bio lab

Chromosomes and Chromosomal Behavior

  • Chromosomal Copying and Structure

    • Humans have two copies of each chromosome: one from father and one from mother.
    • Initial duplication involves the complete set of chromosomes duplicating, resulting in four copies per cell, categorized as sister chromatids.

  • Meiosis Overview

    • During meiosis, chromosomes are altered via a process called crossing over, leading to unique genetic combinations.
    • Starts with one diploid (2n) cell and ends with four haploid (n) cells, each containing only one chromosome from each pair.

Mitosis

  • Mitosis Stages

    1. Interphase (Pre-mitosis)
    • DNA replicates, resulting in two copies of each chromosome (now considered sister chromatids).
    1. Prophase
    • Chromosomes condense and become visible as pairs of sister chromatids.
    • The centromere, located in the center of the chromatids, holds the two chromatids together.
    • The mitotic spindle begins to form, with kinetochores attaching to spindle fibers.
    1. Prometaphase
    • The nuclear envelope disintegrates, allowing spindle fibers to contact chromosomes.
    1. Metaphase
    • Chromosomes align along the metaphase plate (equatorial plane).
    1. Anaphase
    • Spindle fibers pull sister chromatids apart towards opposite poles of the cell.
    1. Telophase
    • Nuclear envelopes reform around the separated chromatids, now individual chromosomes.
    1. Cytokinesis
    • The cell membrane pinches inwards (cleavage furrow in animal cells) leading to the separation into two identical daughter cells.
    • In plant cells, a cell plate forms as the rigid cell wall reconstitutes through the cell division process.

  • Key Differences in Cytokinesis

    • Animal cells: Cleavage furrow is formed due to membrane ingression.
    • Plant cells: Cell plate forms to construct a new cell wall separating daughter cells.

Meiosis

  • Meiosis I and II

    • Meiosis consists of two sequential divisions:
    1. Meiosis I (reduction division):
      • One diploid cell divides to form two haploid cells.
    2. Meiosis II:
      • Each of the two haploid cells divides to produce four haploid cells.

  • Crossing Over and Genetic Diversity

    • Occurs during Prophase I and creates genetic variation by exchanging segments between homologous chromosomes.
    • Resulting cells have a mix of genetic material:
    • For example, if one chromosome has three orange genes while another has red genes, the end result is
      • Chromosome 1: 3 oranges, 1 red
      • Chromosome 2: 1 orange, 3 reds
      • And vice versa for the others resulting from crossing over.

  • This genetic recombination is crucial for diversity among offspring and contributes to why siblings may look different and why no individual is genetically identical to their parents, although they share DNA derived from both sides.

Hands-on Activities and Observations

  • Microscopy Practice

    • Preparation involves viewing onion root tips under a microscope to identify stages of mitosis.
    • Zones in Onion Root Tips:
    1. Zone of Maturation: Mature cells performing functions.
    2. Zone of Elongation: Cells become elongated without dividing.
    3. Zone of Cell Division: Active mitosis is occurring here.

  • Additional Laboratory Work

    • Practical exercises to recreate mitosis stages through diagrams and identifying stages in real cell samples (like fish embryos).
    • Encouragement to research online for better visual aids showing stages in both plant and animal cells.
    • Reference to specific structured drawings illustrating various stages of mitosis and meiosis.