Cell Structure - Elul

Euchromatin vs. Heterochromatin Location in the nucleolus

Euchromatin - located more centrally in the nucleus

Heterochromatin - located more on the periphery

What is a Histone?

A group of 8 sub-units comprised of 4 different types of proteins, H2A-H2B, H3-H4.

DNA wraps around it to form a nucleosome

Chromatin

Contains DNA of the cell
Nucleosomes packed together (add scaffolding proteins and you can eventually get to chromosome structure)

Two types of chromatin

Euchromatin - loosely packed chromatin, transcriptionally active

Heterochromatin - highly condensed chromatin, transcriptionally inactive

Nucleus

Largest organelle of the cell, contains the genetic material (DNA) of the cell. Responsible for RNA synthesis in the nucleoplasm.

Nuclear Envelope

Double membrane that surrounds the nucleus, nuclear pores throughout the membrane

Nuclear pore Function and structure

Pore that allows ribosomes and RNA to move to/from the cytoplasm
Need proteins importin and exportin to facilitate movement
**requires energy

Cytoplasmic Filament

Cytoplasmic ring

Nuclear Ring

Nuclear Basket

Nucleolus

Located within the nucleus (dark spots), site of ribosome/rRNA synthesis

**good histology marker ->shows which cells are actively synthesizing proteins

Nucleoli

Shown in neurons

Indicates high protein synthesis

Importin vs Exportin

-Work with the nuclear pore to facilitate movement.
-Exportins transport molecules from the nucleus to the cytoplasm(Ex. RNA).
-Importins transport molecules form the cytoplasm to the nucleus(Ex. Proteins)

Ribosome

-Composed of a large and small subunit, assembled in the nucleolus and then released into the cytoplasm -> then large and small sub-units come together
-Synthesize proteins from MRNA in the cytosol

Polyribosome (Polysome)

Many (2 or more) ribosomes translating the same strand of mRNA

If a protein is destined to be secreted or is a transmembrane protein the ribosome will associate with the...

Rough endoplasmic reticulum

**these proteins have a signal recognition particle (SRP) that is recognized ->bound to SRP Receptor

Exocytosis

Contents from a cell is released into the environment via vesicle

Rough Endoplasmic Reticulum

Can be found throughout the cell but is in higher density around the nucleus.
Works with ribosomes (free and bound) in order to synthesize proteins to be secreted or intermembrane. Appears to be studded because of all of the ribosomes associated with the membrane

**if protein is to be;
-secreted -> Its secreted into the ER lumen
-transmembrane-> its inserted into the ER membrane

**RER->extensive system of interconnected tubules, cisternae and vesicles

Clinical Correlation: Cystic Fibrosis

Protein mutation causes mis-folding of a transmembrane chloride channel -> degraded before it makes in to the cell -> causes symptoms (salty sweat, dehydrated mucus)

Golgi Apparatus

-Located in-between the golgi and the plasma membrane.
-Modifies, sorts, and facilitates export of proteins synthesized in the Rough ER.
-Three levels of cisternae (Cis, Med, Trans)
-Transitional compartment ERGIC
-Proteins move through the organelle via vesicles (vesicle mediated transport)

Vesicles

Small circular membrane bound structure that can contain extracellular proteins

What are the protein coats(Names and when used)

-Mechanically facilitate budding
-COPII - ER to I
-COPI - within the golgi
-Clathrin - Trans-golgi -> Plasma membrane (regulated secretion, dock at the plasma membrane then fuse when signaled)

**uncoated vesicles from the trans-golgi (non-regulated secretion, have other final destinations)

Vesicle movement (In terms of membrane order, and which protein coat is used in-between)

ER->COPII->ERGIC->COPI->Cis-golgi->COPI->Med-golgi->COPI->Trans-golgi->Clathrin(Regulated)->Plasma Membrane

Endocytosis

A cell ingests macromolecules, matter, and other substances from the extracellular environment.

Receptor-mediated endocytosis

ligand binds to a receptor, initiated endocytosis. The clathrin coat starts to assemble beneath the budding vesicle (intracellularly). Once the vesicle forms the vesicle loses the protein coat and then fuses with the Early Endosome

**vesicle has increased ligand concentration relative to the cell

Early Endosome

Located near the PM. (CURL) Compartment for Uncoupling Receptors and Ligands (ph 6) -> once uncoupled, receptor returns to pm in small vesicles. The Ligands are sent to the Late Endosome

Late Endosome

Located near the golgi complex. Contains ONLY material to be digested (so only ligands are sent here)

Lysosome

Cellular waste bin (pH of 5), digests and hydrolyzes molecules that were taken in by the cell, this is where ligands are completely digested into small diffusible molecules -> then transferred to cytosol to be used by the cell to exported.

**Contains many acid hydrolyses

Tay-Sachs Disease

Lysosomal disorder where lysosomes cannot digest proteins -> Undigested proteins begin to accumulate in the lysosome causing cells to swell ->cannot function properly

**diagnosed by red macula in retina

Mitochondria

Generates energy (ATP) and reactive oxygen, on the inner-membrane the ETC generates H+ and pumps it into the inter-membrane space, then H+ moves back into the matrix (down its concentration gradient) and this drives the production of ATP (by ATP Synthase)

Outer membrane contains porins with allow a passage was for most macromolecules

Inner membrane impermeable to many things ->cardiolipin (4 FA tails) -> facilitate H+ gradient

**more cristae->higher surface area->more ATP production

**mitochondria has its own DNA

Cytoskeleton

Systems of tubules and filaments that define shape, motility and cellular organization.

**Microtubules, Intermediate Filaments, Actin (localized differently)

Microtubules

Structural support. Located near nucleus.
Radiate out of the centrosome (microtubule organizing center), grow from pericentriolar fuzz to cell periphery.

13 protofilaments (Alpha/beta dimers) around lumen
+/- end (grows from + end)

**centrosome contains centrioles
**dynein and kinesin travel on microtubules

Actin

Cell motility. Located at cell periphery. 2 filaments that wrap around each other. +/- end, (+ end ATP bound (growing end), - end ADP and it is unstable (shrinking end))

Made up of g-actin

**can Interact with myosin(muscle myosin to generate movement)

Intermediate Filaments (+ major classes)

Mechanical Support to plasma membrane.
Ex. Keratin - Epithelial cells
Desmin - Muscle Fibers
Vimentin - Fibroblasts, Endothelial cells
Neurofilaments - Neurons
Nuclear lamins - Nuclear Envelope