Cellular Structures and Reproduction: Mitosis and the Nucleus

Cellular Motion Structures

Cells can move using either changes in the viscosity of the cytoplasm (pseudopods) or by means of flagella and cilia.

• Pseudopods: These are cytoplasm elongations produced by changes in cytoplasm viscosity, which, in turn, are due to changes in proteins of the cytoskeleton.
• Cilia and Flagella: These are mobile organelles attached to the cell membrane. They consist of a basal corpuscle and a string-like structure that projects out of the cell, composed of the cell membrane and a series of microtubules.
• The Centrosome: This organelle is constituted by two small cylinders positioned perpendicular to each other. Each cylinder is called a centriole, which is made up of nine sets of three microtubules. The centrosome controls the movement of cilia and flagella, and the movement of chromosomes during cellular division.

The Cell Nucleus

The nucleus is considered a special organelle. In eukaryotic cells, the genetic material is surrounded by a double membrane, forming the nucleus.

• Interphase Nucleus Components

  During interphase, the nucleus contains:

  • Nuclear Envelope: A double membrane with numerous pores that allow certain substances to enter and exit the nucleus.
  • Nucleoplasm (or Karyoplasm): A liquid similar to hyaloplasm.
  • Chromatin: The main component, primarily made up of DNA.
  • Nucleolus: A spherical structure that participates in the synthesis of the genetic message (rRNA synthesis).

• The Nucleus During Cell Division

  • The nuclear envelope and the nucleoli disappear.
  • Chromatin condenses into visible structures called chromosomes.
  • A chromosome is made up of two identical strings of chromatin tightly twisted, each one forming a chromatid joined together by a centromere.

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Cellular Reproduction: The Cell Cycle

1. Interphase Stages

   Prior to mitosis, the cell seems to be resting. Three stages can be differentiated during interphase:

   • G1 (Gap 1): The cell enlarges.
   • S (Synthesis): DNA replicates.
   • G2 (Gap 2): Centrioles replicate.

2. Division of the Nucleus (Karyokinesis)

   This division can produce either two new nuclei identical to each other (Mitosis) or four new nuclei with half the chromosomes (Meiosis).

   Mitosis (Strict Sense)

   Mitosis produces two new nuclei identical to each other and to the parent nucleus. Mitosis occurs in 4 phases.

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