Cell Cycle and Cell Division | Class 11 Biology | Neet Free Notes
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1. Stages of Cellular Division and Replication Sequence
Cell division is a very important process that happens in all living organisms. When a cell divides, it also goes through DNA replication and cell growth. These steps must happen in a coordinated and organized way so that the new cells get the correct genetic material.
The complete set of steps by which a cell copies its DNA, makes new parts, and then splits into two daughter cells is called the cell cycle. Even though cell growth (increase in cytoplasm) happens all the time, the process of DNA synthesis happens only during a specific stage of the cell cycle.
After the DNA is copied, the chromosomes are carefully given to each new cell through a complex process during cell division. This whole process is controlled by genes, meaning it is under genetic contro
2. Sequential Stages in Cellular Life Cycle Progression
In human cells grown in labs, the cell cycle usually takes about 24 hours to complete. However, this time can be different depending on the organism and type of cell. For example, yeast cells can finish the cycle in just 90 minutes.
The cell cycle has two main phases: the Interphase and the M Phase (also called Mitosis phase). The M Phase is when the cell actually divides. The Interphase is the time between two divisions. In human cells, cell division takes about 1 hour, and the Interphase takes more than 95% of the total cycle time.
The M Phase begins with the nuclear division called karyokinesis, where the chromosomes separate. It usually ends with cytokinesis, which is the division of the cytoplasm.
Although Interphase is sometimes called the resting phase, the cell is actually very busy. It is preparing for division by doing cell growth and DNA replication in an organized way.
The Interphase is divided into three phases:
- G1 Phase (Gap 1)
- S Phase (Synthesis Phase)
- G2 Phase (Gap 2)
The G1 Phase happens after mitosis and before DNA replication begins. During this phase, the cell is active and grows, but it does not copy its DNA.
In the S Phase, the DNA is copied or replicated. The amount of DNA in the cell becomes double. If the original DNA amount was 2C, it becomes 4C after replication. However, the number of chromosomes does not change. If the cell was diploid (2n) before S Phase, it remains 2n after it.
In animal cells, during the S Phase, the DNA replication happens in the nucleus, and the centriole makes a copy of itself in the cytoplasm.
In the G2 Phase, the cell makes proteins needed for mitosis, and the cell continues to grow.
Some cells in adult animals (like heart cells) do not divide anymore. Other cells divide only when needed, for example, after an injury or when cells die. These cells leave the G1 Phase and go into a non-dividing stage called the G0 Phase or quiescent stage.
In the G0 Phase, the cells do not divide, but they still do normal cell activities. They can start dividing again if the organism needs it.
In animals, mitosis usually happens only in diploid somatic cells. But there are some exceptions like male honey bees, where haploid cells also divide by mitosis.
In plants, both haploid and diploid cells can divide by mitosis. You may recall this from studying alternation of generations, where certain haploid plant stages also show mitosis.
3. Mitotic Phase of Cellular Division
The M Phase is the most dramatic part of the cell cycle. During this phase, the cell goes through a major reorganization of almost all its internal parts.
Because the number of chromosomes stays the same in both the parent cell and the new cells, this type of division is also called an equational division.
For better understanding, mitosis is divided into four stages of nuclear division, which is also known as karyokinesis. But in reality, cell division is a continuous process, and the boundaries between stages are not very sharp or clearly separated.
The four stages of karyokinesis are:
- Prophase
- Metaphase
- Anaphase
- Telophase
3.1 Initial Stage of Chromosomal Condensation in Mitosis
Prophase is the first stage of karyokinesis during mitosis. It comes after the S and G₂ phases of interphase. In those earlier phases, the new DNA molecules are made, but they are still mixed and tangled together, not clearly visible.
In prophase, the chromosomal material starts to condense. This means the chromatin begins to untangle and form tight, visible structures called chromosomes.
At the same time, the centrosome, which already made a copy of itself during the S phase, starts to move toward opposite poles of the cell.
By the end of prophase, several important events happen:
- The chromosomal material condenses and forms compact mitotic chromosomes. Each chromosome is made up of two chromatids that are joined at a point called the centromere.
- The centrosomes move to opposite sides of the cell. Each centrosome gives out tiny fibers called microtubules, which are known as asters. The two asters, along with spindle fibers, together make the mitotic apparatus that helps in cell division.
When prophase ends, and the cell is viewed under a microscope, some cell parts disappear. These include the Golgi complexes, endoplasmic reticulum, nucleolus, and the nuclear envelope.
3.2 Midway Alignment Stage of Chromosomes During Cell Division
Metaphase is the second stage of mitosis. It starts when the nuclear envelope is completely broken down. Because of this, the chromosomes are now freely spread in the cytoplasm of the cell.
By this time, the chromosomes are fully condensed, and they can be seen clearly under a microscope. This makes metaphase the best stage to study the shape and structure of chromosomes.
Each metaphase chromosome has two sister chromatids, which are held together at a point called the centromere.
At the centromere, there are small disc-like structures called kinetochores. These kinetochores are the places where the spindle fibers attach to the chromosomes.
The spindle fibers help move the chromosomes to the center of the cell. At metaphase, all the chromosomes are arranged in a straight line at the equator of the cell. This arrangement is known as the metaphase plate.
Each sister chromatid is attached to spindle fibers from opposite poles through its kinetochore—one to the left pole and the other to the right pole.
The main features of metaphase are:
- Spindle fibers attach to the kinetochores of chromosomes.
- Chromosomes line up at the spindle equator and are arranged on the metaphase plate, attached to both poles.
3.3 Separation Phase of Sister Chromatids Toward Opposite Poles
Anaphase is the third stage of mitosis. It begins when the chromosomes that were lined up on the metaphase plate are split at the same time.
The two daughter chromatids of each chromosome are now called daughter chromosomes. These daughter chromosomes start to move toward opposite poles of the cell.
As the chromosomes move away from the center, their centromeres lead the way, pointing toward the poles, while the arms of the chromosomes trail behind.
The main features of anaphase are:
- The centromeres split, and the chromatids separate.
- The chromatids (now called daughter chromosomes) move to opposite poles of the cell.
3.4 Final Stage of Nuclear Division
Telophase is the final stage of karyokinesis in mitosis. At the start of this stage, the chromosomes that have moved to the opposite poles of the cell begin to decondense, meaning they uncoil and lose their clear structure.
The individual chromosomes are no longer visible. Instead, the chromatin material (loosely packed DNA) starts to gather at each pole of the cell.
The important events during telophase are:
- Chromosomes gather at the opposite spindle poles and lose their clear shape, so their identity as separate chromosomes disappears.
- A new nuclear envelope forms around each chromosome cluster, creating two daughter nuclei.
- The nucleolus, Golgi complex, and endoplasmic reticulum (ER) are re-formed inside each new nucleus.
4. Division of Cytoplasm
Cytokinesis is the process where the cell’s cytoplasm is divided into two daughter cells. It happens after mitosis, which already separated the chromosomes into two nuclei. When cytokinesis is done, the entire cell division is complete.
In animal cells, cytokinesis starts with the formation of a furrow (a small groove) in the plasma membrane. This furrow becomes deeper and deeper, and finally splits the cell into two parts, dividing the cytoplasm equally.
In plant cells, there is a rigid cell wall, so the process is different. Instead of a furrow, a new wall starts forming in the center of the cell and grows outward until it joins the existing side walls.
This new wall begins with the cell plate, which is the starting structure of the wall. The cell plate becomes the middle lamella, which separates the two new plant cells.
During cytoplasmic division, cell parts like mitochondria and plastids are also shared between the two new cells.
In some organisms, cytokinesis does not happen after karyokinesis. This results in a multinucleate cell, called a syncytium. An example of this is the liquid endosperm in coconut.
5. Importance and Role of Cell Division by Mitosis
Mitosis, also called equational division, usually happens in diploid cells. But in some lower plants and certain social insects, haploid cells can also divide through mitosis.
This type of cell division is very important in the life of an organism. In most cases, mitosis produces diploid daughter cells that are genetically identical to the parent cell.
The growth of multicellular organisms mainly happens because of mitotic division. When a cell grows, the balance between the nucleus and cytoplasm is disturbed. To restore this balance, the cell must divide.
Another important role of mitosis is cell repair. For example, the cells in the upper layer of the skin, the lining of the gut, and blood cells are constantly being replaced through mitosis.
In plants, mitotic division happens in meristematic tissues like the apical meristem and lateral cambium. This allows plants to keep growing throughout their entire life.
6. Reductional Cell Division (Meiosis Process)
Meiosis is a special type of cell division that happens during sexual reproduction. In this process, two gametes (reproductive cells) come together, and each gamete has a haploid set of chromosomes.
These gametes are formed from special diploid cells. In meiosis, the number of chromosomes is reduced by half, so the daughter cells are haploid. This is why meiosis is also called a reductional division.
Meiosis creates the haploid phase in the life cycle of organisms that reproduce sexually. Later, during fertilization, the haploid cells combine to restore the diploid phase.
Meiosis happens during gamete formation (gametogenesis) in both plants and animals, and it leads to the creation of haploid gametes.
The main features of meiosis are:
- Meiosis has two rounds of nuclear and cell division: meiosis I and meiosis II, but DNA replication happens only once before meiosis begins.
- Meiosis I starts after the DNA has already been copied in the S phase, making identical sister chromatids.
- Homologous chromosomes pair up, and genetic exchange (called recombination) happens between non-sister chromatids of these paired chromosomes.
- At the end of meiosis II, four haploid cells are produced.
The different phases of meiosis are divided into two stages:
- Meiosis I:
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
- Meiosis II:
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
7. First Division Phase of Meiosis (Reductional Division Stage)
Meiosis I
Prophase I is the first stage of meiosis I. It is longer and more complex than the prophase of mitosis. It is further divided into five sub-stages based on chromosome behavior:
1. Leptotene
In this stage, chromosomes start becoming visible under a microscope. They slowly become shorter and thicker. This process of condensation continues through leptotene.
2. Zygotene
Here, homologous chromosomes (same type from each parent) begin to pair up. This pairing process is called synapsis.
The paired chromosomes are now called bivalents or tetrads. Under an electron microscope, a special structure called the synaptonemal complex can be seen holding the homologous chromosomes together.
3. Pachytene
At this stage, all four chromatids of a bivalent become clearly visible.
Small structures called recombination nodules appear. These are the sites where crossing over happens — a process where genetic material is exchanged between non-sister chromatids of homologous chromosomes.
Crossing over is controlled by an enzyme called recombinase, and it results in genetic recombination, increasing genetic diversity.
By the end of pachytene, crossing over is complete, and the chromosomes remain linked at the crossover sites.
4. Diplotene
Now, the synaptonemal complex dissolves, and homologous chromosomes begin to separate, but they stay connected at crossover points called chiasmata.
In some organisms, especially in female egg cells (oocytes), this stage can last for months or even years.
5. Diakinesis
This is the final stage of prophase I. In this phase:
- The chiasmata move toward the ends of chromosomes (called terminalisation).
- Chromosomes become fully condensed.
- The spindle begins to form.
- The nucleolus disappears.
- The nuclear membrane breaks down.
This marks the end of prophase I and the beginning of metaphase I.
Metaphase I
In this stage:
- The bivalent chromosomes (paired homologous chromosomes) line up at the center (equator) of the cell.
- Spindle fibers from opposite poles attach to the kinetochores of the homologous chromosomes.
Anaphase I
Here, homologous chromosomes separate and move to opposite poles of the cell.
However, the sister chromatids remain attached at their centromeres.
Telophase I
In this stage:
- The nuclear membrane and nucleolus reappear.
- Cytokinesis (division of cytoplasm) happens, forming two daughter cells, each with half the chromosome number.
This stage is called the dyad stage.
Chromosomes may slightly loosen, but do not fully unwind like in interphase.
Interkinesis
This is the short rest phase between meiosis I and meiosis II.
- There is no DNA replication in this phase.
- It is followed by Prophase II, which is much simpler and shorter than Prophase I.
8. Second Division Phase of Meiosis (Equational Division Stage)
Meiosis II is the second phase of meiotic division, and it begins shortly after the completion of cytokinesis following Meiosis I. Unlike the first meiotic division, Meiosis II closely resembles mitosis, as it involves the separation of sister chromatids rather than homologous chromosomes. It consists of four main stages: Prophase II, Metaphase II, Anaphase II, and Telophase II.
During Prophase II, the nuclear envelope starts to break down and the chromosomes, which may have slightly decondensed after Meiosis I, once again condense and become compact. This stage is generally brief and occurs rapidly after cytokinesis, often before the chromosomes have completely elongated.
In Metaphase II, the chromosomes align themselves along the equatorial plane (middle) of each of the two haploid cells formed in Meiosis I. Each chromosome consists of two sister chromatids connected at the centromere. Spindle fibers, composed of microtubules, extend from opposite poles of the cell and attach to the kinetochores of the sister chromatids.
Anaphase II is initiated when the centromeres split simultaneously in each chromosome. This separation allows the now individual sister chromatids (now called daughter chromosomes) to be pulled apart toward opposite poles of the cell. The spindle fibers shorten, helping in this movement and ensuring that each daughter cell receives a complete set of chromosomes.
Finally, in Telophase II, the separated chromosomes reach the poles, and new nuclear membranes form around each group, resulting in the reformation of two nuclei in each of the two cells. This is followed by cytokinesis, which divides the cytoplasm and leads to the formation of a total of four haploid daughter cells, each with half the number of chromosomes compared to the original diploid parent cell. These four cells are genetically distinct due to the genetic recombination events that occurred during Meiosis I.
9. Biological Importance of Meiosis in Life Processes
Meiosis plays a crucial role in maintaining the constant chromosome number of a species across generations, especially in sexually reproducing organisms. Even though meiosis involves the reduction of chromosome number by half, it ensures that when gametes fuse during fertilization, the original diploid number of chromosomes is restored in the offspring. This helps preserve the genetic stability of a species over time.
Another major significance of meiosis is the introduction of genetic variation. During processes like crossing over in Prophase I and independent assortment of chromosomes, new gene combinations are created. These variations contribute to genetic diversity within a population, which is essential for the adaptability and evolution of species. Over generations, such diversity allows populations to better respond to environmental changes and selection pressures, thereby driving the process of evolution.
10. Over View Of This Chapter
According to the cell theory, all cells originate from pre-existing cells through a process known as cell division. In all sexually reproducing organisms, life begins as a single-celled zygote, and this cell continues to divide and grow to form the complete organism. Cell division does not stop after maturity but continues throughout the organism’s life. The complete sequence of events in a cell’s life from one division to the next is called the cell cycle. It is divided into two main phases: Interphase, where the cell prepares for division, and the M phase (mitosis), which is the actual division phase. Interphase includes three stages – G1 phase, S phase, and G2 phase. In G1, the cell grows and performs normal metabolic activities, while organelle duplication begins. During S phase, DNA replication occurs, resulting in the duplication of chromosomes. In G2, there is further cytoplasmic growth and preparation for mitosis. The mitotic phase is further subdivided into Prophase, Metaphase, Anaphase, and Telophase. In Prophase, the chromosomes condense, centrioles move to opposite poles, the nuclear envelope and nucleolus disappear, and spindle fibers begin to form. Metaphase is recognized by the alignment of chromosomes along the equatorial plate. During Anaphase, the centromeres split, and sister chromatids move to opposite poles. In Telophase, the chromatids reach the poles, decondense, and new nuclear membranes and nucleoli are formed. This is followed by cytokinesis, where the cytoplasm divides, resulting in two genetically identical daughter cells. Mitosis is called an equational division because the chromosome number remains unchanged.
In contrast, meiosis occurs in diploid cells that produce gametes, and it is known as a reductional division because it reduces the chromosome number by half. This is important for maintaining the chromosome number across generations through sexual reproduction. Meiosis occurs in two successive divisions: Meiosis I and Meiosis II. During Meiosis I, homologous chromosomes pair up to form bivalents and undergo crossing over. Prophase I is a long and complex stage subdivided into leptotene, zygotene, pachytene, diplotene, and diakinesis. This is followed by Metaphase I, where bivalents align at the equator, and then Anaphase I, where homologous chromosomes, each with two chromatids, move to opposite poles, reducing the chromosome number to half. Telophase I leads to the reformation of the nuclear membrane and nucleolus, followed by interkinesis. Meiosis II is similar to mitosis and involves the separation of sister chromatids during Anaphase II. The process ends with Telophase II and cytokinesis, resulting in the formation of four haploid daughter cells, each with a unique genetic composition.
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