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Through which three means do cells communicate?
Juxtacrine, paracrine, and endocrine
Ligand
A molecule that forms a complex with another molecule to fulfill a biological function; in this case, sending a signal
Receptor
A molecule that receives a signal
Ligand-Receptor Complex
The complex created when a ligand and a receptor combine
Juxtacrine Signaling
Signaling through direct contact between cells; can occur through membrane-bound ligand-receptor complexes or cytoplasmic connections
At which stage in a virus's lifecycle would a cell be recognized as an infected cell by the immune system?
When the virus exits the infected cell; cells do not know what is occurring on the inside of another cell, so they are signaled only when something happens externally
What are the two types of cytoplasmic connections?
Gap junctions in animals and plasmodesmata in plants; allow for chain-like reactions
Paracrine Signaling
Short-distance communications between neighboring cells
What is the development of body parts most often regulated by?
Paracrine signaling; leads to different expression of developmental genes
How is an action potential carried over from nerve cell to nerve cell?
Neurotransmitters are released into the synapse (empty space between cells) and attach to the receiving cell's receptors
Endocrine Signaling
Long-distance communications that cannot travel as far simply through diffusion
Are receptors perfect?
No; they are selective but not perfect
Does every cell have a receptor for each ligand?
No; the ligand will have no effect on cells without the correct receptor
What is an example of different cells exhibiting different responses to the same ligand?
Epinephrine causes heart rate to increase, blood vessels to dilate, and the liver to release glucose
What are the four steps of signal transduction?
Signaling, reception, transduction, and response
Signal Transduction
The transmission of a molecular signal from outside to inside the cell to induce a biological response
What are the three methods of reception?
Intracellular receptors, ligand-gated ion channels, and cell surface receptors
What are the three responses to reception?
Cellular metabolism (cytoplasmic enzyme), cell shape/movement (cytoskeletal elements), and gene expression (nuclear gene)
Pathway
A series of molecular interactions in a biological system
Cascade
A signal pathway; upstream = towards the signal, downstream = towards the response
What are the common patterns in pathways?
Secondary messengers, phosphorylation, and signal amplification
Secondary Messengers
Small, non-protein molecules that pass along a signal; can be assembled and disassembled quickly
What are some examples of secondary messengers?
Na+, Cl-, Ca+ (created by adenylyl cyclase)
Is the original ligand in a pathway considered a secondary messenger?
No; it is the primary messenger
Phosphorylation
Proteins are turned on/off by adding a phosphate and off/on by removing a phosphate
Which amino acids are phosphates typically linked to, and why?
Tyrosine, threonine, and serine; they have hydroxyl groups in their side chains
Kinase
Enzyme that adds a phosphate
Phosphatase
Enzyme that removes a phosphate
Signal Amplification
The signal from one ligand-receptor complex can be amplified as the signal moves downstream
What is the benefit of signal amplification?
One signal molecule can evoke a large response
What is the importance of molecules like MAPKKK?
They strengthen responses to a signal because more proteins become phosphorylated
Intracellular Receptors
The ligand (primary messenger) passes through the cell membrane and forms a complex with an intracellular receptor in the cytoplasm or nucleus
Lactase Expression
Lactose forms a complex with Lac-9 which activates the expression of lactase
Ligand-Gated Ion Channels
Ligand opens an ion channel; ion acts as a secondary messenger
Ca+ and Muscle Contraction
Na+ acts as a secondary messenger, connecting electrical impulses to muscule cells' contractile proteins (sarcomeres)
Cell Surface Receptors
A ligand binds to a receptor; G protein-coupled receptors and enzyme-linked (catalytic) receptors
Catalytic (Enzyme-Linked) Receptors
Enzymatic active site on the cytoplasmic side of the membrane; enzyme activity initiated by ligand binding at the extracellular surface
Insulin Receptor
After binding insulin, a pathway is initiated that allows the cell to grow, synthesize lipids, and import glucose
G Protein-Coupled Receptors
A peripheral protein that consists of three subunits and a guanosine phosphate (GTP or GDP)
What is the transduction process in G protein-coupled receptors?
G protein with GDP associates with the receptor, ligand binds, GDP is swapped with GTP, G protein subunits dissociate, and secondary messengers are activated
How does epinephrine transduction work in the liver?
Epinephrine leads to glycogen breakdown in the liver
Homeostasis
The tendency to resist change in order to maintain a stable, relatively constant internal environment
Setpoint
The level at which a condition is being maintained
What are examples of homeostasis?
Maintaining osmolarity, pH, or temperature
Metabolic Regulation
The signal is broken down
How do the liver and kidneys contribute to metabolic signal regulation?
Hormones are broken down in the liver and kidneys, putting a time limit on hormone effectiveness
How can kidney and liver disease affect signal regulation?
It often leads to hormone imbalance
What is mRNA given for its lesser stability compared to DNA?
5' cap and 3' poly-A tail
What is located in the cytoplasm to regulate mRNA?
De-capping proteins and exonucleases
Exonucleases
Enzymes that remove nucleotides one at a time
Negative Feedback Loops
Counters signal; maintains homeostasis at the setpoint
What type of feedback is sweating and shivering an example of?
Negative feedback
Positive Feedback Loops
Amplifies signals; moves away from homeostsis and towards completion at the endpoint
Insulin (Negative Feedback Loop)
High blood sugar causes pancreas to release insulin and cells to uptake glucose; low blood sugar causes pancreas to release glucagon and liver to release glucose
Oxytocin (Positive Feedback Loop)
Oxytocin leads to contractions, which lead to more oxytocin; birth ends the cycle
What are the two phases of the cell cycle?
Interphase (cell growth) and M Phase (cell division)
What are the phases in interphase?
G1, S Phase, G2
What are the phases in the M Phase?
Mitosis (nuclear division) and cytokinesis (cytoplasmic division)
What are the phases in mitosis?
Prophase, metaphase, anaphase, and telophase
In which phases are there checkpoints in the cell cycle?
G1, G2, M
Interphase
Cell growth
In which phase do most cells spend most of their time?
Interphase
What are chromosomes like in interphase?
Uncondensed chromatin
G1 (Gap 1)
Cell increases in size, organelles are duplicated, and proteins for S Phase are translated
S Phase (Synthesis Phase)
DNA is replicated, centrosomes are replicated in animals
Centrosome
Microtubule-organizing centers in animals only
Centriole
Complex cylinders of tubulin at the center of a centrosome
What shape is eukaryotic DNA?
Linear; linear segments are called chromosomes
Centromere
Central region of chromosomes where two sister chromatids connect
G2 (Gap 2)
Preparation of M Phase and last-minute DNA repair
Does more growth occur in G1 or G2?
G1
Early Prophase
Chromosomes condense; mitotic spindle forms through microtubules that form between centrosomes; nucleolus breaks down
Are all spindle fibers microtubules?
Yes
Are all microtubules spindle fibers?
No
Late Prophase (Prometaphase)
Chromosomes condense more; nuclear envelope breaks down; microtubules start to capture chromosomes
Kinetochore
Proteins surrounding the centromere to which microtubules bind onto in late prophase; attachment site
Metaphase
Kinetochores are attached to spindle fibers; chromosomes align along the metaphase plate
Anaphase
Proteins connecting sister chromatids dissolve, forming individual chromosomes; chromosomes move to opposite poles
How do microtubules attached to kinetochores change in anaphase?
They shorten
How do microtubules attached to other microtubules change in anaphase?
They elongate
Telophase
Mitotic spindle is broken down; nuclear envelope and nucleolus reform; chromosomes decondense; cytokinesis begins
What forms during cytokinesis in animals?
Cleavage furrow
What happens during cytokinesis in animals?
Cell membrane is pulled together by actin and myosin filaments
Why can't cytokinesis happen at the same time as anaphase?
Spindle fibers are still attached to chromosomes
What forms during cytokinesis in plants?
Cell plate
What happens during cytokinesis in plants?
Vesicles filled with cell wall components are aligned in the middle
Checkpoints
Stages where requirements must be met for a cell to proceed through the cell cycle
G1 Checkpoint
Checks for size, nutrients, DNA integrity, and cell signals
What happens if a cell fails the G1 checkpoint?
Cell enters G0 until it passes check; G0 is not necessarily a bad thing, most cells are in G0
G2 Checkpoint
Checks for DNA synthesis and DNA damage
What happens when DNA can't be repaired?
The cell undergoes apoptosis
Apoptosis
Programmed cell death
What types of cells undergo apoptosis?
Cells in development, injured cells, cells with DNA damage, and virally-infected cells
Necrosis
Unprogrammed cell death; messy and harmful
What types of cells undergo necrosis?
Cells with DNA damage and virally-infected cells
M Checkpoint (Spindle Checkpoint)
Checks that spindle fibers are connected to all kinetochores, makes sure anaphase starts on time
What could happen if anaphase begins too early?
Chromosomes may be left behind
Cyclins
Four types of cell cycle regulators that are present at different stages of the cell cycle; activate CdKs
Cyclin-dependent Kinases (CdKs)
Present throughout the cell cycle; need to pair with cyclin to be active; activate proteins through phosphorylation
What phase of the cell cycle does a Eukaryote spend most of its time in?
Interphase
