Cell Nucleus and Chromosome Structure

Cell Nucleus: Structure and Function

The nucleus, a defining structure of eukaryotic cells, controls cellular activities and transmits hereditary information across generations.

Morphology

The nucleus can mirror the cell's shape or be irregular.

Size

Nuclear size correlates with cytoplasmic volume and the cell's proliferative capacity; it tends to be larger in actively dividing cells.

Structure

The nucleus exhibits two distinct periods: interphase and mitosis. During interphase, the following are present:

• Nuclear Membrane: A double-layered structure enclosing the nucleus, separating it from the cytoplasm. It regulates the passage of ions and small molecules.
• Nucleoplasm: This consists of nuclear sap, the nucleolus, and chromatin.

Chromosomes

Chromosomes become visible during cell division. They possess a defined organization, are capable of self-replication (autoduplication), and transmit hereditary traits.

Chromosome Structure

Chromosomes consist of several key parts:

1. Centromere: The constricted region of the chromosome.
2. Chromatids: The arms of the chromosome. Within the chromatids, one can observe:

• Chromonemata
• Chromomeres

Secondary Constriction: A constriction other than the centromere. Satellite: A segment of the chromosome separated from the main body by a secondary constriction.

Types of Chromosomes

Based on their shape, chromosomes are classified as:

1. Telocentric: Rod-shaped.
2. Subtelocentric: One very short arm.
3. Metacentric: V-shaped.
4. Submetacentric: J-shaped.

Irritability: Cellular Response to Stimuli

Irritability is the ability of a cell to respond to environmental changes.

Stimuli

Stimuli are any environmental modifications that act on the cell's protoplasm, triggering a specific reaction.

Classification of Stimuli

Stimuli are classified based on their origin and nature:

1. By Origin:
   • External: Light, gravity, pressure, heat, etc.
   • Internal: Levels of nutrients, water, waste products.
2. By Nature:
   • Mechanical: Contact with rough surfaces.
   • Physical: Light, heat, sound, electricity, temperature.
   • Chemical: Changes in the chemical composition (oxygen, hydrogen, hormones, vitamins, etc.).
   • Biological: Interactions between biological components.
   • Psychological: Certain psychological states can have an effect.

Reactions

Reactions are the responses of the protoplasm to stimuli.

1. Morphological: Involve changes in cell shape.
2. Motor: Changes in the intensity or direction of movement.

• Tropism: Directional movement in response to a stimulus.
  • Phototropism: Stimulus is light.
  • Geotropism: Stimulus is gravity.
  • Chemotropism: Orientation movements in plants stimulated by chemicals.
  • Thigmotropism: Stimulus is a nearby object.