying different cell types and relating their structure to their function.
• Be familiar with the main theories of aging and the role of telomeres and telomerase.