Cell division

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 which 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

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 as plant cells do not have centrioles and centrosomes, and the microtubule organising centre regulates mitosis.

Early and late prophase

  • In the early prophase, the cell initiates cell division by breaking down some cell component 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 organises 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 late prophase which is also called prometaphase, the mitotic spindle starts to organise the chromosomes.

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

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

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

Metaphase

  • Metaphase starts when the mitotic spindle organises 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.

  • 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 it allows them to separate.

  • Microtubules which 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 which 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 chromosomes pairs may then exchange parts of DNA through crossing over or recombination.

  • In the end, 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 centre of the cell.

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

Anaphase I

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

  • In meiosis I, 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.

FAQ (Frequently Asked Questions)

1. What is the cell division cycle?

The cell division cycle or the cycle is a series of events that takes place in a cell which causes the cell into two daughter cells. It consists of two primary phases


  1. Interphase

  2. M phase

 

Interphase: It is an active phase between subsequent cell divisions. This consists of multiple phases. These are


  • G0: It is the resting phase between cell divisions

  • G1: The cell shows metabolic activity and grows continuously

  • S phase: DNA duplication takes place in this phase

  • G2: Protein synthesis takes place

  • Quiescent stage: The cell does not undergo further division and exits the G1 stage and enter the inactive stage.


M phase: Cell division takes place in this phase. It consists of 2 phases:

  1. Karyokinesis

  2. Cytokinesis

2. Why is cell division necessary?

Cell division serves as a means of cell reproduction in both unicellular and multicellular organisms. In unicellular organisms, reproduction takes place through binary fission which is a type of mitotic division. In multicellular organisms, cell division assists in the formation of gametes which combine to produce organisms. Cell division also plays a role in the growth and development of an organism and repairs injuries.