Nucleus and Mitosis

Question 1

Where in the nucleus is most RNA being synthesized?

Answer 1

The nucleolus is the site of most rRNA synthesis. A small fragment is synthesized in the rest of the nucleus.

Question 2

What is happening in the dense fibrillar zone of the nucleolus?

Answer 2

rRNA is being transcribed

Question 3

What is happening in the granular zone of the nucleolus?

Answer 3

The granular zone is the site of ribosomal packing and processing.

Question 4

What is the structure of the nuclear envelope?

Answer 4

The nuclear envelope has a double membrane. It is continuous with the endoplasmic reticulum and the cisternal space is continuous with rER lumen.

Question 5

What is the importance of lamin?

Answer 5

Lamin is an intermediate filament lining the inside of the nuclear envelope. It attaches to chromatin and keeps it in place n relation to the envelope. It is phosporylated during mitosis, which results in a disruption of the lamins and the attached envelope (into numerous, lamin bound vesicles.

Question 6

How is DNA packed?

Answer 6

DNA consists of a pair of helices wraped around two tetramers of histone proteins. These nucleosomes are coiled into a solenoid (one wrap of nucleosomes) and these are folded into chromosomes.

Question 7

What does condensin do?

Answer 7

Protein that helps to create solenoids in the chromosome

Question 8

Describe the nuclear pore complex?

Answer 8

Protein with NLS is binding by importin (need GTP hydrolysis) -> Ran GDP on outside of nucleus and Ran GTP on inside of nucleus

Question 9

What is the longest phase in mitosis?

Answer 9

G1: initial growth phase

Question 10

How is the cell cycle regulated?

Answer 10

The cell cycle is regulated by the appearance and disappearance of cyclins. Cyclins interact with cyclin-dependent kinases (Cdks) that phosporylate a large number of proteins that contribute to the progress of the cycle. Cyclins will activate different CDKs (M-cyclin for mitosis, S-Cdk for DNA sythesis, for example). At the end of its part of the cycle, the cylin will be bound to ubiquitin, that will tag it for destruction by proteosomes.

Question 11

What are the components of interphase in the cell cycle?

Answer 11

G1 (Gap 1; growth of cell), S (DNA replication), G2 (Growth of Cell and replication of organelles).

Question 12

What are “checkpoints” and when do they occur?

Answer 12

These are places at which the cell cycle may be interrupted if things are not right. The checkpoint at G1 detects signals saying that the environment sufficient for division, that DNA is undamaged. The checkpoint at G2 evaluates if the cell is big enough and the DNA is replicated. The checkpoint at M, metaphase, evaluates if all chromosomes lined up at the metaphase plate

Question 13

What is the role of p53?

Answer 13

This protein is phosporylated in the presence of damaged DNA. It turns on the gene for p21, which produces a protien that will arrest the cell cycle until the DNA is repaired.

Question 14

What occurs during prophase?

Answer 14

The chromatin condenses to chromosomes, centrosomes migrate, MT assemble; nucleolus disappears

Question 15

What occurs during prometaphase?

Answer 15

There is breakdown of the nuclear envelope with phosphorylation of nuclear lamins. Spindle fibers (microtubules) attach to the kinetechores of the chromosomes (kinetochore microtublues).

Question 16

What occurs during metaphase?

Answer 16

Chromosomes are lined up at the metaphase plate. Kinetochore microtubules are attached to spindle poles (centrosomes) and the (inter)polar microtubules are overlapping with those form the other spindle pole.

Question 17

What happens during anaphase:

Answer 17

he paired sister chromatids separate, moving toward the spindle poles (centrosomes). Dynein on the astral microtubules pull, kinesins on the (inter)polar microtubules push the chromosomes apart and progressive depolarization of the kinetochor microtubules pull the chromosomes apart.

Question 18

What occurs during telophase?

Answer 18

Chromosomes approximate the centrosomes. The nuclear envelope reforms as lamins are dephosphorylated and associated vesicles reform. Cytokinesis (division of the cytoplasm) is complete.

Question 19

What process is necessary for cytokinesis?

Answer 19

A contractile contractile ring comprised of acting filaments can interact with myosin to pinch off the cell into two daughter cells.

Question 20

What is apoptosis and why is it important?

Answer 20

This is programmed cell death. It is not inflammatory and is due to activation of intracellular mechanisms that cause destruction of the cell from the inside, with phagocytosis of the resultant debris. It is important for normal development, since it will get rid of unwanted cells withut damaging surrounding tissue.

Question 21

What intracellular signaling molecule is key in induction of apoptosis?

Answer 21

Caspases are the most important signaling molecules for apoptosis. These can be activated by signals from outside the cell or by various intracellular processes (such as release of cytochrome c from the mitochondria).

Question 22

Distinction between centromeres and kinetichores?

Answer 22

Centromere is a region of DNA where sister chromatids attach

Kinetochores are a group of proteins in the centromere region where microtubules attach.