Chapter 8: Cellular Reproduction – Essentials of Biology Study Notes

Cellular Reproduction

Overview of Cellular Reproduction

Cellular reproduction is the process by which cells produce new cells. In multicellular organisms, this process is essential for growth, development, and tissue repair. All cells arise from pre-existing cells, as stated in the cell theory.

• Asexual reproduction: Involves a single parent and produces genetically identical offspring. Example: Binary fission in bacteria.

• Sexual reproduction: Requires the fusion of sperm and egg, resulting in genetic diversity.

• Cellular reproduction is necessary for both the production of new cells (growth, repair) and new organisms (reproduction).

• Two main processes:

  • Growth: The cell duplicates its contents, including DNA and organelles.

  • Cell division: The parent cell divides into two daughter cells.

Chromatin and Chromosomes

Genetic material in eukaryotic cells exists in two forms:

• Chromatin: DNA and associated proteins (histones) appear as thin threads; this is the normal state of DNA in non-dividing cells.

• Chromosomes: During cell division, chromatin condenses into thick, visible chromosomes. DNA replication ensures each daughter cell receives a full set of genetic material.

• Humans have 46 chromosomes in each somatic cell.

Example: DNA is periodically wound around histone proteins to form nucleosomes. Before cell division, chromatin condenses into chromosomes for easier distribution.

The Cell Cycle: Interphase, Mitosis, and Cytokinesis

Phases of the Cell Cycle

The cell cycle is an orderly sequence of stages that a cell goes through from its formation until it divides into two daughter cells. The main phases are:

• Interphase: The cell performs its normal functions and prepares for division. This phase occupies the majority of the cell cycle.

• M (Mitotic) phase: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Stages of Interphase

• G1 (Gap 1) phase: The cell grows, doubles its organelles, and accumulates materials for DNA synthesis. The cell decides whether to divide. If not, it may enter a resting state (G0).

• S (Synthesis) phase: DNA replication occurs, resulting in each chromosome consisting of two sister chromatids.

• G2 (Gap 2) phase: The cell synthesizes proteins needed for cell division and prepares for mitosis.

M (Mitotic) Phase

Mitosis ensures that each daughter cell receives an identical set of chromosomes. It is followed by cytokinesis, which divides the cytoplasm.

• Each sister chromatid contains identical DNA.

• After mitosis, daughter nuclei are genetically identical to each other and to the parent nucleus.

Moving the Chromosomes

• Spindle fibers: Structures made of microtubules that pull chromatids apart during mitosis. They are part of the cytoskeleton.

• Centrosome: The primary microtubule organizing center in animal cells; contains centrioles and an aster.

• Spindle fibers may overlap at the spindle equator or attach to duplicated chromosomes at the centromere.

Phases of Mitosis in Animal and Plant Cells

Mitosis is a continuous process, traditionally divided into four phases:

• Prophase: Chromosomes condense and become visible as sister chromatid pairs. The nuclear envelope breaks down, and spindle fibers form.

• Metaphase: Chromosomes align at the cell's equatorial plate (middle).

• Anaphase: Sister chromatids separate and move to opposite poles of the cell.

• Telophase: Nuclear envelopes reform around the daughter chromosomes, and chromosomes decondense. Cytokinesis often begins during this phase.

Example: In animal cells, the spindle apparatus forms from centrosomes containing centrioles and asters. In plant cells, centrosomes lack centrioles.

Comparison of Mitosis in Plant and Animal Cells

• Plant cells: Have centrosomes but lack centrioles. Cytokinesis occurs via the formation of a cell plate from the center outward.

• Animal cells: Each centrosome has two centrioles and an aster. Cytokinesis occurs via a cleavage furrow, which pinches the cell from the outside inward.

Cytokinesis in Animal and Plant Cells

• Animal cells: A cleavage furrow forms as anaphase ends. A contractile ring of actin filaments constricts the cell, dividing it like a drawstring.

• Plant cells: The rigid cell wall prevents furrowing. Instead, vesicles from the Golgi apparatus form a cell plate, which develops into new plasma membranes and cell walls between daughter cells.

• Mitosis without cytokinesis results in multinucleated cells (e.g., muscle cells, embryo sac in plants).

The Cell Cycle Control System

Cell Cycle Checkpoints

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

• G1 checkpoint: The cell commits to division if conditions are favorable. If not, it may enter G0. DNA integrity is checked; if damage is irreparable, apoptosis (programmed cell death) occurs.

• G2 checkpoint: Ensures DNA has been replicated correctly and repairs any damage.

• Mitotic (M) checkpoint: Occurs between metaphase and anaphase; ensures all chromosomes are properly attached to the spindle before proceeding.

Internal and External Signals

• Signals: Molecules that stimulate or inhibit cell cycle events.

• External signals: Come from outside the cell (e.g., growth factors like EGF, hormones like estrogen).

• Internal signals: Come from within the cell (e.g., cyclins, kinases).

• Kinases: Enzymes that transfer phosphate groups from ATP to other molecules, regulating cell cycle progression.

• Cyclins: Proteins whose levels fluctuate during the cell cycle; their destruction at the right time is necessary for normal progression.

Cell Cycle Signals and Apoptosis

• Contact inhibition: Cells stop dividing when they touch each other.

• Telomeres: Repeating DNA sequences at chromosome ends; their shortening limits the number of cell divisions.

• Apoptosis: Programmed cell death, triggered by internal or external signals. It helps maintain appropriate cell numbers and is a normal part of development (e.g., removal of tadpole tail, webbing between human digits).

Cell Cycle and Cancer

Regulation and Cancer

The cell cycle is regulated by signals that promote or inhibit division. Cancer results from an imbalance in these signals, leading to uncontrolled cell division.

• Cancer: A disease in which cellular reproduction occurs repeatedly without end.

• Carcinogenesis: The development of cancer.

Genetic Control of the Cell Cycle

• Proto-oncogenes: Genes that code for proteins promoting the cell cycle and inhibiting apoptosis. When mutated, they become oncogenes (cancer-causing genes).

• Tumor suppressor genes: Code for proteins that inhibit the cell cycle and promote apoptosis. Mutations in these genes remove cell cycle inhibition.

Other Genetic Changes and Cancer

• Absence of telomere shortening: Allows cancer cells to divide indefinitely.

• Chromosomal rearrangements: Instability can lead to loss, duplication, or translocation of DNA segments.

• Other genes associated with cancer: BRCA1, BRCA2, RB gene, RET gene.

Characteristics of Cancer Cells

• Lack of differentiation: Cancer cells do not contribute to normal body function and may be "immortal" (divide indefinitely).

• Abnormal nuclei: Often have abnormal numbers of chromosomes.

• Do not undergo apoptosis: Continue dividing despite errors.

• Form tumors: Masses of cells that do not respond to inhibitory signals.

• Metastasis: Cancer cells can spread to start new tumors in other tissues.

• Angiogenesis: Formation of new blood vessels to supply the tumor.

• Benign tumors: Confined to a capsule; malignant tumors: Invasive and may spread.

Cancer Treatment and Prevention

• Treatment: Removal of tumors, radiation therapy, chemotherapy (target rapidly dividing cells), and hormone therapy.

• Prevention:

  • Avoid smoking and excessive sun exposure.

  • Limit alcohol consumption.

  • Maintain a healthy diet rich in vitamins A and C, and include cruciferous vegetables (e.g., cabbage family).

  • Avoid salt-cured or pickled foods.

Summary Table: Comparison of Mitosis in Plant and Animal Cells

Feature	Plant Cells	Animal Cells
Centrosomes	Present, but lack centrioles	Present, each with two centrioles and an aster
Cytokinesis	Cell plate forms from center outward	Cleavage furrow forms from outside inward
Spindle Formation	Without centrioles	With centrioles

Additional info: This summary expands on the provided notes with definitions, examples, and context for clarity and completeness, suitable for exam preparation in a General Biology course.