Mitotic phase, Practice Problems and FAQs

Have you ever looked down at your leg or arm to find some cut that has already healed but you have no idea where it has come from or how you got it? You might have just looked at your nails and noticed that they are a lot longer than the last time you trimmed them. And how often have you looked at old pictures and wondered how I grew up this big?

What do all these incidents have in common? All these events are a result of a very crucial process in our body, that is, cell division. The most common type of cell division seen in the majority of our body cells is mitosis. 

The healing of our wounds, the growth of our nails, the growth of our entire body is due to mitotic cell division in our body cells. But if we can heal wounds and grow due to cell division, then do we have superpowers to heal all wounds and keep growing indefinitely? No, we don’t because cell division is a very tightly regulated process and cells divide only when signalled to do so. So how does mitotic cell division occur? Let us try and understand the process of mitosis through this article.

Table of contents:

• Introduction to Cell Cycle
• M Phase
• Karyokinesis
• Cytokinesis
• Significance of mitosis
• Practice Problems
• FAQs

Introduction to Cell Cycle

The cell cycle is the entire series of events that take place in a cell due to which it duplicates its cell content and genome and divides into two daughter cells is known as cell cycle. There are two phases in the cell cycle -

Interphase: A phase of cell cycle when the cell prepares itself for cell division.

Mitotic phase or M phase: A phase of cell cycle when the actual division of cell occurs.

M phase of the cell cycle occurs after the interphase. The division during M phase can be mitotic or meiotic. Meiotic division occurs during formation of gametes in the gonads and results in the formation of four daughter cells with half the number of chromosomes compared to parent cells. In somatic cells, M phase involves the mitotic division of a cell in which two similar daughter cells are formed with the same number of chromosomes as that of the parent cell. Since the daughter cells have the same number of chromosomes as the parent cell, it is also known as equational division.

Fig: Schematic representation of mitosis

Mitotic phase (M phase)

The mitotic phase or M Phase occurs in almost all cells of a growing embryo and in later stages it occurs in all the cells of the body of higher organisms, except for the gonads. Mitosis is divided into two stages:

• Karyokinesis - division of nucleus

• Cytokinesis - division of cytoplasm

GIF: Different stages of mitosis

Karyokinesis

The word karyokinesis is derived from two words:

• Karyon- Means nucleus
• Kinesis- Means movement

This stage of mitosis involves division of nucleus which occurs through following stages:

• Prophase
• Metaphase
• Anaphase
• Telophase

Prophase

It is the first stage of karyokinesis. This stage is the longest of all phases in terms of duration. This stage is recognised by the formation of condensed chromosomes each having a pair of sister chromatids.

GIF: Prophase

Changes that occur in prophase

Early Prophase

Refractivity and viscosity of cytoplasm increases. Nucleus becomes spheroidal. Animal cells become spheroidal due to depolymerisation of cytoskeletal microtubules. Chromatin fibres thicken and shorten to form long stainable chromosomes. The sister chromatids which were intertwined during the G2 phase of interphase become untangled during chromatin condensation. The ends of elongated chromosomes are not visible and the chromosomes overlap each other and appear like a ball of wool. This is known as the spireme stage. 

The two centriole pairs which were so far close to each other start moving away from each other and towards the poles. The centrioles start radiating microtubular fibrils known as astral rays. The centrioles and the radiating astral rays together form the star-shaped aster.

In plant cells, centrioles are absent. An area present just near the nuclear membrane called Microtubule Organising Centre (MTOC) can serve to organise and assemble the microtubules required for spindle fibre formation.

Mid Prophase

Further condensation of chromosomes makes them shorter and thicker and their ends become more prominent. Each chromosome has a duplicate sister chromatid attached to it at the centromere. The free surface of the centromere is covered by a protein complex known as the kinetochore. Nucleolus starts disappearing. Asters move farther away from each other due to elongation of microtubular fibrils between them.

Fig: Condensation of chromosomes

Late Prophase

Nucleolus disappears completely and the asters reach the poles. Disintegration of endoplasmic reticulum and golgi apparatus occurs. The nuclear membrane disintegrates into vesicles. The fibrous lamina of the nucleus dissolves. Chromosomes become randomly distributed in the former area of the nucleus.

The mitotic spindles start emerging between the two poles. The spindle fibres converge at the poles and are broad at the centre known as the equator. In animal cells the spindle fibres arise from the astral rays and the poles of the spindles are formed by the asters. As there are two asters in animal cells, the spindle apparatus of animal cells is considered to be amphiastral. Spindle apparatus in plant cells is known as anastral as they lack asters. 

Each fibre is composed of 4-12 microtubules. In the equatorial region, some overlapping fibres get connected to form continuous fibres. Others remain discontinuous fibres. Some may bind to chromosomes during prometaphase and serve as chromosomal fibres.

Fig: Mitotic spindle apparatus

Prometaphase

The nuclear envelope disappears completely. Chromosomes gets attached to chromosomal spindle fibres with the help of their kinetochores. The chromosomal fibres start contracting and the chromosomes start moving towards the equator of the cell. This process is called congression. Chromosome condensation continues up to the end of prometaphase.

Metaphase

It is the second stage of karyokinesis. By this stage, condensed chromosomes are clearly visible under the microscope. This stage is considered the best of all stages to study morphology of chromosomes and count it in numbers.

GIF: Events in etaphase

Changes that occur in metaphase

Each chromosome is connected to both the poles of the spindle apparatus by separate chromosomal fibres from each pole being attached to its kinetochore at the centromere. Contraction of the spindles results in aligning the chromosomes at the equatorial region. Centromeres of all the chromosomes lie over the equator to form an apparent plate called metaphasic plate.

Fig: Metaphase

Anaphase

It is the third stage of karyokinesis. This stage is characterised by the separation of sister chromatids of chromosomes. 

At the beginning of anaphase, a cell cycle checkpoint exists which is mediated by a Mitotic Checkpoint Complex (MCC) composed of proteins. It checks whether all the chromosomes are attached to two chromosomal fibres, one from each pole, at their centromeres. The MCC prevents the activation of the Anaphase Promoting Complex (APC) and prevents the cell from entering into anaphase unless all the chromosomes are properly attached to the spindle fibres at their centromeres.

Changes that occur in anaphase

In the beginning of anaphase, an anaphase promoting complex (APC) dissolves cohesin protein binding the centromeric complex. This causes the centrosome to split and the sister chromatids to separate from each other. The separated chromatids are now referred to as daughter chromosomes. 

Chromosomes are pulled from the centromere while the arms of chromosomes trail behind.

GIF: Anaphase

According to position of the centromere, chromosomes can be of following types:

• Metacentric: Centromere is at the centre and the chromosomes are V shaped.
• Submetacentric: Centromere is close to the centre and the chromosomes are J shaped.
• Acrocentric: Centromere is close to the terminal end and chromosomes are L shaped.
• Telocentric: Centromere is terminal and chromosomes are I shaped.

Fig: Different types of chromosomes

The separating chromosomes may be pushed towards the poles by development and elongation of special interzonal fibres between them. By the end of anaphase, the daughter chromosomes reach the respective poles.

Telophase

It is the fourth and last stage of karyokinesis. This stage is characterised by the disappearance of condensed chromosomes as they loosen up into chromatin fibres. 

Changes that occur in telophase

• Chromosomes assemble at the poles of the cell.
• Spindle fibres disappear.
• Chromosomes decondense into chromatin fibres.
• Organelles like Golgi apparatus, endoplasmic reticulum, nucleolus etc. start to reappear.
• The nuclear envelope develops again.

GIF: Telophase

Cytokinesis

The word karyokinesis is derived from two words:

• Cytos- Means cell
• Kinesis- Means movement

This stage of mitosis involves division of cytoplasm.

After the segregation of chromosomal material, the next step is the division of cytoplasmic material of the cell. The process of cytokinesis differs in a plant cell and animal cell.

GIF: Cytokinesis

Cytokinesis in animal cell

Cytokinesis in an animal cell occurs by the formation of a cell furrow. Furrow starts appearing from the plasma membrane and gradually moves towards the centre of the cell centripetally, hence dividing cell cytoplasm in two halves. Microfilaments and microtubules aid formation of the cell furrow.

GIF: Formation of cell furrow in an animal cell

Cytokinesis in plant cell

Cytokinesis in a plant cell occurs by the formation of a cell plate. Growth of the cell plate occurs in a centrifugal manner. Vesicles from the Golgi complex fuse together to form a cell plate which starts to appear from the centre of the cell and gradually grows outwards. Cell plate represents the middle lamella which lies between the cell wall of two plant cells.

GIF: Formation of cell plate in a plant cell

Syncytium

Syncytium is a multinucleate condition which occurs when karyokinesis is not followed by cytokinesis. Example: Liquid endosperm of coconut

Fig: Syncytium

Significance of mitosis

• Growth and development in multicellular organisms occur with the help of mitosis.

Fig: Growth in plants

• Mitosis helps in repair of old, worn out or damaged cells.

GIF: Repair of a wound

• Regrowth of a lost organ or any part of the body is seen in organisms like Planaria and starfish which is only possible because of mitosis.

GIF: Regeneration in starfish and Planaria

• Reproduction in unicellular organisms like bacteria occurs through mitosis.

GIF: Binary fission in bacteria

• Smaller cells have higher surface area to volume ratio and are therefore more efficient in exchange of materials. Mitosis helps in division of a cell into two daughter cells once it grows to a certain limit. This helps to maintain the surface area to volume ratio.
• For efficient control of the cell, a particular ratio between size of nucleus and cytoplasm has to be maintained. Increase in cell size will disturb this ratio and hence cell division helps to maintain it.

Practice problems

Q1. If you want to count the number of chromosomes, which is the best stage to do so?

A. Telophase
B. Interphase
C. Prophase
D. Metaphase

Solution: Metaphase is the second stage of karyokinesis. By this stage, condensed chromosomes are clearly visible under the microscope. This stage is considered the best of all stages to study morphology of chromosomes and count it in numbers. Thus, the correct option is d.

Q2. After how many mitotic divisions will a cell reach a count of 256?

A. 8
B. 16
C. 20
D. 34

Solution: . Mitosis is the process in which a cell divides into two identical daughter cells with the same number of chromosomes as that of the parent cell. Since the daughter cells have the same number of chromosomes as the parent cell, it is also known as equational division. 

There is a simple formula for calculating the number of cells produced during mitosis.. it is 2n where ’n’ is the number of times the cell was made to divide. Here in the question 

2n = 256

2n  = 28 

Thus, n = 8.

Thus the correct answer is option a.

Q3. In which of the following phases does the nuclear membrane disappear?

A. Metaphase
B. Prophase
C. Telophase
D. Prophase

Solution: During the mid prophase the nuclear membrane starts disintegrating and it completely disappears by late prophase. This makes option (b) correct. 

Q4. If a diploid cell is undergoing mitosis, what will be its ploidy during the anaphase of mitosis?

A. n
B. 4n
C. 2n
D. 3n

Solution: Although during the S phase of interphase, the DNA replicates and the chromatin fibres form duplicate copies of themselves, the ploidy of the cell still remains the same because the duplicated chromatin fibres remain coiled with the original ones and do not separate out as individual chromosomes. All through prophase the chromatin fibres condense and although the duplicated sister chromatids become visible, they are still connected at the centromeres to form single chromosomes. Thus, the cell is still considered to be diploid. Even in metaphase the sister chromatids remain united as a single chromosome which results in diploidy. But, in anaphase the each sister chromatid separates and forms two daughter chromosomes which move to the poles. Thus, the number of chromosomes doubles and hence the ploidy of a diploid cell doubles from 2n to 4n.

Thus, the correct answer is option b.

FAQs

Question 1. What is amitosis?
Answer: Amitosis is a method of direct cell division in which the nucleus simply constricts and divides followed by division of cytoplasm to form two daughter cells. There is no formation of spindles or differentiation of chromosomes involved in amitosis.

Question 2. How does colchicine affect the mitotic phase of the cell cycle?
Answer: Colchicine is a drug that causes depolymerisation of the microtubules and hence prevents the spindle fibres from pulling the sister chromatids apart to form individual chromosomes. Thus, the cells get arrested at metaphase.

Question 3. What are mitocytes and mitospores?
Answer: Mitocytes are cells that undergo mitosis and mitospores are haploid or diploid spores that are formed as a result of mitosis.

Question 4. Which signals tell the cell to stop dividing?
Answer: Cells can start or stop dividing based on certain signals received from the cell cycle regulator proteins known as cyclins. Lack of positive signals can cause a cell to stop dividing. If a cell detects that it is surrounded on all sides by other cells, then it stops dividing. This is known as contact inhibition. For example, if you have a cut in the skin, the cells in that area will divide as long as they sense there is a gap around them. Once the gap is filled, the cells stop dividing.

Also, most cells have a predetermined number of times that they can divide programmed into their genetic constitution. Once the cells have divided for a sufficient number of times, they slow down their division and enter cellular senescence. Damaged DNA due to defective telomerase (enzyme which protects ends of chromosomes) activity are some signals which signal the cell to enter senescence.

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