test 4

1. Signaling Types
  • Paracrine signaling: A form of cell-to-cell communication in which a cell produces a signal to induce changes in nearby cells, altering the behavior of those cells. Signaling molecules travel a short distance.
  • Endocrine signaling: Cells secrete signaling molecules (hormones) that travel long distances through the bloodstream to reach target cells throughout the body.
  • Autocrine signaling: A cell secretes a signaling molecule that binds to receptors on its own surface, leading to changes within the same cell. It essentially signals to itself.
  • Contact-dependent signaling: Cells make direct physical contact through molecules bound to their surfaces. This type of signaling often involves transmembrane proteins on one cell interacting with receptor proteins on an adjacent cell.
2. Requirements for Signal Transduction

Signal transduction requires the following steps:

  1. Reception: A signaling molecule (ligand) binds to a specific receptor on or in the target cell.
  2. Transduction: The binding of the ligand to the receptor activates a sequence of intracellular relay molecules, often involving phosphorylation cascades.
  3. Response: The transduced signal triggers a specific cellular response, such as gene activation, enzyme activation, or changes in cell shape.
3. Polar vs. Nonpolar Signaling Molecules
  • Polar signaling molecules: These are hydrophilic and cannot easily pass through the lipid bilayer of the plasma membrane. Their receptors are typically located on the cell surface (e.g., G protein-coupled receptors, receptor tyrosine kinases).
  • Nonpolar signaling molecules: These are hydrophobic and can readily diffuse across the plasma membrane. Their receptors are typically located inside the cell (e.g., steroid hormone receptors).
4. What is a Tissue?

A tissue is a group of similar cells and their extracellular matrix that work together to perform a specific function within an organism.

5. Cytoskeletal Proteins and Their Functions
  1. Microtubules: Hollow rods made of tubulin. They maintain cell shape, facilitate cell motility (cilia, flagella), guide organelle movement, and form the spindle fibers during cell division.
  2. Microfilaments (Actin Filaments): Solid rods made of actin. They maintain cell shape, are involved in muscle contraction, cell motility (amoeboid movement), and cytoplasmic streaming.
  3. Intermediate Filaments: Fibrous proteins (e.g., keratin) supercoiled into thicker cables. They maintain cell shape, anchor organelles, and are more permanent than microtubules and microfilaments.
6. Adhesion Proteins
  • Integrins: Transmembrane receptor proteins that connect the extracellular matrix (ECM) to the cytoskeleton, mediating cell-ECM adhesion. They bind to ECM proteins like collagen, fibronectin, and laminin.
  • Cadherins: Glycoproteins that mediate cell-to-cell adhesion in a calcium-dependent manner. They bind to identical cadherins on adjacent cells.
  • Desmosomes: Cell junctions that provide strong adhesion between cells. They are made of cadherin proteins that link to intermediate filaments within the cell, forming a strong, rivet-like connection between neighboring cells.
7. What is a Motor Protein?

A motor protein is a molecular motor that converts chemical energy in the form of ATP into mechanical energy (force and movement). They work with cytoskeletal filaments to move organelles, vesicles, and chromosomes within cells, and are responsible for muscle contraction and cell motility (e.g., myosin, kinesin, dynein).

8. Difference between Gap Junctions and Plasmodesmata
  • Gap junctions: Found in animal cells, these are protein channels (connexons) that allow for direct cytoplasmic communication and the passage of small molecules and ions between adjacent cells. They enable rapid communication and coordination of activities.
  • Plasmodesmata: Found in plant cells, these are channels that traverse the cell walls of adjacent plant cells, connecting their cytoplasm and allowing the passage of water, solutes, and some macromolecules (e.g., proteins, RNA). They are lined by the plasma membrane and contain a desmotubule (a modified endoplasmic reticulum tubule).
9. Prokaryotic vs. Eukaryotic Genomes and Replication

Genomes:

  • Prokaryotic genomes: Typically a single, circular chromosome located in the nucleoid region. Often contain plasmids (small, extra-chromosomal DNA molecules). The DNA is naked (not associated with histones).
  • Eukaryotic genomes: Multiple, linear chromosomes located within a nucleus. DNA is tightly wound around histone proteins to form chromatin.

Replication:

  • Prokaryotic replication: Occurs in the cytoplasm. Has a single origin of replication. Replicates bidirectionally until the two replication forks meet. Relatively fast.
  • Eukaryotic replication: Occurs in the nucleus. Has multiple origins of replication along each linear chromosome. Replication forks move bidirectionally, and the process is precisely regulated for DNA content. Slower than prokaryotic replication due to the larger, more complex genome.
10. DNA Replication: Conservative or Semi-Conservative?

DNA replication is a semi-conservative process. This means that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. This mechanism ensures that genetic information is accurately passed from one generation to the next, as each parental strand serves as a template for a new strand.

11. Role of Molecules in DNA Replication
  • Primase: An RNA polymerase that synthesizes a short RNA primer (about 5−105-10 nucleotides long) on the DNA template. This primer provides a free 3′3' hydroxyl group for DNA polymerase to add nucleotides.
  • Helicase: Unwinds the DNA double helix at the replication fork, separating the two parental strands by breaking hydrogen bonds.
  • Topoisomerase: Relieves the strain caused by the unwinding of the DNA double helix by helicase. It does this by cutting, swiveling, and rejoining DNA strands, preventing supercoiling.
  • DNA polymerase: Enzyme responsible for synthesizing new DNA strands. It adds nucleotides to the 3′3' end of a growing DNA strand, using the parental strand as a template. It also has proofreading activity.
  • Telomerase: An enzyme that extends the telomeres (ends of chromosomes) in eukaryotic cells. It contains an RNA template to add repetitive DNA sequences to the 3′3' end of the lagging strand template, preventing shortening of chromosomes during replication.
  • Telomeres: Repetitive nucleotide sequences at the ends of eukaryotic linear chromosomes. They protect the genetic information from being lost during successive rounds of DNA replication.
  • Ligase: An enzyme that forms phosphodiester bonds to join DNA fragments (e.g., Okazaki fragments on the lagging strand, or newly synthesized DNA segments after primer removal). It essentially