Cell Development and Signaling

Cell Development and Signaling

• Cell development is orchestrated by cell signaling and modulated by gene expression. This process leads from fertilization to a fully developed organism.
• Stem cells divide and differentiate into specialized cells.
• Differentiated cells cannot further divide and perform specific functions (e.g., muscle cells generate force, neurons transmit signals, bone cells provide structural support).
• Mesenchymal stem cells (MSCs) can differentiate into various cell types including fat, bone, muscle, tendon, ligament, cartilage, and skin.
• External and internal signals (neighboring cells, cell membrane components) guide cell differentiation.
• Apoptosis is programmed cell death.
• Transcription factors regulate gene expression and cell communication.
• Cell communication involves signaling molecules released by signaling cells binding to receptor proteins on responding cells, initiating signal transduction pathways.
• The extracellular matrix and adjacent cells influence cell behavior.
• The MAPK/ERK pathway is activated during cell division, stimulating gene expression for cell renewal; its dysregulation can lead to cancer. Raf and MEK are oncogenes within this pathway.
• The EGF pathway, frequently mutated in cancers, regulates cell division; a constitutively active receptor can cause uncontrolled growth. Ras protein, a key component, is often affected by mutations reducing GTPase activity.
• Signal termination mechanisms include kinase dephosphorylation by phosphatases, receptor endocytosis, and the use of inhibitors. Ubiquitin-mediated proteolysis targets proteins for degradation in the proteasome.
• Kinases and phosphatases regulate protein activity; positive and negative feedback loops control signaling pathways. The firefly flash is an example of a negative feedback loop (oscillatory pathway).

Cell Cycle Control, Mitosis, and Meiosis

• The cell cycle includes interphase (G1, S, G2, G0 phases) and mitotic phases. The G0 phase is a quiescent state; cells may re-enter the cycle.
• Chromatin condenses into chromosomes during cell division. Humans have 23 pairs of chromosomes (22 somatic pairs, 1 sex pair).
• Haploid number refers to the number of unique chromosomes (23 in humans); diploid number refers to the total number of chromosomes (46 in humans).
• Centromeres join sister chromatids; kinetochores are attachment sites for microtubules during cell division.
• Uncontrolled cell division leads to cancer.

Cell Structure and Membrane Dynamics

• The cell theory states that all organisms are composed of cells, cells are the basic units of life, and cells come from pre-existing cells. Viruses are not considered living organisms.
• The plasma membrane defines cells, separating them from their environment and facilitating compartmentalization (in eukaryotes) and communication.
• Cell membranes are composed of phospholipids, which are amphipathic (both hydrophilic and hydrophobic). Phospholipids form bilayers in aqueous environments. Micelles form if the hydrophilic portion is more prominent.
• Membrane fluidity is influenced by temperature and fatty acid composition (saturated vs. unsaturated). Cholesterol (animals), sitosterol (plants), and hopanoids (bacteria) modulate fluidity.
• Membrane proteins are integral (span the membrane) or peripheral (attach to one side). The fluid mosaic model describes the dynamic nature of membrane components.
• Transport across membranes can be passive (simple or facilitated diffusion, osmosis through aquaporins) or active (primary or secondary, requiring ATP). The Na+/K+ pump is an example of primary active transport.

Organelles and Endosymbiosis

• Mitochondria produce ATP through cellular respiration; chloroplasts (in plants) conduct photosynthesis.
• Mitochondria and chloroplasts possess features suggesting bacterial ancestry (circular DNA, double membranes, ribosomes, independent replication), supporting the endosymbiotic theory.