Cell Biology: Structure, Types, and Division Processes

Posted on Dec 14, 2024 in Biology

The Discovery of the Cell: The Cell Theory

Robert Hooke discovered the cell when looking through a microscope, made by himself, at a very thin sheet of cork. In the nineteenth century, the improvement of microscopes and the discovery of techniques to stain preparations allowed for the observation of new cellular structures.

  • The botanist Robert Brown, in 1831, found a particle inside plant cells which he called the nucleus.
  • Johannes Purkinje, in 1838, introduced the term protoplasm to describe the liquid that fills the cell.
  • The botanist Matthias Schleiden, in 1838, and the doctor Friedrich Schwann, in 1839, stated that all plants and animals are made of cells.
  • The doctor Rudolf Virchow, in 1855, established that every cell comes from another existing one. “Omne ex ovo” to “omne cellula ex cellula”.

Cell theory:

  • The cell is the structural unit of life.
  • All living things are composed of one or more cells.
  • The cell is the smallest functional unit of living beings.
  • Every cell comes from an existing cell.

Types of Cellular Organization

  • Prokaryotic cells: They have no nucleus. Their genetic material is distributed throughout the cytoplasm.
  • Eukaryotic cells: They have a nucleus that contains genetic material and numerous organelles.

Prokaryotic Cell Structure

  • Cell wall: A rigid and strong sheath made of polysaccharides and proteins that shape the bacteria.
  • Plasma membrane: It lies within the cell wall and controls the entry and exit of substances. In some regions, it forms folds called mesosomes (structures in which important metabolic processes such as respiration occur).
  • Bacterial chromosome: This consists of a single circular DNA molecule that contains all the genetic information of the cell. It is located in an area called the nucleoid.
  • Ribosomes: Small organelles in which protein synthesis occurs.
  • Flagella: Extensions of the cytoplasm involved in movement.
  • Fimbriae: Numerous short structures that fix bacteria to the substrate.

The General Structure of the Eukaryotic Cell

  • Cytoplasmic membrane: A layer around the cell, isolating and regulating the exchange of substances with the outside.
  • Nucleus: Contains genetic material inside and is separated from the rest of the cell by a porous double membrane, the nuclear envelope, which allows the exchange of substances.
  • Cytoplasm: The area between the membrane and the nucleus. It consists of an aqueous medium and a network of protein fibers that are involved in movement and cell division and constitute the cytoskeleton.
  • Centrosome: A region near the nucleus which controls the movement of the fibers of the cytoskeleton and is involved in cell division.
  • Organelles: Cellular elements that are responsible for different functions:
    • Endoplasmic reticulum: A set of flattened sacs and tubular conduits. If it has ribosomes, it is called rough; if not, smooth.
    • Golgi apparatus: A set of flattened and stacked tanks. They accumulate substances from the endoplasmic reticulum and secrete them to the outside through small vesicles that are formed in its periphery.
    • Ribosomes: Small-sized particles composed of RNA and proteins. They perform protein synthesis.
    • Mitochondria: Their shape is spherical or elongated, like a double membrane. Cellular respiration takes place in them, the process by which the cell gets energy.
    • Lysosomes: Membrane vesicles, from the Golgi apparatus, which contain digestive enzymes that turn complex molecules into simpler ones by hydrolysis.
    • Vacuoles: Membrane vesicles that accumulate different products, reserve substances, or pigments.

Two Models of Eukaryotic Cells

  • Animal eukaryotic cell: They show in the centrosome two small cylinders called centrioles, constituted by protein tubules.
  • Plant eukaryotic cell: They have a cell wall surrounding the plasma membrane. They have channels called plasmodesmata that connect with other cells. They have large vacuoles and chloroplasts, oval-shaped organelles delimited by a double membrane where photosynthesis takes place.

The Cell Nucleus

Components:

  • Nuclear envelope: Formed by a double membrane, the external and internal, separated by an intermembrane space. The outer membrane is connected to the endoplasmic reticulum. Both membranes are interconnected by nuclear pores, through which the interchange of substances occurs.
  • Nucleoplasm: Aqueous internal environment where other nuclear components are immersed.
  • Nucleolus: Spherical corpuscle devoid of a membrane that can only be seen when the cell is not dividing. Its main mission is creating ribosomes.
  • Chromatin: This consists of strands of DNA at different degrees of condensation, associated with proteins and scattered throughout the nucleoplasm. When the cell is going to divide, the chromatin filaments organize and condense to form thicker structures called chromosomes.

Chromosome Numbers

Each species has a characteristic number of chromosomes. There may be:

  • Haploid organisms: They possess a single set of chromosomes in their cells.
  • Diploid organisms: They possess a pair of chromosomes in their somatic (non-reproductive) cells. They are called homologous chromosomes, and each is from the gamete of a parent.

The Cell Cycle

  • Interphase or non-division phase: The initial phase of long duration in which you can see the cell nucleus, is called the interphase nucleus. At the end of this phase, DNA replication or duplication occurs, necessary so that after division, each daughter cell can receive the same amount of DNA as the parent cell.
  • Division phase / M phase: The final stage of short duration in which the nucleus disappears and chromosomes become visible. It involves the division of the nucleus, or mitosis, and cytoplasmic division, or cytokinesis.

Cell Division in Eukaryotic Cells

During the division phase, from a stem cell, two daughter cells arise with the same number of chromosomes as the parent. In eukaryotic cells, cell division involves two processes:

Mitosis, or Nuclear Division

Before starting the process, the DNA is duplicated, but chromosomes still cannot be seen because chromatin condensation has not yet occurred. The centrosome also doubles, and if the cell is animal, the centrioles do as well.

4 phases:

  • Prophase: Chromatin condenses and turns into chromosomes, the nucleolus and the nuclear membrane disappear.
  • Metaphase: The chromosomes are placed in the center of the cell to form an equatorial plate.
  • Anaphase: The sister chromatids of each chromosome separate and are placed at opposite ends of the cell.
  • Telophase: The nucleolus returns, the nuclear membrane begins to form, and chromosomes condense, forming chromatin again.

Cytokinesis, or Cytoplasmic Division

Once the nucleus divides, the division of the cytoplasm and the distribution of organelles between the two new daughter cells take place.

  • Animal: Performed by strangulation of the cytoplasm at the cell equator.
  • Vegetables: A partition called a phragmoplast forms between the daughter cells.

Meiosis

Meiosis is a type of reduction division, as from a diploid stem cell (2n), haploid daughter cells (n) are formed, i.e., with half the DNA of the parent cell. It takes place in the gonads, and the cells formed are gametes.

Comparison Between Mitosis and Meiosis

Mitosis:

  • It is produced in somatic cells.
  • It is a short process.
  • It states that the chromosomes are paired, so it can occur in both haploid and diploid cells.
  • The nucleus divides only once.
  • No crossing over occurs.
  • During anaphase, the sister chromatids are separated.
  • Two identical daughter cells are produced, with the same chromosomes as the mother.

Meiosis:

  • It only occurs in stem cells of the gametes.
  • It is a long process.
  • It occurs only in cells with a diploid chromosome number because these are precisely matched counterparts.
  • The nucleus divides twice.
  • During the first meiotic division, homologous pairs of chromosomes separate. In the second division, chromatids are separated.
  • Four daughter cells originate that are genetically distinct, with half the chromosomes as the parent cell.

The Significance of Mitosis and Meiosis

Mitosis and meiosis are two mechanisms to split a cell with different biological meanings.

  • In multicellular organisms: Cell division by mitosis is the growth of the individual through successive divisions from a single cell, the zygote. It is also the form of renewal of lost or damaged cells.
  • In unicellular organisms: Mitosis is a mechanism of asexual reproduction that can increase the number of individuals in the population. In this case, individuals are identical to the parent.
  • Through meiosis: Haploid gametes originate. Upon fertilization, the resulting zygote is diploid. Thus, meiosis is an essential mechanism for sexual reproduction. In addition, because of the genetic variability of offspring due to the exchange of genetic information that occurs in meiosis between each pair of homologous chromosomes. New combinations also occur as a result of the independent segregation process, as the maternal and paternal chromosomes are combined at random in each gamete.

Formation of Gametes (Gametogenesis)

The process by which gametes are formed. There are two types:

  • Spermatogenesis: Formation of male gametes. It takes place in the testes. Each diploid cell will produce 4 different haploid cells, which by a process of cellular differentiation will give rise to 4 male gametes or sperm. Two of them carry one X chromosome and two carry one Y chromosome.
  • Oogenesis: Formation of female gametes. It takes place in the ovaries. Each stem cell will give rise to 4 haploid cells, a female gamete or ovum, which receives almost all the cytoplasm, and 3 polar bodies. All eggs carry an X chromosome.