Specialized structures that join cells.

3 types in animal cells:Adhesive Junctions, Tight Junctions, and Gap Junctions

In Plant cells: Plasmodesmata

Type of Adhesive Junction that are points of strong adhesion between adjacent cells.

Beneath the plasma membrane of each adjoining cell is a plaque (fibrous material including plakoglobin and desmoplakin which associate with intermediate filaments extending into the cell interior).  The plaque is connected to the transmembrane proteins, desmocollin and desmoglein (both cadherins; glycoproteins that complex with Ca⁺² to join cells;

Type of Adhesive junction that connects a cell to the extracellular matrix instead of another cell.

Beneath the plasma membrane is a plaque (fibrous material including the protein plectin) which associates with intermediate filaments (keratin) that extend into the cytoplasm. Integrins (transmembrane proteins) link the plaque to the extracellular matrix.

A type of Adhesive Junction that involves actin microfilaments instead of intermediate filaments.

Plaques contain numerous proteins which associate with microfilaments that extend into the cytoplasm and transmembrane cadherins which link cells together.  In epithelial cells, adherens junctions often form a continuous adhesion belt around the cells

 involved.

Tight Junctions

What do they make possible?

Found as continuous belts that serve as seals preventing water/solute movement between cells.

They make cell compartmentalization possible.

Six transmembrane protein (connexin) molecules associate to form a hollow cylindrical structure in the plasma membrane (connexon).  Connexons in the membranes of adjacent cells align to form a pipe connecting two cells.  The resultant hydrophilic channel conducts water, ions, and other small molecules

1) measuring the current flowing between cells in response to a voltage potential.

2) by monitoring the movement of dye from one cell to another.

At low calcium concentration and high pH the channels open.

At high calcium concentration and low pH the channels close.

Plasmodesmata

What do they allow?

Cytoplasmic connections between plant cells that aid in communication.

They allow for the intercellular transport of water and solutes such as ions sugars, amino acids, and small proteins. Excluded are large proteins and RNA molecules.

Extracellular matrix (ECM)

What is the ratio of cells to matrix?

Consists of various materials produced and secreted by cells. Is found in all 5 kingdoms where it plays an architectural, supportive and other funtional roles.

The ratio of cells to matrix is highly variable.

In animals the ECM is produced primarily by Fibroblasts, Ondontoblasts, osteoblasts, and chondroblasts.

Major groups of ECM molecules include

 glycosaminoglycans & proteoglycans, fibrous proteins, and adhesive

 glycoproteins.

What are GAG's composed of?

 what do they attract?

what are they linked to?

composed of repeating chains of disaccharides, one of which is always an amino sugar (dermatan sulfate and hyaluronan)

The negative charges of the carboxyl and sulfate groups attract cations (esp Na+) allowing water to move into the matrix.

They are often covalently linked to core proteins forming proteoglycans.

What are proteoglycans linked to?

What are their functions?

can be linked to hyaluronan (hyaluronic acid) to form enormous complexes.

Function as molecular sponges

bedding for other extracellular components

sites of attachment

enzyme regulators

what does collagen assembly require?

What are elastins?

Collagen assembly requires proteolytic cleavage of procollagen before incorporation into fibrils

Elastins are covalently linked polypeptides that can adopt alternative configurations to stretch and recoil.

G1 (1st gap) - growth

S - DNA synthesis

G2 (2nd gap)

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis (division of the cytoplasm)

(mitosis = nuclear division)

What produces MPF and how does MPF activate?

mitotic cyclin combines with a cyclin dependent kinase (cdk) to produce the maturation promoting factor (mpf)

MPF activation requires the kinase and phosphotase activity of other regulatory proteins.

Activated MPF moves cells from G2 to M phase by initiating:

disassembly of nuclear lamina

chromosome condensation

spindle fiber formation

anaphase promoting complex (APC) which tags proteins (anaphase inhibitors and mitotic cyclin) for degredation.

external signals recieved from the extracellular environment

internal signals related to the physiological state of the cell.

destroying numerous caspases (including MAP KKK and Raf)

destroying nuclear lamins

activating DNase

inactivating DNA repair mechanisms

Defects in apoptosis/the cell cycle can lead to cancer.

cancer usually results from mutations in the following genes:

proto-oncogenes (converted into oncogenes)

tumor supressor genes

DNA repair genes

Oncogenes are genes that cause cancer when present. Often they code for altered growth factors, receptor tyrosine kinases, G proteins, Kinases (like raf), transcription factors, cyclins, and Cdk's

Altered oncogene proteins lead to uncontrolled cell proliferation.

What does UV light do?

Cells increase levels of p53 gene product present (by decreasing degredation) which functions as a transcription factor to increase p21 gene transcription.  The p21gene product is a Cdk inhibitor that blocks G₁ cyclin-Cdk complex activity.  Cells cannot move from G₁ into the S phase which gives cells time to repair

 damaged DNA.

If the damage is beyond repair, or if the cell has alreadypassed the G₁ checkpoint, p53 triggers apoptosis by aninternal pathway.  p53 recruits Bax (Bcl-2 family member)which forms homodimers that facilitate the release of cytochrome c from mitochondria.  Cytochrome c with

 Apaf-1 activates caspases which kill the cell.

Human papillomavirus encodes for protein E7 which

 inactivates the Rb protein so that it no longer binds to the

 E2F transcription factor (refer to handout).  E2F facilitates

 the transcription of genes required for DNA synthesis

 leading to cell proliferation and cervical cancer (fig 20-43).