Cell Communication & The Cell Cycle (11)

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