Question

1) Explain the role of the centrosome and mitotic spindle in cell division. How does dynamic instability of microtubules play a role in its formation?

2) It is common misconception that the cell is not doing anything during G1 or G2. What is the cell doing and how long does interphase take in relation to M phase?

3) Compare cell division in plants to animal cells. Why and how is it different?

4) Predict what would happen to the cell cycle in a cell if the following mutations took place:

1. A mutation alters Cdk so that it doesn’t need cyclin to be active.
2. A mutation inactivates the kinase part of the cyclin-Cdk complex (no phosphorylation).

5) Only damaged cells will undergo apoptosis. Explain why this is true or false.

Answer 1

Answer:

1. Microtubule nucleating sites exert a major influence on spindle assembly. Most animal cells contain a single microtubule nucleating structure, the centrosome, which consists of a pair of centrioles surrounded by amorphous material that harbors templates for microtubule nucleation. The polarity of microtubule growth from centrosomes, with their minus-ends tethered and their plus-ends extending outward, facilitates proper organization of the spindle.

Essential to the process of cell division is the mitotic spindle, which partitions a complete set of chromosomes to each daughter cell. The spindle consists of microtubules, polar dynamic fibers that polymerize from tubulin subunits, as well as hundreds of other proteins that function together to orchestrate chromosome segregation. These include a large set of microtubule-based motor proteins that use ATP hydrolysis to generate movement, or alter microtubule dynamics.

2. In eukaryotic cells, or cells with a nucleus, the stages of the cell cycle are divided into two major phases: interphase and the mitotic (M) phase.

Interphase is the longest phase of the cell cycle. Cell growth is central to the cell cycle, and this is the primary purpose for interphase. At the end of this phase, there is double the amount of DNA, centrioles have replicated, and the cell is big enough for cell division. Interphase is divided into the first growth (G1), Synthesis (S), and the second growth (G2) phases. The growth phases are, as you may have suspected, for the growth of the cell, during the synthesis phase the DNA replication occurs in preparation for the second growth phase.

During G1 phase, also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps.

During G2 phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G2 phase ends when mitosis begins.

3. Plant cell division:-

Plant cell division is the production of two daughter plant cells from a mother cell. Plant's vegetative cell division occurs by mitosis and gametes are produced by meiosis. During the mitotic division of plant cells, they undergo usual M phase and cytokinesis begins after the late stages of the M phase. The cytokinesis is significantly different in plant cells due to the presence of a cell wall. Plant cells form a new cell wall in between the two cells. The new cell wall is identified as the cell plate.

Animal cell division:-

Animal cell division is the production of daughter animal cells from a mother cell. Animals utilize mitosis as the vegetative cell division and meiosis as the reproductive cell division. The phases of mitosis and meiosis are almost the same except the differences in their cytoplasmic division, cytokinesis.

The key differences between plant and animal cell division is that plant cells form the cell plate in between the two daughter cells in mitosis, whereas the cell membrane forms the cleavege furrow in between the two daughter cells in animal cells.