Questions & explanations
1. Explain how the phragmoplast ensures that the cell plate forms exactly between the two daughter nuclei.
The phragmoplast assembles from the remnants of the mitotic spindle that are left between the two sets of chromosomes. Microtubules of the phragmoplast are oriented with their plus ends pointing toward the center, where the cell plate will form. Motor proteins and other factors help transport vesicles along these microtubules to the midline. The phragmoplast also contains proteins that mark the division plane, ensuring that the cell plate grows precisely at the equator. As the plate expands, the phragmoplast microtubules shorten and reorganize, always staying at the leading edge of the growing plate. This dynamic behavior keeps the plate centered between the nuclei.
2. What is an actin-binding protein (ABP)?
An actin-binding protein is a protein that attaches to actin filaments and controls their behavior. ABPs can cap the ends of filaments to stop growth, sever filaments into pieces, crosslink filaments into bundles or networks, or start new filament formation (nucleation). For example, capping protein binds to the plus end of actin to prevent addition of new actin subunits. Severing proteins like cofilin cut filaments to create more ends for assembly or disassembly. Crosslinking proteins like filamin link filaments side-by-side, while nucleation factors like the Arp2/3 complex start new branches off existing filaments.
3. What is the first source of energy for a muscle during a short, intense burst of exercise like a 100-meter sprint?
The first source is phosphocreatine, a stored molecule in muscle that quickly gives up its phosphate to make ATP (the main energy currency of cells). This system provides energy for about 10 seconds. After that, the muscle switches to glycolysis, which breaks down glucose without oxygen to make ATP quickly but produces lactic acid. For longer exercise, oxidative phosphorylation in mitochondria uses oxygen to make ATP from glucose or fat. Different muscle fiber types use these systems differently: fast-twitch fibers rely more on phosphocreatine and glycolysis, while slow-twitch fibers use oxidative phosphorylation.
4. Why is cohesin at the centromeres protected from cleavage during meiosis I?
Cohesin at centromeres is protected by a protein called shugoshin, which prevents separase from cleaving it. Shugoshin recruits a phosphatase that keeps the cohesin subunit phosphorylated, making it resistant to cleavage. This protection ensures that sister chromatids stay together until meiosis II. If centromeric cohesin were cleaved in meiosis I, sister chromatids would separate too early, leading to unequal chromosome distribution. The protection is removed in meiosis II, allowing sister chromatids to separate. This stepwise removal is critical for the two rounds of division in meiosis.
5. What is the main difference between cortical microtubules in plant cells and microtubules in animal cells?
In plant cells, cortical microtubules are arranged just under the plasma membrane and guide the deposition of cellulose fibers in the cell wall. Animal cells have microtubules that radiate from a central centrosome and are involved in cell division and shape. Plant cells lack a centrosome, so their microtubules organize differently. Cortical microtubules also form a preprophase band that marks where the cell will divide. This band disappears before division, but it helps position the new cell wall. In animal cells, microtubules form the mitotic spindle directly without a preprophase band.
6. What is the phragmoplast and what is its main job during plant cell division?
The phragmoplast is a structure made of microtubules and other proteins that forms between two daughter nuclei during plant cell division. Its main job is to guide the formation of a new cell wall, called the cell plate, which splits the cell into two. The phragmoplast acts like a scaffold, directing vesicles carrying cell wall materials to the middle of the cell. These vesicles fuse together to create the cell plate, which grows outward until it reaches the existing cell wall. This process is unique to plants because they have rigid cell walls that cannot be pinched like animal cells.
7. Compare the removal of cohesin in meiosis I versus meiosis II.
In meiosis I, cohesin is removed from chromosome arms but not from centromeres. This allows homologous chromosomes to separate while sister chromatids remain attached. In meiosis II, the centromeric cohesin is also cleaved, allowing sister chromatids to separate. The timing is controlled by different regulatory mechanisms. In meiosis I, separase is activated but shugoshin protects centromeric cohesin. In meiosis II, shugoshin is degraded, so centromeric cohesin becomes vulnerable. This difference ensures the two distinct divisions: reductional in meiosis I and equational in meiosis II.
8. How is Notch signaling different from signaling that uses second messengers?
Notch signaling does not use second messengers like cAMP or calcium. Instead, the receptor itself is cut and a piece goes directly to the nucleus. This is much simpler and faster. Second messenger systems amplify the signal, but Notch does not amplify; one receptor activation leads to one NICD. Notch signaling is also irreversible because the receptor is destroyed. In contrast, second messenger signals are reversible. Notch is used for short-range, contact-dependent communication, while second messengers can spread signals inside a cell. Both are important for different purposes.
9. Compare the roles of ATP hydrolysis in actin treadmilling and in microtubule dynamic instability.
In actin, ATP hydrolysis happens after a subunit adds to the filament, turning ATP-actin into ADP-actin. This makes the filament less stable at the minus end, promoting treadmilling. In microtubules, GTP hydrolysis occurs after tubulin adds, turning GTP-tubulin into GDP-tubulin. This creates a GTP cap at the plus end that stabilizes the microtubule; loss of the cap leads to rapid shrinking (catastrophe). Both use nucleotide hydrolysis to create instability, but actin uses it for steady treadmilling while microtubules use it for dynamic switching between growth and shrinkage.
10. Compare the actions of cAMP and IP3/DAG pathways.
Both cAMP and IP3/DAG are second messengers that relay signals from the cell surface. cAMP is made from ATP and activates PKA, which phosphorylates proteins. IP3 and DAG are made from a membrane lipid; IP3 releases calcium, and DAG activates PKC. cAMP acts mainly in the cytoplasm, while IP3/DAG involve both membrane and calcium signals. Both pathways can change gene expression, but they use different enzymes. Some signals activate both pathways at the same time, leading to a combined response. They are examples of how cells use different messengers for different signals.
11. What is juxtacrine signaling?
Juxtacrine signaling is when a signal is passed between two cells that are touching each other. The signaling molecule is on the surface of one cell, and the receptor is on the surface of the other cell. For example, in the Notch pathway, the Delta ligand on one cell binds to the Notch receptor on the neighboring cell. This direct contact allows very precise communication. Juxtacrine signaling is important in development, where cells need to talk to their immediate neighbors to decide their fate. It is different from paracrine signaling, which uses secreted signals.
12. How does the cell plate grow from the center to the edge of the dividing plant cell?
The cell plate starts as a small disk in the middle of the cell, right between the two new nuclei. Vesicles from the Golgi apparatus, filled with pectins and other cell wall components, travel along phragmoplast microtubules to the center. They fuse with each other and with the growing plate, adding more material. As more vesicles arrive, the plate expands outward in a ring-like fashion. The phragmoplast microtubules also rearrange, forming a ring that moves toward the cell edges. Eventually, the cell plate fuses with the parent cell wall, completing the separation.