Cell division

Cell division is the process in which a cell duplicates itself by dividing its genetic material. For prokaryotes, this process follows simple binary fission in reproduction. In a eukaryotic cell, division for sexual reproduction or vegetative growth occurs through a process involving the replication of DNA, followed by two rounds of division without an intervening round of DNA replication. Students can understand different types of cell division at the organelle level by learning about the appearance of each organelle during interphase and prophase. Our experts at Vedantu have covered everything about cell division for Class 11 students, be it different types of mitosis, meiosis, or any other forms of cell division that you would need to know about.

What is Cell Division?

Cell division can be defined as a process by which a cell distributes its genetic material and cytoplasm and gives rise to new daughter cells. It is a part of the larger cell cycle and has a direct role in cell reproduction.

In well-developed organisms, there are two types of cell division observed, mitosis and meiosis. These are very complex processes that are carried out through different phases. However, if simplified, mitosis can be defined as the exact duplication of a cell where the daughter cells will have the same genetic information as the parent cell. In meiosis, the daughter cells will only have half of the genetic information of the original cell. The common end phase in both processes is cytokinesis and the division of the cytoplasm. We will discuss both types of cell division in this topic.

Cell Division- Mitosis and Meiosis

The two well-documented types of cell division are:

1.Mitosis

2. Meiosis

3. Binary Fission

Mitosis

It is the type of cell division where one cell divides to produce two genetically identical daughter cells. A great majority of cell divisions that take place in our body is mitosis. The process is integral to an organism's body growth and development, and it takes place throughout the organism's lifetime. For some single-celled organisms such as yeast, mitotic cell division is the only way they can reproduce. In the following, we will learn about the mitotic process of cell division.

The cell division phases of mitosis are:

1. Early and late prophase

2. Metaphase

3. Anaphase

4. Telophase

Before mitosis begins, the cell is in a state called interphase and it copies its DNA and so the chromosomes in the nucleus consist of two copies which are called sister chromatids. In animals, the centrosome is also copied. Centrosomes control mitosis in animal cells. It should be mentioned here, that plant cells do not have centrioles and centrosomes, and the microtubule-organizing center regulates mitosis.

Early and Late Prophase

• In the early prophase, the cell initiates cell division by breaking down some cell components and building other components and then the chromosome division starts.

• In this stage, the chromosomes start to condense which helps them to separate easily in later stages

• Afterwards, the mitotic spindle starts to form, a structure made of microtubules. It organizes the chromosomes and moves them around during mitosis. The mitotic spindle grows between the centrosomes of the cell as they move towards different poles.

• The nucleolus then disappears which is a sign that the nucleus is getting ready to break down.

• In the late prophase which is also called prometaphase, the mitotic spindle starts to organize the chromosomes.

• Once the chromosomes finish condensing, they form a compact structure.

• Then the nuclear envelope breaks down and the chromosomes are released.

• At the end of the prophase, the mitotic spindle grows, and some microtubules start to capture and organize chromosomes.

Metaphase

• Metaphase starts when the mitotic spindle organizes all chromosomes and lines them up in the middle of the cell to divide.

• All chromosomes align at the metaphase plate.

• At this stage of metaphase, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles. Before proceeding forward to anaphase, the cell will check if all kinetochores are properly attached to microtubules and it is called spindle checkpoint.

• The spindle checkpoint ensures that the sister chromatids are split equally into two daughter cells.

Anaphase

• In this stage, the sister chromatids separate from each other and move towards the opposite poles of the cell. The protein glue that holds them breaks and allows them to separate.

• Microtubules that are not attached to chromosomes elongate and push apart. In doing so they separate the poles and makes the cell longer. These processes are controlled by motor proteins and these proteins carry the chromosomes and microtubules as they move.

Telophase

• In this stage, the cell is almost divided and starts to re-establish its normal cellular structures as cytokinesis takes place.

• The mitotic spindle breaks down into its building blocks and two new nuclei are formed, one for each set of chromosomes. 

• The nuclear membrane and the nucleoli then reappear and the chromosomes begin to de- condense to return to their normal form.

Cytokinesis

• In animal cells, cytokinesis is contractile. There's a pinch-like formation within the cell which divides it in two like a coin purse with a 'drawstring'. The "drawstring" is a band of actin protein filaments. The pinch crease is called the cleavage furrow.

• Plant cells can't be divided like this as they have a rigid cell wall and are too stiff. A cell plate forms down the middle of the cell which splits the daughter cells. 

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Meiosis

In meiosis, a single cell divides twice to produce four cells that contain half of the original amount of genetic material. It can be observed in sperm cells in males and egg cells in females. 

There are 9 meiotic cell division phases. These are discussed below:

Interphase

• Similar to mitosis the genetic material of the cell is copied and two identical sets of chromosomes are formed.

• The centrosomes and the centrioles are also copied and in this phase, the microtubules extend from centrosomes.

Prophase I

• The two sets of chromosomes condense into an X-shaped formation 

• Each chromosome consists of two sister chromatids which contain identical genetic information.

• All chromosomes pair up. For example, both copies of chromosome 1 and both copies of chromosome 2 are together.

• The chromosome pairs may then exchange parts of DNA through crossing over or recombination.

• In the end, in this stage, the nuclear membrane dissolves and releases the chromosomes.

• The meiotic spindle which consists of microtubules and other proteins extends across the cell.

Metaphase I

• The chromosome pairs align next to each other along the center of the cell.

• The centrioles move at the opposite poles of the cell and the meiotic spindles extend from them. Their fibers attach to one chromosome of each pair. 

Anaphase I

• The chromosome pairs are then separated by the meiotic spindle and move one chromosome to opposite poles of the cell.

• In meiosis, the sister chromatids of the cell stay together.

Telophase I and Cytokinesis

• The chromosomes move to opposite poles of a cell and each pole has a full set of chromosomes.

• A nuclear membrane starts to form around each set of chromosomes to form two new nuclei.

• Cytokinesis takes place and two daughter cells are produced.

Meiosis II

Prophase II

• At the end of meiosis, there are two daughter cells with 23 chromosomes

• The chromosomes condense again and form visible X-shaped structures 

• The nuclear membrane will dissolve releasing the chromosomes.

• The centrioles duplicate and the meiotic spindle is formed.

Metaphase II

• Similar to metaphase I, the sister chromatid align along the center of the cell

• The centrioles move to opposite poles of the daughter cells.

• Meiotic spindle fibers attach to individual sister chromatids.

Anaphase II

• The sister chromatids are separated and moved to opposite poles by the meiotic spindle and they become individual chromosomes.

Telophase II and Cytokinesis

• The chromosomes move to opposite poles of the cell and each pole has a full set of chromosomes.

• A nuclear membrane starts to form again and two new cell nuclei are formed.

• Cytokinesis takes place.

• Once cytokinesis is completed there are four new cells, with a haploid set of chromosomes

• In males, all four cells are sperm cells

• In females, one new is an egg cell and the others are polar bodies

Binary Fission

When a cell divides, it first duplicates its DNA, then divides. This is how the day-to-day growth of the human body occurs, which requires new cells to be created for tissue repair and maintenance through cell division. For prokaryotes (bacteria), the process of binary fission is a simple duplication of the DNA followed by division into two cells. For eukaryotes (plants and animals), the process of cell division is more complicated. The first step in cell division for most cells is the duplication of the chromosomes. A single set of chromosomes in a normal human cell contains approximately three billion base pairs or six billion nucleotides.

Conclusion

Cell Division can be simply defined as the process that results in two daughter cells, each with the same number of chromosomes as the parent cell. The chromosomes are duplicated first, and then the cell divides. In prokaryotes, binary fission is a simple duplication of the DNA followed by division into two cells. For eukaryotes, the process of cell division is more complicated. The first step in cell division for most cells is the duplication of the chromosomes. A single set of chromosomes in a normal human cell contains approximately three billion base pairs or six billion nucleotides.

In meiosis, I, the sister chromatids of the cell stay together. When the cells divide, it forms four new cells, with a haploid set of chromosomes. In males, all four cells are sperm cells. One new is an egg cell in females, and the others are polar bodies. So, meiosis is important in the process of sexual reproduction. One should know that meiosis II is complete and the cells divide into four new daughter cells.