Mitosis

Key Concepts

• Eukarotic
• Prokaryotic
• Nucleus
• Chormosome
• Chromatid
• Telomere

Introduction

Majority of the cells are less than 100 µm (100 × 10–6 m) in diameter, which is smaller than the full stop at the end of the sentence. Do you know why most of the cells are so tiny?  

Cell size 

There are many factors that affect cell size. The main component that controls the size of a cell is the ratio of its surface area to volume. The surface area of the cell represents the region covered by the plasma membrane. The volume means the space occupied by the interior matter of the cell, including the organelles in the cytoplasm and the nucleus.  

As the cell grows up, its volume increases much more quickly than the surface area. This means that the cell might have problem in supplying food material and ejecting unwanted products. If cells are small, it will have a higher ratio of surface area to volume and can maintain themselves very easily.  

The small size of cells can manage the movement of material very easily.  It enables faster diffusion and, transport of nutrients and waste products with the help of motor proteins. Small cell size provide more effective transport systems. 

Cell size affects a cell’s capability to communicate instructions for cellular activity/functions. If the cell becomes large, it becomes practically difficult for cellular communications, many of which involve the movement of substances and signals to various organelles, to take place efficiently.  

The Cell Cycle 

As soon as a cell reaches its size limit, either cell will stop growing or it will divide. Most cells will ultimately divide. Cell division not only inhibits the cell from becoming too large, but it also helps the cell to reproduce. Cells reproduce by a cycle of growth and division called the cell cycle. Every time a cell completes one cycle, it becomes two cells. When the cell cycle is repeated continuously, it results in the continuous production of new cells.  

The cell cycle is divided into 3 stages: Interphase, mitosis and cytokinesis. 

Interphase: During interphase, cell grows and duplicates its DNA, and prepares for the next stage of the cell cycle. Interphase is divided into 3 steps: G1, S, and G2. 

G1 phase: It is the first step of interphase. Cell division is immediately succeeded by the G1 phase. During G1 phase a cell grows, carryout normal cell functions, and prepares for DNA replication. Cells of muscle and nerve, leave the cell cycle after G1 phase and do not divide again. 

S Phase (Synthesis of DNA) 

Second phase of interphase is ‘S’ phase. The synthesis phase is the longest phase because of the intricacy of the genetic material being duplicated. In the course of interphase, nuclear DNA stays in a semi-condensed chromatin structure. In the S phase, DNA replication results in the formation of same pairs of DNA molecules, sister chromatids, that are strongly attached to the centromeric area. During S phase, the centrosome is duplicated. The two centrosomes will produce mitotic spindle. It helps the movement of chromosomes during mitosis. At the center of each animal cell, the centrosomes are associated with a pair of rod-like structures called the centrioles. Centrioles are positioned at right angles to each other. They help to manage cell division. Centrioles are absent in the centrosomes of plants and most fungi.  

G2 Phase (Second Gap) 

The G2 phase is the phase when the cell prepares for the cell division of its nucleus. A protein that makes microtubules for cell division is produced at this stage. During G2 phase, the cell gets ready to start with mitosis. When all these processes are completed, the cell starts the next stage of the cell cycle, i.e., mitosis. 

Mitosis 

During mitosis, the cell’s duplicated genetic material separates and the cell prepares to divide into two cells. The main activity of mitosis is the exact separation of the cell’s replicated DNA. This allows the cell’s genetic information to pass to the new cells, resulting in two daughter cells that are genetically same. In multicellular organisms, the method of mitosis increases the number of cells. Mitosis is also useful in replacement of the damaged cells. 

The stages of mitosis and cytokinesis comes after interphase. In mitosis, the cell’s nuclear material divides and splits into opposite ends of the cell. In cytokinesis, the cell divides into two daughter cells with the same nuclei. The first part of the mitotic stage is called nuclear division or karyokinesis. The second part of the mitotic stage is known as cytokinesis. It is the physical separation of the cytoplasmic parts into two daughter cells. 

The Stages of Mitosis 

Like interphase, mitosis is divided into stages: prophase, metaphase, anaphase, and telophase. 

Prophase

It is the first step of mitosis. In this phase the cell’s chromatin tightens into chromosomes and then these chromosomes form a shape like X. At this point, each chromosome is a single structure that contains genetic material duplicated in the interphase. Each half part of this X (chromosome) is known as a sister chromatid. Sister chromatids are structures that consist of same copies of DNA. The sister chromatids are attached at the centre with the help of centromere. This structure ensures that a complete copy of the replicated DNA will become part of the daughter cells at the end of the cell cycle.  

As prophase continues, the nucleolus disappears and spindle fibers form in the cytoplasm. In many protist cells and animal cells centrioles move to the ends or poles of the cell. Aster fibers are starlike structures that come out of the centrioles. The entire structure, including the spindle fibers, centrioles, and aster fibers, is known as the spindle apparatus. The spindle apparatus helps in moving and arranging the chromosomes before cell division. In plant cells, centrioles are not component of the spindle apparatus whereas only spindle fibers are present. At the end of prophase, the nuclear envelope disappears. The spindle fibers attach to the sister chromatids of each chromosome on both sides of the centromere and then attach to opposite poles of the cell. This

arrangement confirms that every new cell gets one complete copy of the DNA. 

Metaphase

During metaphase, the sister chromatids are pulled by motor proteins across the spindle apparatus near the center of the cell and line up in the center, or equator, of the cell. Metaphase is the shortest phase of mitosis, but when completed successfully it ensures that the new cells have exact copies of the chromosomes. 

Anaphase 

In the third stage of mitosis, i.e., anaphase, the chromatids move apart. In anaphase, the microtubules of the spindle apparatus start to become short. This shortening pulls at the centromere of each sister chromatid, making the sister chromatids to split into two equal chromosomes. All of the sister chromatids separate at the same time, even though the correct mechanism that controls this is not known. At the end of anaphase, the microtubules, with the help of motor proteins, move the chromosomes to the poles of the cell. 

Telophase

Telophase is the last stage of mitosis during which the chromosomes reach at the poles of the cell and begin to decondense. As shown in Figure 5, two new nuclear membranes start to form and the nucleoli appear again. The spindle apparatus pulls apart and some of the microtubules are recycled by the cell to develop different parts of the cytoskeleton. Even though the four phases of mitosis are complete and the nuclear material is divided, the procedure of cell division is not yet complete. 

Cytokinesis 

Just before the end of mitosis, the cell begins another process known as cytokinesis that will split the cytoplasm. This results in two cells, each with same nuclei. During the later phases of mitosis, microtubules are developed that will be involved in cytokinesis. In animal cells, cytokinesis is achieved by using microfilaments to constrict the cytoplasm.  Plant cells have an inflexible cell wall covering their plasma membrane. Instead of pinching in half, a new structure, known as a cell plate, forms between the two daughter nuclei. Then on either side of the cell plate the cell wall forms. So, the two genetically identical cells form. Prokaryotic cells, divide by binary fission, so the cell division occur in a different way. When prokaryotic DNA is duplicated, both copies connect to the plasma membrane. As the plasma membrane grows, the connected DNA molecules are pulled apart. The cell finishes fission, and produces two new prokaryotic cells. 

Results of Mitosis  

Two important factors about mitosis are:  

1. It is the nuclear division.  

2. It makes two new nuclei that are same.  

Every new nucleus has the similar number and type of chromosomes. Each cell in our body, except for sex cells, possesses a nucleus with 46 chromosomes i.e, 23 pairs. 

Summary

• The cell cycle is the living cycle of a cell.
• In eukaryotic cells, the cell cycle is divided into two – interphase and the mitotic (M) phase.
• In the course of interphase, the cell grows and creates a copy of its DNA.
• In the mitotic (M) phase, the cell divides its DNA cytoplasm into two groups, creating two new cells.
• During cytokinesis, the physical division of the cytoplasmic components into two daughter cells takes place.