Cell Structure: From Theory to Chromosomes
1.1. The Cell Theory
Thanks to Schleiden and Schwann, the development of cell theory began:
• The cell is the structural unit of all living things.
• It is the functional unit that performs all life processes.
• Every cell comes from an existing one.
• It represents the genetic unity of all living things, containing the hereditary material passed to daughter cells.
3.1. Prokaryotic Cell Structure
• Cell wall: a rigid casing formed by polysaccharides and proteins that give shape to bacteria.
• Plasma membrane: located inside the cell wall, it controls the entry and exit of substances. It folds into mesosomes, which are involved in metabolic processes like respiration.
• Bacterial chromosome: consisting of a single circular DNA molecule, it contains all the genetic information of the cell. It is located in the nucleoid.
• Ribosome: small organelles that carry out protein synthesis.
• Flagella: extensions of the cytoplasm involved in movement.
• Fimbriae: short, numerous structures that fix bacteria to the substrate.
3.2. General Structure of Eukaryotic Cells
• Plasma membrane: a layer around the cell, isolating it and regulating the exchange of substances with the outside environment.
• Nucleus: contains genetic material and is separated from the rest of the cell by a nuclear envelope, allowing the exchange of substances with the rest of the cell.
• Cytoplasm: located between the plasma membrane and the nucleus. It is formed by an aqueous medium and a network of protein fibers (cytoskeleton) involved in movement and cell division. Cellular organelles are found within the cytoplasm.
• Organelles:
– Endoplasmic reticulum: formed by a group of flattened sacs and tubular conduits. If it has ribosomes attached, it is called rough endoplasmic reticulum; otherwise, it is smooth. The rough ER synthesizes proteins, and the smooth ER synthesizes lipids.
– Golgi apparatus: formed by sets of flat, stacked cisterns. They accumulate substances from the endoplasmic reticulum and export them through small secretory vesicles.
– Ribosomes: small particles composed of RNA and protein. They perform protein synthesis.
– Mitochondria: spherical or elongated organelles with a double membrane. Cellular respiration takes place within them, providing energy for the cell.
– Lysosomes: membrane vesicles originating from the Golgi apparatus. They contain digestive enzymes that break down complex molecules into simpler ones through hydrolysis.
– Vacuoles: membrane vesicles that accumulate various products, such as water, reserve substances, or pigments.
3.3. Two Models of Eukaryotic Cells
• Animal cells: possess a centrosome, which consists of two small cylinders called centrioles, made up of protein tubules.
• Plant cells: have a cell wall surrounding the plasma membrane externally. Its function is to protect the cell and maintain its shape. They have channels called plasmodesmata that connect adjacent cells. Plant cells also have large vacuoles and chloroplasts.
4. Cell Nucleus
The most prominent structure in eukaryotic cells. It houses most of the DNA. Cells typically have a single nucleus, but some, like striated muscle cells, are multinucleated (polynuclear). Others, like mammalian red blood cells, lack a nucleus.
4.1. Nuclear Components
• Nuclear envelope: formed by a double membrane separated by an intermembrane space. The outer membrane is connected to the endoplasmic reticulum. Nuclear pores penetrate the envelope, allowing the exchange of substances between the nucleus and cytoplasm.
• Nucleoplasm: the aqueous environment in which other nuclear components are immersed. DNA replication occurs here.
• Nucleolus: a spherical structure lacking a membrane. Its main function is the formation of ribosomes.
• Chromatin: consists of strands of DNA and associated proteins, dispersed throughout the nucleoplasm. When the cell prepares to divide, the chromatin filaments organize and condense to form thicker structures called chromosomes.
5. Chromosomes
Filamentous structures that appear during cell division. They carry the genetic information from the DNA of the parent cell to the daughter cells. Chromosomes are made of a highly coiled strand of DNA and various proteins that maintain their shape. They are composed of two chromatids joined at the centromere, each formed by a condensed chromatin molecule (sister chromatids). Each chromatid has an arm, which can be of the same or different length.
5.1. Number of Chromosomes
Each species has a characteristic number of chromosomes.
• Haploid organisms: possess a single set of chromosomes in their cells, represented by the letter ‘n’.
• Diploid organisms: have an even number of chromosomes in their somatic cells. These are called homologous chromosomes, with each one coming from a parent’s gamete. This is represented by ‘2n’.
5.2. Types of Chromosomes
• Metacentric: the centromere is located in the middle of the chromosome, resulting in arms of approximately equal length.
• Submetacentric: the centromere is displaced to one side, resulting in slightly unequal arms.
• Acrocentric: the centromere is located far to one end of the chromosome, resulting in very unequal arms.
• Telocentric: the centromere is located at one end of the chromosome, so only one arm is visible.
6. The Karyotype
The karyotype is the complete set of chromosomes of a species. There are two types of chromosomes:
• Heterosomes or sex chromosomes: involved in sex determination. One is called X, and the other is called Y. Men are XY, and women are XX.
• Autosomes: all other chromosomes, which are the same in both sexes. Human somatic cells possess 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.