Nucleoli are present during _____.

A.Metaphase.

B.Anaphase.

C.Interphase.

D.Prometaphase.

E.Prophase.

Diagram of the cell cycle
Diagram of the cell cycle

The correct answer is C. Interphase.

Cells divide for purposes of reproduction or growth and repair of tissues. Eukaryotic cells have a cycle of stages that occur in a specific sequence. The stages of interphase happen when the cell is not actively dividing.

Interphase has three stages that always occur during which many reactions and activities occur, these stages are G1, S, and G2. Mitosis occurs after G2 and before G1 occurs and is the time when a parent cell actively divides to form two new separate daughter cells.

G1 is a time when protein synthesis occurs and the cell goes through a checkpoint in which the decision is made to progress or not. If the decision is made to divide then the cell enters the S stage when DNA is synthesized.

The final checkpoint before mitosis occurs happens in G2. The DNA is checked for damage and if it is damaged it is either repaired or the cell self-destructs. If the cell passes this checkpoint it enters the M stage.

The M stage is mitosis when the cell actively divides and passes through the phases of prophase, prometaphase, anaphase, telophase, and cytokinesis.

It is during the mitotic stages that the nucleolus and nuclear envelope disintegrate at the start of the process and then form again at the end.

The eukaryotic cell cycle

The cells of eukaryotic organisms have membrane-bound organelles such as a nucleus containing a nucleolus. This means that the division of these cells is a complex process, more so than when prokaryotic cells divide.

Eukaryotic cells undergo a complex cell cycle during which there is a sequence of events that occur. The stages that occur in these cells when not dividing are known collectively as interphase. This is the time when cells are growing, forming proteins and in some cases, preparing for cell division.

Interphase consists of three stages which occur in a specific order: G1, S and G2 stages. It is after G2 and before G1 that the M stage occurs when the cell actually divides.

Interphase

During interphase, the nucleus and nucleoli are still present in the cell. No breakdown of these structures occurs until the cell leaves interphase and begins mitosis.

Many activities occur during the G1 (Gap 1) stage including the transcription and translation of proteins in the cell. Several biochemical reactions occur in the cell during this stage of interphase.

Proteins known as cyclins along with cyclin kinase enzymes regulate the progression of the cell through this stage of the cycle. If conditions are not right then the cell sometimes enters a stage known as G0 during which it becomes dormant.

There is a checkpoint in G1 when the cell is checked to see if conditions are favorable for entry into the next stage, S, of interphase. If conditions are favorable, different cyclins are translated in preparation for entry into this next stage.

The S stage of interphase is when DNA is synthesized so that extra chromosomal material is present. This is necessary so that when the cell does divide there will be enough genetic material present for the daughter cells.

The G2 (Gap 2) stage is the final stage of interphase that occurs before the cell divides. Microtubular spindle fibers begin to be made during this stage in preparation for the next stage. There is also an additional checkpoint when the DNA is checked for damage.

If the genetic material is damaged then the cell may either attempt to repair the DNA or trigger apoptosis, which is programmed cell death.

Mitosis

The cell enters the M stage or mitosis at the end of the G1 stage of the cell cycle. Mitosis is the stage when the cell divides to form two daughter cells from one parent cell. This form of division occurs in body cells for growth and repair of tissues.

The stages of mitosis occur in a set order which is as follows: prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.

Prophase is the first phase that occurs in the mitotic cell division stage of the cell cycle. During this time the chromatin material reorganizes so that the chromosomes become visible. The mitotic spindle also begins to become visible at this time and the nucleolus also starts to break down.

During prometaphase, the nuclear membrane starts to disintegrate and kinetochores form on each chromosome. The spindle fibers attach to the kinetochore regions on the chromosomes.

There is a kinetochore on each chromatid of the chromosome, and there are two chromatids in each chromosome.

In the next stage of mitosis, metaphase, the chromosomes that are now all attached to spindle fibers, align at the equator of the cell. They line up in a single row with a spindle extending from each chromatid to the end of the cell.

Anaphase to cytokinesis

The spindle fibers consist of several proteins, including microtubular proteins. The proteins contract causing these fibers of the spindle to shorten. This, in turn, acts to pull the chromatids of each chromosome apart.

It is important to remember that a spindle fiber is attached to each kinetochore and each kinetochore is on a separate chromatid of the chromosome. The chromatids thus move apart to opposite sides of the cell.

Telophase is the stage when all the chromatids are at the opposite ends of the cells. Now the nucleolus starts to reform, chromosomes form chromatin material again, and nuclear membrane is deposited. At the end of telophase, the cell is ready for the final step of cell division.

This is the cytokinesis in which the cytoplasm itself divides at the center of the cell. In plant cells, vesicle produced by the Golgi apparatus lineup in the middle of the cell to form a cell plate which will become a cell wall.

In animal cells, the cytoplasm pinches in at the center of the cell until two separate cells are formed, each with their own genetic material, membrane-bound organelles, and cell membrane.

References

  1. Editors of Encyclopedia Britannica (2018). Cell cycle. Retrieved from Encyclopedia Britannica.
  2. J Lukas, J Bartkova, J Bartek (1996). Convergence of mitogenic signalling cascades from diverse classes of receptors at the cyclin D-cyclin-dependent kinase-pRb-controlled G1 checkpoint. Molecular and Cellular Biology.
  3. LE Rosenberg, DD Rosenberg (2012). Growth, development and Retrieved from sciencedirect.com.
  4. RH Raven, RF Evert, SE Eichhorn (1987). Biology of plants, 4th edition. New York: USA, Worth Publishers.
  5. TD Pollard, WC Earnshaw, J Lippincott-Schwartz, G Johnson (2017). Cell Biology, 3rd  USA: New York, Elsevier Publishers.

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