Cellular Processes and Protein Synthesis

Chapter 04: Cellular Processes

Section 1: The Cell Cycle

Overview of the Cell Cycle
  • Definition: The cell cycle is the series of events that take place in a cell leading to its division and duplication (replication). This includes both interphase and the mitotic phase.
    • Interphase: Comprises three subphases: G1, S, G2.
    • G1 Phase (First Gap Phase):
      • Duration: Interval between cell birth (from division) and DNA replication.
      • Functions: Cell growth, normal metabolic roles, and accumulation of materials necessary for the next phase.
    • S Phase (Synthesis Phase):
      • Function: DNA replication. The cell duplicates all nuclear DNA and centrioles.
    • G2 Phase (Second Gap Phase):
      • Duration: Interval between DNA replication and cell division.
      • Functions: Cell repairs DNA replication errors, grows, and synthesizes enzymes controlling cell division.
    • M Phase (Mitotic Phase):
    • Defines the actual division of the cell where it replicates its nucleus and divides into two daughter cells.
    • G0 Phase: Describes cells that have exited the cycle and do not divide for extended periods or permanently.
Duration of the Cell Cycle
  • The duration of the cell cycle varies significantly between different cell types.

Section 2: Mitosis

Functions of Mitosis
  • Definition: Mitosis is the process of cell division that results in two genetically identical daughter cells.
  • Functions: This includes:
    • Development from a zygote to an estimated 50 trillion cells.
    • Growth of tissues and organs post-birth.
    • Replacement of dead cells.
    • Repair of damaged tissues.
Phases of Mitosis

  1. Prophase:

    • Chromatin condenses into compact chromosomes (46 chromosomes, each with two chromatids).
    • Nuclear envelope disintegrates.
    • Centrioles sprout spindle fibers which push them apart and some attach to kinetochores at the centromeres of the chromosomes.

  2. Metaphase:

    • Chromosomes are aligned along the equator of the cell.
    • The mitotic spindle forms a distinct shape (lemon-shaped).
    • Shorter microtubules form an aster anchoring themselves to the cell membrane.

  3. Anaphase:

    • Enzymes cleave sister chromatids apart at their centromeres.
    • Daughter chromosomes, now single-stranded, migrate toward opposite poles of the cell.

  4. Telophase:

    • Chromosomes gather at each pole.
    • Rough endoplasmic reticulum (ER) forms a new nuclear envelope around each chromosomal cluster.
    • Chromosomes decondense back into chromatin and the mitotic spindle disintegrates.
Cytokinesis
  • Definition: Division of the cytoplasm that occurs at the conclusion of mitosis.
  • Overlaps with telophase, forming a cleavage furrow around the cell's equator, eventually pinching the cell into two.

Section 3: Regulation of Cell Division

Mechanisms Activating Cell Division
  • Cells divide when:
    • Sufficient cytoplasm is available for two daughter cells.
    • DNA has been accurately replicated.
    • Nutrient availability and presence of protein factors are adequate.
    • Molecular timing is appropriately aligned.
    • Proper checkpoints are passed.
When Cell Division Ceases
  • Cells will stop dividing when:
    • They snugly contact neighboring cells (contact inhibition).
    • They lack necessary nutrients or growth factors.
Molecular Timing and Checkpoints
  • Cyclins: Proteins that regulate the cell cycle; absent at certain times and form during interphase.
  • Cyclin-dependent Kinases (Cdks): Activated by cyclins to phosphorylate other proteins enhancing cell cycle progression.
  • Checkpoint Phases:
    • G1 Checkpoint: Evaluates the readiness of the cell to proceed to S phase.
    • G2/M Checkpoint: Determines if the cell can advance to mitosis.
    • Metaphase to Anaphase Checkpoint: Monitors whether all chromosomes are correctly attached to the spindle before proceeding with separation.

Section 4: Chromosomes and Heredity

Definitions
  • Heredity: The transmission of genetic characteristics from parent to offspring.
  • Karyotype: A visual arrangement of an organism's chromosomes to observe chromosomal number and structure; typically organized by size.
  • Homologous Chromosomes: Pairs of chromosomes of the same length and shape, each carrying genes for the same traits - one from each parent.
  • Autosomes: 22 out of the total 23 pairs of chromosomes that are not sex chromosomes.
Human Chromosome Makeup
  • Each human has:
    • 46 total chromosomes, grouped into 23 pairs.
    • 22 pairs of autosomes.
    • 1 pair of sex chromosomes (XX for females, XY for males).

Section 5: Cancer

Malignant Tumors
  • Characteristics:
    • Rapid growth, ability to metastasize, leading to the spread of cancerous cells and formation of secondary tumors.
    • Differentiated from benign tumors which grow slowly and are encapsulated.
Types of Cancer
  • Carcinomas: Developed in epithelial tissues.
  • Sarcomas: Found in connective tissues such as bone or muscle.
  • Lymphomas: Occur in lymph nodes.
  • Leukemias: Originate in blood-forming tissues.
  • Melanomas: Arise in pigment cells of skin (melanocytes).
Carcinogens
  • External factors causing cancer include:
  • Radiation: UV rays, X-rays.
  • Chemical agents: Cigarette tar, food preservatives.
  • Viruses: HPV, hepatitis C.
  • Genetic mutation (5-10% of cases) often triggered by carcinogens.
Genetic Factors
  • Oncogenes: Genes promoting uncontrolled cell division.
  • Tumor-Suppressor Genes (TS): Genes that inhibit tumors; mutations or silencing of TS leaves oncogenes unopposed, leading to cancer.
Cancer's Effects on the Body
  • Malignant tumors replace vital tissue and impede organ function.
  • Cachexia: Severe wasting due to nutrient theft by tumors.
  • Reduction of immunity increasing susceptibility to infections.

Section 6: Cellular Respiration

Overview of Cellular Respiration
  • Three primary processes:
    1. Glycolysis
    2. Citric Acid Cycle
    3. Electron Transport Chain
Glycolysis
  • The breakdown of glucose into energy, producing pyruvic acid and energy carriers (ATP).
    • Reaction: C6H{12}O6 + 6 O2
      ightarrow 6 CO2 + 6 H2O + ATP
Citric Acid Cycle and Electron Transport Chain
  • Processes that further convert pyruvic acid to energy, producing ATP and carbon dioxide as by-products.
Importance of Glycolysis
  • Glycolysis occurs in the cytoplasm and is crucial as an early step in cellular respiration, yielding 2 ATP for each glucose.
Key Features of Enzymes in Glycolysis
  • Specific pathways regulated by enzymes facilitate glucose's conversion to ATP while maintaining energy efficiency.
Transcription and Translation in Protein Synthesis
  • Transcription: The process where a gene's DNA is transcribed to RNA.
  • Translation: The process where mRNA is translated into a protein using tRNA and ribosomes.
  • Each step in protein synthesis involves complex coordination between nucleotides and amino acids, leading to the production of functional proteins essential for cellular functions.
Post-Translational Modifications
  • Proteins may undergo folding, modification, and packaging within the cell to achieve functional conformations before being released or utilized internally.

Conclusion

  • The intricate processes of cellular respiration, the cell cycle, heredity, and protein synthesis form the foundation of cellular function and organismal growth, exhibiting the elegant complexity of life at the molecular level.