Cell Biology - GRE

The chloroplast has a very simular structure to the mitochondrion, as it is a double-membraned organelle. The chloroplast is used to house the processes of photosynthesis. The light reactions take place in the thylakoid membrane, while the light independent reactions take place in the stroma.

The endoplasmic reticulum, while crucial to protein synthesis, is also an intracellular calcium storage organelle, and calcium can be released in response to signaling cascades as needed. None of the other organelles listed participates in storage of calcium.

The smooth endoplasmic reticulum lacks ribosomes on its surface, meaning it does not participate in protein translation. It does play a key role in the production of various lipids, such as phospholipids and triglycerides.

The endoplasmic reticulum (ER) is the cellular organelle responsible for membrane synthesis. Products from the ER are moved to the Golgi, where they are tagged and shipped off to their final destination. Lysosomes are important for degradation and ribosomes play a role in translation. Mitochondria are for energy production in the form of ATP.

Second messengers are the molecules in a signal transduction pathway that will activate an intracellular response. Epinephrine is a hormone that will bind a receptor on the exoplasmic face of the cell, making is a first messenger. G subunits interact with adaptor proteins that will then stimulate the production of second messengers. Receptor tyrosine kinases are examples of receptor proteins that will bind first messengers. cAMP, however, is a widely used second messenger that is involved in the activation of many pathways and signal amplification in the cytosol.

A second messenger response is created in cells that use signal transduction, meaning that the signaling molecules attach to a receptor on the outside of the cell. Steroid hormones are largely nonpolar, and can enter the cell in order to affect cellular processes at the level of transcription. As a result, they do not need to rely on second messenger pathways in order to elicit a response.

The phosphorylation of Erk is the final step of the protein cascade of EGF pathway, and phosphorylated Erk enters the nucleus to increase transcription of genes that induce proliferation. If Erk is constitutively active, it will likely lead to higher proliferation rate.

Preventing EGF from binding to EGFR or disrupting EGFR's ability to enter the membrane would abolish EGF pathway activity and reduce proliferation. Likewise, abolishing kinase activity of RAF would terminate the signal transduction and lead to reduced proliferation.

Erythropoietin, released by the kidney, stimulates the production of red blood cells, which becomes necessary if circulating has decreased. Tumor necrosis factor stimulates systemic inflammation and regulates the immune system. Transforming growth factor beta 1 controls cell growth, proliferation, differentiation and other processes. Interferon type II modulates immune functions. Interleukin 2 also modulates the immune cells.

Erythropoietin is a glycoprotein that is crucial for the production of red blood cells, a process also called "erythropoiesis." Each of the other answers contains a growth factor, but none of these have a primary function in red blood cell production.

The correct answer is master regulators promote the deposition of methyl or acetyl groups to mark inactive or active enhancers. Master regulators bind enhancer regions that have been created by pioneer factors to establish the chromatin state of the cell by deposition of methyl or acetyl groups on chromatin. Methylation correlates with inactive enhancers, whereas acetylation correlates with active enhancers. The fingerprint of active/inactive enhancers and its effect on gene expression establishes cell subtypes. Some, but not all master regulators function as pioneer factors to bind nucleosome rich DNA to promote euchromatin formation.

Bax and Bak dimerize to form a pore in the mitochondria outer membrane, which allows cytochrome c to escape into the cytosol. When cytochrome c is found in the cytosol, procaspase becomes activated and is cleaved into caspase. Once the caspase cascade begins the cell is destined for death.

Bax and Bak have nothing to do with the apoptosome and, while Bcl2 does block Bax and Bak from dimerizing, Bax and Bak do not prevent the action of Bcl2.

Interactions between , , , and PKC do indeed occur downstream of activation of PLC to contribute to numerous downstream cascades primarily initiated by protein kinase C (PKC). However, it is important to understand that the second messengers are and , which are specifically formed by the cleavage of , and each of the other molecules is considered an effector of those second messengers in this context.

A GEF activates a small GTPase by exchanging a bound GDP (which confers an inactive state) for a GTP (which is higher energy, and activates the protein). A GAP performs the opposite; GAPs enhance the intrinsic GTPase activity of the small GTPase, which causes hydrolysis of the GTP on the active protein, thus converting it back to GDP and an inactive state.

Proteins have different level of protein structure, termed primary, secondary, and tertiary (quarternary is also a type in certain proteins). The 3D shape of proteins is largely due to the tertiary structure of a protein. This level is dictated by the specific amino acid sequence of the protein.

Mannose-6-phosphate is a post-translational modification found on proteins important to the functionality of the lysosome (such as acid hydrolases). Ubiquination is a signal for proteins to be brought to the proteosome and degraded. Myristoylation involves the addition of a fatty acid chain, and is often seen in proteins targeted to the plasma membrane. Acetylation is a common modification found on histones that can help make genes transcriptionally active.

An isomerase is an enzyme that catalyzes the rearrangement of bonds in a single molecule. For example glucose-6-phosphate isomerase catalyzes the conversion of glucose-6-phosphate into fructose-6-phosphate during glycolysis.

A hydrolase catalyzes a hydrolytic cleavage reaction, a kinase catalyzes the addition of a phosphate group, and a polymerase catalyzes polymerization reactions.

The correct answer is ubiquitination. Ubiquitin is added to the substrate protein to target the protein for degradation by the proteasome, serving as an efficient mechansim to control cellular protein levels. Myristoylation, palmitoylation, isoprenylation, and glycosylation are all post-translational protein modifications that involve the addition of a 14-carbon saturated acid, a 16-carbon saturated acid, an isoprenoid group, and a glycosyl group, respectively. These modifications have diverse functions, however, do not initiate the degradation of the protein.

While each of these molecules could potentially be bound to a protein as a post-translational modification, the only one listed that is a fatty acid is palmitate. Thus, this is the correct answer.

The correct answer is adenosine triphosphate (ATP). In order to phosphorylate a substrate, kinases catalyze the hydrolysis of ATP to adenosine diphosphate (ADP) and inorganic phosphate. This released phosphate by the hydrolysis reaction is covalently added to an amino acid residue on the substrate. Nicotinamide adenine dinucleotide phosphate, flavin adenine dinucleotide, and nicotinamide adenine dinucleotide are proton carriers. Guanine nucleotide exchange factor aids in exchanging guanine diphosphate for guanine triphosphate in a substrate.

The correct answer is all of the answers are cellular processes in which ubiquitination is involved. Post-translational ubiquitination of proteins initiates many cellular processes by altering protein activity and the proteins that interact with the ubiquitinated protein.