Understanding Chromosomes, Cell Reproduction, and Mitosis
Chromosome Structure
Chromosome: These are stick-shaped structures representing chromatin condensation during mitosis. Their size varies according to species.
Centromere: This is the primary constriction point, marking the start of the distal arms. Telomeres may have secondary constrictions near them.
Kinetochore: This protein structure appears at the centromeres and acts as a microtubule organizing center, where the spindle fibers attach.
Telomeres: These are protective caps at chromosome ends, preventing degradation, gene loss, and chromosome fusion. There are three positions: medial (metacentric), subterminal (acrocentric), and at the extremes (telocentric).
Function: Chromosomes facilitate the sharing of genetic information from the parent cell to its offspring. Each species has a specific number of chromosomes in its cells. Most cells are diploid, where “n” represents the number of chromosomes. Only gametes have a haploid number. Chromosomes are distinguished by length and centromere position. A graphic representation of the complete set of metaphase chromosomes in a cell is called a karyotype, with homologous chromosomes arranged in pairs from highest to lowest, known as an idiogram.
Cell Reproduction
The cell reproduction rate depends on the cell type.
Cell Cycle
The cell cycle covers the period from the time a cell originates until it divides into two daughter cells. It consists of four phases: G1, S, G2, and M. The M phase falls into two stages: a long phase distinguished in the nucleus and a short phase where chromosome division includes the nucleus and cytoplasm to originate daughter cells. The G1, S, and G2 phases are biochemically very active periods where synthesis of all the cell’s own substances occurs, including DNA, while in the M phase, cell activity focuses on the distribution of said DNA between the two daughter cells and lasts one-tenth of the cell cycle.
Phases of the Cell Cycle
- G1 Phase: Initial growth phase after division, with intense transcription and translation activity where specific genes determine cellular differentiation.
- S Phase: DNA synthesis and duplication of hereditary material.
- G2 Phase: DNA synthesis ends, including the production of histones and spindle microtubules, i.e., transcription and translation of genes necessary for cell division.
Cell cycle control is achieved through the phosphorylation of a protein that initiates replication. These phosphorylations are conducted through kinases, which are activated by proteins present at certain times, called cyclins. Cells can only divide when circumstances demand it.
Apoptosis, or programmed cell death, occurs when a set of processes leads to cell death. This helps achieve equilibrium in a multicellular organism, and its alteration can lead to uncontrolled proliferation (tumor).
Mitosis
- Prophase: Chromosomes become visible and are formed by sister chromatids joined by centromeres. The nucleolus disappears, and the nuclear membrane breaks down. The mitotic spindle begins to form. In plant cells, which lack centrioles, the spindle forms from a diffuse centromere region. The kinetochore develops.
- Metaphase: Chromosomes are arranged in the equatorial plane of the cell. Metaphase chromosomes have a characteristic structure for each species.
- Anaphase: The separation of chromatids begins, moving each chromosome toward the spindle poles as a result of the shortening of the microtubules.
- Telophase: Daughter cell nuclei are reconstructed with the formation of a new nuclear envelope. Nucleoli reappear, and the chromosomes decondense.