Apoptosis: Cellular Suicide Mechanisms and Nuclear Components

Apoptosis: Cellular Suicide

Apoptosis, or programmed cell death, is a crucial mechanism in multicellular organisms. It plays a vital role in development, removing unnecessary cells during embryonic stages (e.g., interdigital tissue), and in adulthood, eliminating damaged or potentially harmful cells. This process involves a series of changes leading to cell death. The cell shrinks, loses water, and forms bubble-like protrusions. The nucleus fragments, and the resulting apoptotic bodies are ingested by neighboring cells. Apoptosis is triggered by signals that activate endonucleases, which in turn produce lethal proteins, hydrolases, and proteases.

The Interphase Nucleus

The interphase nucleus houses the genetic material and controls all cellular activities. In animal cells, it is centrally located, while in plant cells, it is not. The nucleus is separated from the cytoplasm by a nuclear envelope. Within the nucleus, there are areas of granular chromatin, a spherical nucleolus, and the nuclear matrix.

Nuclear Components

Nuclear Envelope

The nuclear envelope consists of two membranes separated by a small space and traversed by numerous tiny pores. The outer nuclear membrane may have ribosomes attached to its cytosolic face. The inner nuclear membrane organizes chromatin and regulates the nuclear envelope after mitosis. Pores are dynamic structures that open and close to regulate the transport of macromolecules.

Nucleoplasm

The nucleoplasm is the internal environment of the nucleus, composed of mineral salts, nucleotides, RNA, and proteins.

Nucleolus

The nucleolus is a dense, almost spherical structure with an irregular outline, surrounded by nucleoplasm. It is the site where 45S nucleolar RNA is synthesized, which is then processed into 28S, 18S, and 5.8S ribosomal RNA, essential for ribosome construction. It is visible under a light microscope during interphase. The nucleolus has three zones: a fibrillar center containing DNA loops, a dense fibrillar component where nucleolar RNA copies bind to proteins, and a granular component where precursors of the two ribosomal subunits originate.

Chromatin

DNA in the nucleus is always associated with proteins. During interphase, chromatin fibers exhibit different levels of organization, including nucleosomes, the 'pearl necklace' structure, and 30nm fibers. There are two types of chromatin:

• Euchromatin: This is less condensed and transcriptionally active, allowing for better gene expression.
• Heterochromatin: This is more condensed and transcriptionally inactive. There are two types:
  • Optional: Represents genes that are inactivated in a cell-specific manner.
  • Constitutive: DNA that is never transcribed.

Mitotic Chromosomes

Mitotic chromosomes are highly condensed and portable structures, each consisting of two chromatids. They move during cell division to ensure that each daughter cell receives a complete set of chromosomes. Key parts include: chromatids, centromere, secondary constrictions, bands, and telomeres. Chromosomes are classified as: metacentric, submetacentric, acrocentric, and telocentric. Cells can be diploid (two sets of chromosomes, one from each parent) or haploid (one set, as in algal spores and gametes). Triploid and tetraploid cells also exist.