Question

14. What role does the transcriptome (and specifically microRNAs) play in the cell?

15. Describe the process of microRNA function.

20. How does ubiquitination play a role in mitosis?

21. How does Aurora B regulate chromosome alignment? Which stage of mitosis is it active?

23. What are the differences between condensin and cohesin? At what stages of the cell cycle is each added to the DNA and then removed?

Answer 1

14) Two types of miRNA inhibit the E2F1 protein, which regulates cell proliferation. miRNA appears to bind to messenger RNA before it can be translated to proteins that switch genes on and off.

15) The function of miRNAs appears to be in gene regulation. For that purpose, a miRNA is complementary to a part of one or more messenger RNAs(mRNAs). Animal miRNAs are usually complementary to a site in the 3' UTR whereas plant miRNAs are usually complementary to coding regions of mRNAs. Perfect or near perfect base pairing with the target RNA promotes cleavage of the RNA.This is the primary mode of plant miRNAs. In animals the match-ups are imperfect.

20) ​Fig: Regulation of mitosis by ubiquitin system. Faithful regulation of mammalian mitotic division (prophase, prometaphase, metaphase, anaphase, and telophase) requires precise coordination of structural transitions. This machinery involves many critical factors and enzymes that need to be precisely regulated in time and space. Thus, both their timely expression and correct localization to the subcellular compartments must be achieved during different morphological transitions. Blue indicates nucleus and chromosomes; green (representing the CPC component, Aurora B) marks the centromere structures during prometa- and metaphases and midzone and midbody regions during anaphase and telophase, respectively; and red marks microtubules and spindle structures. The critical and major ubiquitin E3-ligases (blue) responsible for coding ubiquitin on mitotic factors are depicted. APC/C E3-ligase is a subject of fidelity control by the spindle assembly checkpoint (SAC) network. The ubiquitin decoders, DUBs (green) and UBPs (red), contribute to fidelity and directionality of mitotic division at specific stages and transitions.

21) Chromosomal alignment during mitosis as well as meiosis is regulated by kinases and phosphatases. The Aurora kinases associate with microtubules during chromosome movement and segregation. Aurora kinase B localizes to microtubules near kinetochores, specifically to the specialized microtubules called K-fibers, and Aurora kinase A localizes to centrosomes.

Aurora B localizes to the chromosomes in prophase, the centromere in prometaphase and metaphase, and the central mitotic spindle in anaphase.

23) Condensin

Condensin I: present in cytoplasm during interphase, has access to chromosomes after prophase. Contributes to condensed chromosome assembly during prometaphase and metaphase.

Condensin II: present in nucleus during interphase, involved in chromosome condensation. Contributes to condensed chromosome assembly during prometaphase and metaphase.

Cohesin:It keeps sister chromatids connected with each other during metaphase and ensures that each sister chromatid segregates to opposite poles. "It facilitates spindle attachment onto chromosomes. It facilitates DNA repair by recombination."

Cohesions are added during late G1/S-phase of mitosis and are removed during the late metaphase/anaphase during chromosome segregation.

Condensins are used in the separation of sister chromatids, which occurs during late metaphaseII/anaphase II.
The reason that I put late G1/S-phase is because research shows that they are added after DNA is duplicated which occurs during the S-phasse of mitosis; whether they are added during G1 or S-phase is still up for debate. Also, they are removed when chromosomes separate and/or sister chromatids, and are required for separation. I also say late metaphase/anaphase I and II because separation or segregation can occur as early as late metaphase and as late as anaphase. I think it is important to just remember that they cohesions are added during interphase and are removed during mitosis/meiosis. Also, that condensins are added during mitosis/meiosis. Histones are removed for DNA replication and are added when chromatin recondense into chromosomes.