Introduction to Cell Biology

Posted on Oct 30, 2024 in Biology

Cytoskeleton

The cytoskeleton is dynamic, adaptable, and consists of three networks:

  • Microtubules
  • Microfilaments
  • Intermediate Filaments

These three are ATP catalysts.

Microtubules

Composed of the protein tubulin, measuring 25 nm, microtubules form the mitotic spindle and are used in vesicle transport. They are found throughout the cell from the core to the membrane.

Unlike the other two components, microtubules have a channel in the middle. They are organized by proteins called MAPs (Microtubule-Associated Proteins).

Intermediate Filaments

These fibrous proteins hold the cell together, organize its center, and provide mechanical stability. They are found throughout the cell and measure 10 nm.

Microfilaments

Formed by the protein actin, microfilaments are double-stranded helixes. They are dynamic and control cell movement and shape. They are found near the membrane and measure 7 nm.

Cell Cycle

A cell divides to create two identical daughter cells. The cell cycle is divided into three phases:

1. Interphase

The process that precedes mitosis, where chromosomes differentiate.

  • G1: The cell grows and prepares to divide, increasing in size.
  • S: Protein synthesis occurs and the genetic material is duplicated.
  • G2: Components for division (mitosis) are separated.

2. Mitosis

Cell division, divided into:

  • Prophase: The genetic material condenses and forms the mitotic spindle.
  • Metaphase: Chromosomes are separated into two pairs of chromatids and align.
  • Anaphase: The genetic material condenses towards the poles and the nuclear envelope disappears.
  • Telophase: Chromosomes completely separate and come together, forming the nuclear membrane.

3. Cytokinesis

Division of the cytoplasm, where the two cells completely separate. This phase involves protein kinases and phosphatases.

  • Kinase (CAK): Adds enzymes, cyclin-dependent.
  • Phosphatase: Adds enzymes, removes phosphates.

Cyclin + kinase = activated cellular process

M-CDK: Triggers mitosis

CDK: Active cell cycle, regulated by phosphorylation (CAK), dephosphorylation (cdc25), positive feedback.

P53: Recognizes damaged DNA

P21: Inhibits CDK, CDK excludes

Cell Death

Cell death is divided into:

  • Apoptosis: Natural cell death that does not affect neighboring cells. It maintains balance across the cell and is identified by vesicles forming in dying cells. Dead cell pieces are removed by macrophages through phagocytosis. Apoptosis can be extrinsic or intrinsic.
  • Necrosis: Death by trauma to the cell. Cells burst and their contents spill onto neighboring cells.
  • Extrinsic: Cell death induced by external factors, such as a virus. Receptors on the cell surface receive signals from outside the cell, activating procaspase enzymes. The receptor captures the caspase and receives the signal that the cell will die.

Factors that Regulate Cell Amount and Size

  • Mitogen: Secreted proteins and natural or man-made factors that stimulate cells to grow or change their size.
  • Growth Factor: Cell mass increases due to proteins.
  • Survival Factors: Anti-apoptosis factors are activated.

Signal Transduction

Signals are captured by receptors on target cells (cells that receive and execute signals). Signaling molecules can be proteins, peptides, amino acids, nucleotides, steroids, or gases. They can target the cell membrane or the cytoplasm.

There are four types of signaling:

  1. Endocrine: The signaling molecule is secreted and travels through the blood to a distant receiver (e.g., insulin from the pancreas).
  2. Paracrine: The signaling cell produces the signal and the receiver is very close.
  3. Neuron: The neuron receives stimuli from the dendrite and the stimulus travels down the axon, activating the secretion of neurotransmitters.
  4. Contact-dependent: (e.g., immune system)

If there is no receiver, there is no response and the target cell does not act. Two factors influence the signal molecule:

  • Cell Type: One signal can have different effects depending on the cell it acts upon.
  • Receptor Type: One signal can have different receptors.

When a cell does not receive a signal, it dies.

Signaling cascade: Signaling activation leads to an effect.

1 signaling molecule activates 1 receptor, which activates 1 intracellular signaling protein, which activates 1 target protein.

Extracellular Matrix

The extracellular matrix depends on the cellular environment and is made up of three parts:

  • Half sheet
  • Primary cell wall
  • Plasma membrane

Collagen fibers can be simple, triple helix, or collagen fibrils.

Collagen types depend on the type of cell producing the collagen and its location (e.g., fibroblasts in the skin).

Connections and Cell Junctions

  1. Tight Junctions: Involve claudin and occludin proteins, forming a seal between cells.
  2. Adherent Junctions: Involve actin and form adhesion bands that connect actin filaments in adjacent cells.
  3. Desmosomes: Involve cadherin proteins, which anchor intermediate filaments and bind to similar cadherins in neighboring cells.
  4. Hemidesmosomes: Basal junctions that form between the cell membrane and the basal lamina. Intermediate filaments are involved. Three connections are involved: membranal, integrins, and laminins.

Cell Differentiation

Four stages: cell division, cell growth, cell movement, cell specialization.

Cellular communication: Intercellular adhesion, intercellular memory, stem cells, precursor cells in the course of division.

Terminal differentiation: Stem cells have the ability to remain as such and not divide. Free stem cells and precursor cells can be myeloid or lymphoid, which determines the type of cell they will become.

A cell that differentiates sooner may have the capacity to be totipotent (a general cell for cloning).

An unfertilized egg has its nucleus removed and replaced with the nucleus of the cell to be cloned. The cell continues to divide and an early embryo is formed. This embryo can be used for therapeutic or reproductive purposes.

Cancer

Properties of Cancer Cells

  1. Proliferate against normal limitations (benign tumors)
  2. Colonize territories reserved for other cells (malignant, metastasis)

Epidemiology:

  • Environmental factors: Lifestyle and diet
  • Genetics: Genetic susceptibility (inheritance)

Characteristics of cancer cells:

  • Do not depend on external signals
  • Inhibition of apoptosis (cells do not die)
  • P53 mutation: The P53 protein does not function
  • Unlimited proliferation: Cells constantly multiply
  • Genetically unstable
  • Invade tissues abnormally
  • Can survive and proliferate in various tissues

There are two types of cancer cells:

  • Proto-oncogenes: Normal, non-cancerous
  • Oncogenes: Mutated, cancerous
  • A: Oncogene mutation (cell becomes cancerous)
  • B: Tumor suppressor genes (P53)

Proto-oncogene mutations can be caused by DNA sequence mutations, gene amplification, or chromosomal rearrangement.

Cancer therapy:

  • Surgery
  • Radiotherapy
  • Chemotherapy
  • Biological therapy: Antibodies linked to toxins