Cell Biology: Structure, Function, and Division

Cell Structure and Function

A cell is the smallest structural and functional unit of life that can carry out the three vital functions: nutrition, reproduction, and interaction.

• Nutrition: obtaining energy, renewing the cellular structure, and obtaining biomolecules.
• Interaction: cells can adapt to changes in their environment and continue performing the activities they need to live.
• Reproduction: formation of new cells.

Types of Cells

Prokaryotic Cells

Prokaryotic cells do not have membrane-bound structures, are relatively small, have a single circular chromosome, have isolated ribosomes in their cytoplasm, and have cavities in their cytoplasm that function as mitochondria. Organisms belonging to the Monera kingdom have prokaryotic cells.

Eukaryotic Cells

Eukaryotic cells have a cytoplasm that contains a range of structures that perform different functions. These structures are called organelles.

• Genetic material is contained in the nucleus.
• Cytoskeleton: a microscopic network of protein filaments and microtubules.
• Organelles:
  • Lysosomes: contain enzymes that help in digestion.
  • Rough endoplasmic reticulum: synthesizes proteins.
  • Smooth endoplasmic reticulum: synthesizes lipids.
  • Vacuoles: store substances.
  • Golgi apparatus: processes substances from the endoplasmic reticulum.

Cell Division

Mitosis

Mitosis is the process by which a cell divides into two identical daughter cells.

• Prophase: Chromosomes condense and form visible structures, the nucleolus disappears, the mitotic spindle appears, and two centrosomes form.
• Metaphase: Chromosomes are located at the equator of the cell, and the mitotic spindle is connected to the centrosomes.
• Anaphase: Centromeres break, and the fibers of the mitotic spindle pull each sister chromatid to opposite poles of the cell. Each chromatid is now considered a single chromosome.
• Telophase: After the division of the nucleus, the cytoplasm divides in a process called cytokinesis.

Meiosis

Meiosis is the process by which a cell divides into four haploid daughter cells, each with half the genetic material of the parent cell.

First Meiotic Division

• Prophase I: Chromosomes form, and homologous chromosomes exchange genetic information through crossing over.
• Metaphase I: Homologous chromosomes, joined by the crossing over, are connected to the mitotic spindle at the centromere.
• Anaphase I: Homologous chromosomes are separated, and the mitotic spindle pulls them to opposite poles of the cell.

Second Meiotic Division

• Prophase II: The mitotic spindle appears, the nuclear membrane disappears, and the chromosomes are condensed.
• Metaphase II: The mitotic spindle connects to the chromosomes, and the chromosomes are situated at the equator of the cell.
• Anaphase II: The mitotic spindle pulls apart the chromatids of each chromosome.
• Telophase II: The centrosomes disappear, and the nuclear membrane reappears.

At the end of meiosis, four haploid daughter cells are produced, each with half the genetic material of the parent cell and containing different genetic information.

Mendel's Laws of Inheritance

• First Law (Principle of Uniformity): When two pure-bred individuals are crossed, all offspring of the first filial generation have the same genotype and phenotype.
• Second Law (Principle of Segregation): Alleles present in a heterozygote are independently distributed when gametes are formed.
• Third Law (Principle of Independent Assortment): When two individuals that have two or more different characteristics are crossed, each characteristic is transmitted independently of the others.

Comparison of Mitosis and Meiosis