Can you please answer my biology questions?

Question

1. Explain the molecular mechanisms that hlep regulate the cell cycle and the types of disease that can result if a malfunction occurs at any stage

2. how are the chromosomes in a cell at metaphase of mitosis similar to and different from the chromosomes in a cell at metaphase of meiosis 2?

3. Under what circumstances would crossing over during meiosis not contribute to genetic variation among daughter cells?

Answer 1

1. The cell cycle is split into Interphase: G1, S, G2 and Mitosis (P,M,A,T). G1 is a phase of growth and development and nommal cell activity, S is a phase of chrmosome synthesis, and G2 is a phase of mitosis preparation. Before entering a different phase a cell must make sure that all systems are a go. There a checks that a cell must go through before it is ready to entre a new phase of activity. For example. At the end of S phase the cell must ensure that the entire genome has been replicated and that no mistakes were made. After S phase two things will happen, 1) all things are in place, and the cell will proceed to G2 phase, or 2) something is out of the ordinary and the cell will essentiall self-destruct. Why such a dire outcome if S phase failed? Well because a broken genome leads to unwanted mutations. Let's say for example the checkpoint between S phase and G2 failed to recognize a problem. The cell will then entire mitosis in a normal fashion. Let say that a mutation has arisen where the proteins coding for S phase to G2 checkpoint was defunct. Now the cell will essentially continue to replicated with dangerous amounts of mutated genome. This is believed to be one of the leading mechanisms in the appearence of cancer. These check points are essential in ensuring that our genome remains intact.

2. At metaphase of mitosis all chromosomes will align along the centre of the cell and sister chromatids then travel along mitotic spindles to each end of the cell.  In metaphase II all chormosomes also align at the centre and seperate. The difference is that at metaphase II there is only one of the homologues present, they were seperated in meiosis I. In mitosis both homologues are present in one cell.

3. The frequency of crossing over has a lot to do with how far apart genes are from each other on the chromosomes. The closer two genes are the more unlikely they are to seperate, these genes are called 'linked' since they show a less that 50% chance of recombining. The question asks when would crossing over during meiosis not contribute to genetic varation, the only things I can think of right now are double cross over events. Where a cross over happened twice between two genes leading to less recombination.