Cell Biology: Prokaryotes, Eukaryotes, and Cellular Processes

Posted on Nov 12, 2024 in Biology

Prokaryotes

Prokaryotic cells have a more complete and rigid cell wall. Their ribosomes are small particles composed of proteins and RNA, functioning as sites of protein synthesis. These cells lack a defined nucleus, and genetic material transfer occurs directly through division. They are smaller in volume compared to eukaryotes.

Eukaryotes

Eukaryotic cells generally lack a cell wall, although some, like algae and plants, possess one. Their ribosomes are larger and are either bound to the rough endoplasmic reticulum or free in the cytoplasm. Eukaryotes have a defined nucleus, and genetic material transfer occurs through mitosis. They are larger in volume compared to prokaryotes.

Organelles and Their Functions

Mitochondrion

Produces energy.

Ribosome

Synthesizes proteins.

Nucleus

Contains genetic material on chromosomes.

Lysosomes

Contain digestive enzymes that help break down ingested materials.

Vacuoles

Store water and food.

Smooth Endoplasmic Reticulum (SER)

Involved in metabolic reactions related to the synthesis of fatty acids and phospholipids. Also important for drug detoxification.

Rough Endoplasmic Reticulum (RER)

Synthesizes proteins that are part of the plasma membrane, Golgi apparatus, lysosomes, and the endoplasmic reticulum itself.

Chloroplasts

Perform photosynthesis, producing glucose and oxygen.

Golgi Apparatus

Regulates the fate of different proteins synthesized in the RER. Lysosomes and plasma membrane proteins are formed here.

Centriole

At the start of cell division, each centriole is surrounded by radially arranged fibers (aster). They are also associated with the generation of cilia and flagella in certain cells, used for locomotion or to mobilize materials.

Bioelements and Biomolecules

Bioelements

Bioelements are the chemical elements that constitute living beings. Of the approximately 100 elements found in nature, about 70 are present in living organisms. Of these, only 22 are found in all living things. The most important are CHON (carbon, hydrogen, oxygen, nitrogen), comprising 96% of living matter.

Biomolecules

Biomolecules are the constituent molecules of living things. These include organic molecules, synthesized only by living beings and based on a carbon structure. Inorganic molecules are not synthesized by living beings but are indispensable for life, such as H2O and CO2.

Carbohydrates

Formed by C, H, and O. Examples include monosaccharides (glucose, fructose) and polysaccharides (starch).

Lipids

Composed of C, H, and O, with lesser amounts of N, phosphorus, and sulfur. Examples include tallow, butter, and oils.

Proteins

Composed of C, H, O, and N, and may also contain phosphorus and sulfur. They are based on amino acids. Examples include collagen and fibrin.

Nucleic Acids

Monomers composed of a nitrogenous base, a pentose sugar, and a phosphate group. Examples include DNA and RNA.

Cell Replication

Binary Fission

Cell replication in prokaryotes is preceded by replication of the bacterial chromosome. The chromosome is replicated, and each copy attaches to a different part of the cell membrane. When the cell begins to divide, the replicated chromosomes separate. Cytokinesis results in two cells with identical genetic makeup.

Mitosis

The process by which a mother cell produces two daughter cells with the same number and kind of chromosomes as the parent cell. It is a continuous process with four phases:

  • Prophase: Chromatin condenses into visible chromosomes. The nuclear membrane and nucleolus disappear. Each centriole moves to opposite poles of the cell, connected by aster fibers, forming the mitotic spindle.
  • Metaphase: Centrioles occupy opposite poles, and chromosomes align along the equatorial plate of the spindle.
  • Anaphase: Centromeres divide, and each chromatid moves to opposite poles of the cell. The spindle fibers begin to retract. The number of chromatids doubles during this phase.
  • Telophase: Chromosomes decondense and become invisible.

Meiosis

A two-stage process of cell division that reduces the chromosome number by half, resulting in four haploid gametes.

  • Premeiotic Interphase: DNA replicates.
  • Prophase I: Homologous chromosomes synapse to form tetrads. Crossing over occurs. The nuclear envelope degrades.
  • Metaphase I: Tetrads align along the equatorial plane.
  • Anaphase I: Homologous chromosomes separate and move to opposite poles.
  • Telophase I: Chromosomes arrive at each pole. Cytokinesis begins.
  • Prophase II: Chromosomes condense again.
  • Metaphase II: Chromosomes align along the equatorial plane.
  • Anaphase II: Sister chromatids separate and move to opposite poles.
  • Telophase II: Nuclei are formed at each pole.

Haploid Cells

Meiosis results in four haploid gametes.

DNA Replication

DNA replication is a semiconservative process, meaning each new DNA molecule contains one newly synthesized strand and one original strand. DNA replication is carried out by DNA polymerases, using one strand as a template to synthesize the complementary strand using deoxynucleotide triphosphates. The nascent DNA chain grows in the 5′ to 3′ direction.