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Notes on Microscopic Observations and Mitosis in Allium cepa

Microscopic Observations of Mitosis in Allium cepa

Optical Microscope Construction

  • Importance: Essential device in biological laboratories.

  • Components: Two main systems - Optical and Mechanical.

  • Optical System:

  • Combines illumination and magnification components.

  • Lighting: Used for optimal illumination of samples.

  • Magnification: Two-stage process to enhance image visibility.

  • Produces a virtual, magnified image.

Optical Elements of the Microscope

  • Light Source:

  • Transition from mirrors in simple microscopes to built-in light bulbs with reflectors in modern ones.

  • Light intensity must be adjusted based on magnification (weak for low, strong for high).

  • Condenser:

  • Set of 2-3 lenses that concentrate light to form a cone, adequately illuminating the sample.

  • Diaphragm:

  • Regulates light entering the condenser.

  • Lenses:

  • Magnify images; can be dry (up to 60x magnification) or wet (90-150x with immersion oil).

  • Immersion lenses reduce light scattering and increase resolution (up to 0.16 µm).

  • Ocular Lens:

  • Magnifies image between 2-30 times, creating a virtual image.

  • Total magnification = Objective lens magnification × Eyepiece magnification (up to 2000x).

Mechanical Elements of the Microscope

  • Base and Arm: Ensures structural rigidity.

  • Mechanical Stage: Fixes the preparation and allows movement in X and Y axes.

  • Coarse Adjustment Knob: Adjusts the distance between the lens and the preparation for initial focus.

  • Fine Adjustment Knob: Used for precise focusing.

  • Nose Piece: Rotates to switch between objective lenses, altering magnification.

  • Head (Tube): Area where the image is formed between the objective and eyepiece.

  • Condenser Adjustment Mechanism: Adjusts vertical position of the condenser.

Cell Cycle

  • Definition: Ordered sequence of events between mitotic divisions (interphase).

  • Phases:

  • G1 Phase: Cell growth; pre-DNA synthesis.

  • S Phase: DNA replication; chromosomes double.

  • G2 Phase: Pre-division preparation; chromatin condenses.

  • M Phase: Mitosis - includes karyokinesis and cytokinesis.

  • Importance:

  • Ensures accurate DNA duplication

  • Enables organism growth and regeneration.

Interphase Description

  • Longest stage between divisions with active metabolism.

  • Chromosomes in non-condensed form, facilitating gene expression:

  • G1 Phase: Increase in cell mass/volume.

  • S Phase: Chromosomes duplicated (2n to 4n).

  • G2 Phase: Final preparations for mitosis; chromosomes begin to condense.

  • G0 Phase: Resting phase; cells do not divide but remain metabolically active.

Mitosis Phases

  • Karyokinesis: Nucleus division into two daughter nuclei with identical chromosome sets.

  • M Phases:

  • Prophase: Chromatin condenses; spindle formation.

  • Prometaphase: Nuclear envelope breaks down; chromosomes attach to spindle microtubules.

  • Metaphase: Chromosomes align along the spindle equator.

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

  • Telophase: Daughter chromosomes arrive at poles; nuclear envelope re-forms.

  • Cytokinesis: Cytoplasm divides, forming two daughter cells.

Practical Applications

  • Student skills include:

  • Proficient use of optical microscope; focusing with various lenses.

  • Experience with immersion techniques.

  • Ability to document findings with drawings from microscopic observations.

  • Exercises outlined for practical microscopy in onion cell analysis:

  • Identify and draw phases of mitosis in root meristem cells.

  • Use immersion oil to enhance observations.

  • Record phases of cell nuclear division and document findings.

Literature Reference

  • Bruce Alberts, Dennis Bray, Karen Hopkin. Essential Cell Biology. 6th Edition, W. W. Norton & Company, 2023.

  • Chapters on Cells and the Cell Division Cycle are particularly relevant for understanding cell structure and function.