Mitosis and the Cell Cycle

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic phase (M-phase), which includes mitosis and cytokinesis.

• G1 phase: Primary period of cell growth.

• S phase: DNA is duplicated; growth continues slowly.

• G2 phase: Cell prepares for division; growth continues slowly.

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

Mitosis: Phases and Key Events

Mitosis is the process of nuclear division that results in two genetically identical daughter cells. It is divided into four main phases:

• Prophase: Chromosomes condense and become visible; centrioles move to opposite poles; mitotic spindle forms; nuclear membrane disappears; centromeres develop into two structures for each DNA molecule.

• Metaphase: Chromosomes align at the metaphase plate (center of the cell); spindle fibers attach to centromeres.

• Anaphase: Centromeres split; sister chromatids (now daughter chromosomes) are pulled to opposite poles by the spindle fibers; requires ATP and contractile proteins.

• Telophase: Chromosomes arrive at poles; spindle disassembles; nuclear membranes reform; chromosomes uncoil into chromatin.

• Cytokinesis: Cytoplasm divides via a contractile ring, forming a cleavage furrow; results in two identical daughter cells.

Summary of Mitosis and Cytokinesis

Mitosis and cytokinesis ensure that each daughter cell receives an identical set of chromosomes. This process is essential for growth, tissue repair, and asexual reproduction in multicellular organisms.

• Diploid cells: Human somatic cells have 46 chromosomes (23 pairs).

• Examples of mitosis: Epithelial cell renewal, stomach lining repair, red blood cell production in bone marrow.

Abnormal Cell Division

Uncontrolled cell division can lead to cancer. Malignant cells divide uncontrollably and may spread to other tissues.

• Causes: Mutations from carcinogens or viruses.

• Treatments: Surgical removal, radiation, chemotherapy (inhibits cell division).

Meiosis: Formation of Gametes

Overview and Purpose

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing haploid gametes (sperm and eggs). This process introduces genetic diversity through recombination and independent assortment.

• Haploid cells: Contain 23 chromosomes (one set).

• Fertilization: Fusion of haploid gametes restores the diploid number in the zygote.

Stages of Meiosis

Meiosis consists of two successive divisions: Meiosis I and Meiosis II, each with four phases (prophase, metaphase, anaphase, telophase).

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

Prophase I: Crossing Over

During prophase I, homologous chromosomes pair up and exchange genetic material in a process called crossing over, resulting in genetic recombination.

Meiosis in Males and Females

Meiosis produces sperm in males (spermatogenesis) and eggs in females (oogenesis), with distinct differences in the process and outcomes.

• Males: Four equal-sized sperm are produced from each primary spermatocyte.

• Females: One large egg and smaller polar bodies are produced; meiosis II completes only after fertilization.

Comparison of Mitosis and Meiosis

Regulation of Cell Reproduction

Cell Cycle Checkpoints

Cell division is tightly regulated by internal and external mechanisms. Checkpoints ensure that cells only proceed to the next phase when conditions are favorable.

• Cyclins: Proteins that activate regulatory proteins to initiate cell cycle events.

• Checkpoints: Located at the middle of G1, end of G2, and middle of M phase.

• External influences: Nutrient and hormone availability can affect cell cycle progression.

Cell Differentiation and Environmental Influence

Cell Differentiation

Cell differentiation is the process by which cells become specialized in structure and function. This is driven by selective gene expression and can occur at any stage of development.

• Environmental factors: Local signals and conditions influence gene expression and cell fate.

Factors Affecting Fetal Development

External factors can impact embryonic and fetal development, potentially causing abnormalities or affecting survival.

• Cigarettes: Increased cancer risk in offspring.

• Alcohol: Fetal alcohol syndrome.

• Drugs: Thalidomide, marijuana, heroin, cocaine, etc.

• Chemicals: DDT, formaldehyde, PCBs, lead, etc.

• Radiation: Ionizing and non-ionizing radiation.

• Infections: HIV, syphilis, rubella (German measles).