Unit 4 AP Review

Cell Communication

Three Steps and Two Types

Allows cells to respond to signals from their environment by initiating cellular activity

It usually requires the secretion of a ligand, a chemical signal from the initiating cell that travels to the target cell and binds to a specific receptor protein.

The three steps are reception, transduction and response. Two types are cell-to-cell contact and paracrine signaling.

Reception

The ligand binds to the receptor and causes a conformational change, which transmits a signal inside the cell.

Three Types of Membrane Receptors

Ligand-gated ion channels open or close an ion channel upon binding a ligand.

• Acetylcholine opens ion channels that allow for the flow of sodium, depolarizing muscle cells and causing muscle contractions.

Catalytic (enzyme-linked) receptors have an enzymatic active site on the cytoplasmic side of the membrane. Ligand binding at the extracellular surface initiates enzyme activity.

G-protein-linked receptors bind to a ligand on the extracellular side, then change shape and bind to a different G-protein (originally GDP, then GTP when active) on the intracellular side. This activates secondary messengers in the cell, such as cyclic AMP (cAMP).

Transduction

Moves the signal from the cell membrane to the nucleus by activating second messengers such as cyclic AMP (cAMP)

The second messenger will then phosphorylate—activating—the next protein in the chain. This continues until the signal enters the nucleus.

Phosphorylation cascades can amplify signals.

Response

The final molecule in the signal pathway initiates transcription of target genes in the nucleus, creating a protein that causes cell response.

Cell-to-Cell Contact

A mechanism of short-distance ligand delivery that involves physical contact between the signal and receptor proteins of adjacent cells

Paracrine Signaling

A mechanism of long-distance ligand delivery that involves a ligand, a signaling molecule that can travel short distances through diffusion or long distances through the blood stream

Feedback Mechanisms

Two Types

Sensory pathways and responses that maintain homeostasis

• Negative feedback pathways slow down or stop responses by creating products that inhibit the pathway.

• Positive feedback pathways amplify or increase responses by creating products that promote the pathway.

The Cell Cycle

Includes interphase, mitosis and cytokinesis

Interphase

G1 (Gap 1 or Growth 1): The cell grows and produces nucleotides, enzymes and organisms.

Synthesis Phase (S Phase): DNA is replicated, producing identical DNA strands

G2: The cell finishes growing and produces VIP molecules and structures

Mitosis (M Phase)

The cell divides into two genetically identical daughter cells. This process happens quickly to avoid DNA damage, without the nucleus.

1. Prophase: Nucleus dissolves, DNA condenses to visible chromosomes, spindle fibers form

2. Metaphase: Spindle fibers attach to the centromere via the protein complex kinetochore, chromosomes line up on the equatorial plate (the middle of the cell)

3. Anaphase: Spindle fibers pull the sister chromatids apart and move each set of chromosomes to opposite poles of the cell

4. Telophase: 2 nuclei form, DNA de-condenses, cell membrane starts pinching in

Cytokinesis

The cytoplasm divides and the cell membrane pinches in to form 2 genetically identical daughter cells

Checkpoints

Regulate the cell cycle through cyclins and cyclin-dependent kinases (CDKs)

There are three checkpoints, one at the end of G1, G2, and mitosis. Cells check for damage, mutations and proper internal conditions. Damaged or mutated cells enter cell cycle arrest. If the damage is irreparable, the cell undergoes apoptosis to prevent cancer.

• Necrosis is unplanned cell death.

Cyclins and Cyclin-Dependent Kinases (CDKs)

Growth Factor

Proteins that control cell progression

Growth factor signals the cell to produce checkpoint specific cyclins proteins. At a certain concentration, cyclins will bind to its CDK, initiating a phosphorylation cascade that allows the cell to pass the checkpoint.

After, the phase-specific cyclins is broken down to prevent improper signaling.

Cancer

Uncontrolled cell growth leading to tumors

Cancer cells have mutations in the genes that prevent damaged cells from continuing through the cell cycle. These mutations prevent programmed cell death and allow the cancer cell to grow and divide unlimitedly.