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Mitosis is that step in the cell cycle when the newly formed DNA gets separated into two for the daughter cells being formed from the parent cell. The daughter cells contain the identical number and type of chromosomes as the parent cell.
Mitosis is defined as the process of asexual reproduction identified with unicellular organisms. This article explores Mitosis, its meaning, its different stages, and its significance in the cell cycle.
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Read Also: Mitosis and Meiosis
What is Mitosis?
At the cellular level, not only unicellular, but multicellular organisms as well observe cell division. It is the driving process behind cellular replication. In eukaryotic cells, cell division occurs in a manner such that the number of chromosomes (also called ploidy) in the daughter cell remains the same as the parent cell. In germ cells, however, this number gets halved.
Mitosis
Mitosis is a five-stage process, where the nucleus of the cell is divided into two daughter cells with equal and exactly the same kind of chromosomes. This genetic material is generated in the cell, which then undergoes mitosis. Mitosis succeeds the G2 phase of the cell cycle and is followed by the cytoplasmic division after the division of the nucleus.
Features
Some salient features of Mitosis are as follows:
- Mitosis is also known as equational cell division because the number of chromosomes in the parent and daughter cells are the same.
- Mitosis occurs in plants and leads to vegetative growth of plant parts like stem tip, root tip, etc.
- Segregation and combination are not part of the Mitosis part of the cell cycle.
Stages of Mitosis
Interphase
Before technically entering mitosis, the cell spends a significant amount of time replicating the DNA to be divided into the daughter cells. This is an essential part of cell growth and is further divided into three phases.
- G1 Phase: The period before DNA synthesis
- S phase: The phase during DNA synthesis
- G2 phase: The period after DNA synthesis
Prophase
The signature feature of Prophase is the condensation of genetic material. These are compact mitotic chromosomes consisting of two chromatids attached with a centromere. After Prophase completes, it is followed by the initiation of the assembly of the mitotic spindle, the microtubules, and the proteinaceous cytoplasm components that aid the cell cycle. Post this, the nuclear envelope starts disintegrating.
Prometaphase
This stage is characterized by the complete disintegration of the nuclear envelope. Now the microtubules extend from the centromere to the chromosomes. The microtubules go on to attach themselves to structures called kinetochores, which allow general movement around the cell.
Metaphase
Around this stage, the chromosomes have oriented themselves around the middle of the cell, and the microtubules start pulling the chromosomes with equal amounts of force. The resultant region created is called the Metaphase plate. This way, the entire cell gets an entire functioning genome.
Anaphase
It is during this stage that the sister chromatids start splitting. The genetic material of the newly formed daughter cells is composed of these chromatids. The chromosomes are attached to kinetochore fibers, which start pulling them toward the poles of the cell. The centromere moves first, with the arm trailing behind.
Telophase
The last stage of Mitosis consists of the clustered chromosomes coalescing into an undifferentiated mass, and the nuclear envelope developing around it. The other components of the cell, ER, Golgi apparatus start to appear.
With Mitosis finished and nuclear division successful, the cell cycle continues to Cytokinesis, which is the division of Cytoplasm.
Functions and Importance of Mitosis
- Responsible for the development of a zygote into an adult.
- Helps in the replacement of damaged tissues and dead cells.
- Maintains a consistent number of chromosomes in the organism.
- The purity of the genome is maintained as no recombination or crossing-over is involved.
Read Also: Difference Between Chromosome and Chromatid
Sample Questions
Ques: Telophase is the reverse of prophase. Elucidate the statement.
Ans: The condensation of chromosomes is the first step of prophase. During the process of chromatin condensation, the chromosomal material begins to untangle. As mitosis approaches its final stage, i.e., the telophase, upon arrival at the respective poles, the chromosomes decondense and loses their individual significant feature. When observed under microscope, the cells at the end of prophase lack nucleolus, golgi complexes, endoplasmic reticulum, and the nuclear envelope. The nuclear envelope at its telophase stage gathers around the cluster of chromosomes. The ER, golgi complex, nucleolus reform.
Ques: Explain: a) Synaptonemal complex and b) Metaphase plate.
Ans: a) These are zipped structures that are assembled during the prophase of the meiosis – I between homologous chromosomes. This assembly and disassembly is interlinked with the continuous rearrangements of the chromatin during the meiotic prophase such as recombination, condensation, poring, and dysfunction of homologous chromosomes. These regulate the distribution and number of reciprocal exchanges among the homologous chromosomes. They convert crossover into functional chiasmata.
b) The centromeres of the chromosomes in the metaphase gather on the metaphase plate. It is an imaginary line at an equivalent distance from the two centrosome poles. The alignment is even because of the opposite kinetochore microtubules. The chromosomes at this plate, the sister chromatids, in particular, are connected to the package of four to eight spindle fibres.
Ques: A cell having 32 chromosomes undergoes mitotic division. During metaphase, what will be the chromosome number (N) of the cell? During anaphase, what will the DNA content of the cell be?
Ans: Mitosis occurs in the somatic cells of the entities. The number of chromosomes is equal in both parent and daughter cells and remains unchanged even at anaphase. The content of DNA, however, is doubled at the synthetic phase or interphase. The division takes place at anaphase but the number of chromosomes remain unchanged.
Ques: Which is the cell that is captured in the diplotene phase for months and years? How does it complete its cell cycle?
Ans: Diplotene can last for months or even for years in the oocytes of some vertebrates.
- The diplotene chromosome is situated in the diplotene phase of some animal oocytes of amphibians or frogs.
- In meiotic prophase, the lampbrush chromosomes are observed. These chromosomes tend to turn normal after their growth, completing the cell cycle.
Ques: Write the phases of the cell cycle against each of the events
- The disintegration of the nuclear membrane
- The appearance of the nucleolus
- Division of centromere
- Replication of DNA
Ans:
- Prophase.
- Telophase.
- Anaphase.
- S-phase.
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