Cellular Structures and Genetic Processes: An In-Depth Look

Posted on Mar 24, 2025 in Biology

Nucleus

The nucleus contains genetic information. It can be single or multiple, round, cubic, or poly-lobed. It’s located either centrally or laterally within the cell. We distinguish between the interphase nucleus and the nucleus undergoing mitosis.

Parts:

Nuclear membrane: Double membrane (external and internal).
Nuclear pores: Formed by the nuclear pore complex, an octagon of 8 subunits of nuclear pore proteins with a central granule. It allows the exchange of molecules in both directions.
Matrix: Formed primarily by chromatin (genetic material). Chromatin in interphase is decondensed (euchromatin), while in mitosis, it forms heterochromatin or condensed chromatin (chromosomes). The nucleolus, composed of RNA and proteins, synthesizes rRNA. It disappears during mitosis and originates from the nucleolar organizer region (NOR) of the chromosome.

Mitotic Chromosomes

Chromosomes represent the maximum degree of chromatin compaction. In metaphase (phase AB 2), they are formed by two chromatids joined by a central constriction, the centromere. The terminals are the telomeres, and the satellite is attached to the NOR. An anaphase A chromosome has only one chromatid.

Karyotype: The complete set of chromosomes of a cell. The photographic image of the karyotype is the ideogram (chromosomes are placed in homologous pairs). Chromosomes are constant in cells; each type of chromosome is numbered and grouped in homologous pairs (one from the mother and one from the father). Although they carry information for the same characters, the information is not identical. A diploid cell (2n) has a full set of chromosomes, while gametes are haploid (n).

Types of chromosomes:

Metacentric: Centromere in the middle.
Submetacentric: One arm is longer than the other.
Acrocentric: One arm is much longer.
Telocentric: Only one arm exists.

Non-sex chromosomes are called autosomes.

Endoplasmic Reticulum (ER)

The ER is a network of interconnected sacs extending throughout the cytoplasm. It has an internal space (lumen) and communicates with the nuclear membrane and the Golgi apparatus. There are two types:

Rough Endoplasmic Reticulum (RER): Has ribosomes attached to the outer membrane. RER functions: Protein synthesis and storage, protein glycosylation, and formation of the cell wall in plant cells.
Smooth Endoplasmic Reticulum (SER): SER functions: Synthesis of membrane lipids and steroids, calcium storage, and involvement in cellular detoxification processes (metabolizes alcohol and drugs).

Golgi Apparatus

The Golgi apparatus is located near the centrosome and consists of a series of flattened and stacked cisternae forming dictyosomes. It has two faces: cis (connected to the ER) and trans (connected to lysosomes). Function: Protein transport and glycosylation.

Lysosomes

Lysosomes are simple cytoplasmic organelles responsible for cellular digestion. They contain a large number of hydrolases, which require an acidic pH to function. A proton pump introduces H+ into the lysosome, consuming energy.

Types:

Primary lysosomes: Originate from the Golgi apparatus. They release enzymes through exocytosis for the digestion of dead cells and the formation of new lysosomes.
Secondary lysosomes: Involved in cellular digestion and can take various forms:
- Phagolysosome: Formed when a lysosome fuses with a phagocytized bacterium.
- Autophagolysosome: Formed by a lysosome and autophagosomes (derived from the ER or other cellular organelles).
- Endolysosome: Formed by the union of a lysosome with an endocytic vesicle.

When materials in secondary lysosomes cannot be degraded, they become residual bodies.

Mutations

Mutations are alterations in the genetic material. They may go unnoticed or have significant effects. They are a source of genetic variation and, if they occur in gametes, are transmitted to the offspring.