Prokaryotes





 CELl DIVISION IN PROKARYOTES:

Cell division  in the prokaryotes is simple and does not occur by mitosis.However, it is as precise as in eukaryotes. The chromosome divides prior to the cytoplasm




(i) Chromosomal Division:

Replication of chromosome ( DNA molecule) starts at one point and  proceeds in both directions from this point, forming two replication forks. As  the forks complete their circuit around the DNA molecule, two complete circular DNA molecules are formed. Presumably , the attachment point of  the DNA molecule to the  plasma membrane is also duplicated so that the replicated DNA molecules are now joined  to the cell membrane at two nearby points. The ce membrane between  the points of attachment then grows, pushing the two DNA molecules to opposite ends of the cell. This, the ce membrane rather  than a mitotic spindle separates the replicated DNA molecules. 

 (ii) Cytoplasmic Division:

The cytoplasm divides by furrowing as in an animal cell. A circular  constriction  appears in the plasma membrane round the middle of the cell be the two separated DNA molecules ( chromosomes). The constriction  gradually deepens, finally dividing the cytoplasm into two separate halves. The cell wall grows, following the inward extension  of the furrow .As the cell membranes separate, the growing cell wall separate the two daughter  cells.

   The whole process of replication and cell fission is completed in about 20 minutes in rapidly growing  bacteria




What  are prokaryotes ?

Prokaryotes are unicellular organisms that lack a nucleus and other membrane-bound organelles. They are smaller and simpler in structure compared to eukaryotes. 

The basic structure of a prokaryotic cell includes a cell membrane, cytoplasm, ribosomes, and a circular DNA molecule called a nucleoid. Some prokaryotes also have external appendages like pili for adhesion and conjugation, flagella for movement, and a capsule for protection.

The cell membrane is a phospholipid bilayer that surrounds the cell and regulates the movement of substances in and out of the cell. Cytoplasm is a gel-like substance that contains enzymes, nutrients, and molecules required for cellular processes. Ribosomes are small structures that synthesize proteins.

The nucleoid is the region within the cytoplasm where the prokaryotic DNA is located. It lacks a membrane and, therefore, is not considered a true nucleus. The genetic material in prokaryotes is usually a single, circular chromosome. Some prokaryotes also have plasmids, which are small, circular DNA molecules that carry additional genetic information.

Overall, the structure of prokaryotes is simple and efficient, allowing them to carry out necessary functions for survival in various environments.




Types of cell division in prokaryotes 

Prokaryotes reproduce asexually through two main types of cell division: binary fission and budding.

1. Binary fission: This is the most common type of prokaryotic cell division. In binary fission, the parent cell replicates its genetic material (DNA) and divides into two identical daughter cells. The process of binary fission involves the following steps:

- The bacterial DNA replicates, creating two identical copies of the chromosome.
- The cell elongates, and the two copies of the DNA move to opposite ends of the cell.
- The cell membrane then grows inward, dividing the cell into two identical daughter cells.

2. Budding: Budding is a less common method of cell division in prokaryotes, in which a small bud forms on the parent cell and grows until it separates from the parent cell to form a new independent cell. Budding is seen in bacteria such as Caulobacter crescentus.

In addition to these two main types of cell division, prokaryotes can also undergo other forms of cell division, such as multiple fission and fragmentation. Multiple fission occurs when the parent cell divides into several daughter cells simultaneously, whereas fragmentation occurs when the parent cell breaks into smaller fragments that grow into independent cells.

What types of cell division occurs in eukaryotes ?


Eukaryotes undergo mitotic cell division and meiotic cell division. Mitosis is the process by which a single cell divides into two identical daughter cells, each containing the same number of chromosomes as the parent cell. This type of division is involved in growth, repair, and asexual reproduction in eukaryotic organisms. Meiosis, on the other hand, is a specialized type of cell division that occurs in sexually reproducing organisms. It involves the production of haploid gametes, such as sperm and eggs, through two rounds of division, resulting in cells with half the number of chromosomes as the parent cell.




What are the 3 main  stages of the  prokaryotic cell division ?

The three main stages of prokaryotic cell division are:

1. DNA replication: The first stage involves the duplication of the prokaryotic cell's DNA. The circular chromosome is replicated, producing two identical copies.

2. Chromosome segregation: In this stage, the duplicated chromosomes move towards opposite ends of the cell. This process is facilitated by the attachment of the replisome complex to the cell membrane.

3. Cytokinesis: During cytokinesis, the cell undergoes division, separating into two daughter cells. A new cell wall is formed between the two daughter cells, and the cytoplasm is divided, resulting in two separate genetically identical cells.

What is cell division in prokaryotes ?

Prokaryotes are unicellular organisms that lack a nucleus and other membrane-bound organelles. They are classified into two domains, Bacteria and Archaea. Prokaryotes are the oldest and most abundant forms of life on Earth and can be found in various environments, including soil, water, and even inside other organisms. They have a simple cell structure, with a single circular chromosome, no membrane-bound organelles, and a cell wall. Prokaryotes reproduce through a process called binary fission, which involves the division of a single cell into two identical daughter cells.


How does cell division differ in prokaryotes and eukaryotes ? 

Cell division in prokaryotes (such as bacteria) and eukaryotes (such as plants, animals, and fungi) differs in several significant ways:


1. Organization and complexity: Prokaryotic cells ack a nucleus and other membrane-bound organelles, while eukaryotic cells have a distinct nucleus and other complex organelles.


2. DNA organization: Prokaryotes generally have a single circular chromosome, which is not enclosed in a nucleus. In contrast, eukaryotes have multiple linear chromosomes, enclosed within a membrane-bound nucleus.


3. Reproduction: Prokaryotes reproduce asexually through a process called binary fission, where the parent cell divides into two identical daughter cells. Eukaryotes reproduce both asexually and sexually. Asexual reproduction may involve processes like mitosis or budding, while sexual reproduction involves the production of haploid gametes that fuse during fertilization to form a diploid zygote.


4. Cell division process: Prokaryotes undergo a simple cell division process, where DNA replication, segregation, and cytokinesis occur concurrently. In contrast, eukaryotic cell division is a more complex process that involves distinct phases: interphase (growth and DNA replication), mitosis (nuclear division), and cytokinesis (cytoplasmic division).


5. Chromosome separation: Prokaryotes undergo chromosome segregation through the attachment of a protein complex called the replisome to the cell membrane. In eukaryotes, mitosis ensures that chromosomes are aligned and separated properly, involving the assembly and disassembly of the mitotic spindle.


6. Cell size: Prokaryotes are generally smaller and have simpler cell structures. Eukaryotes are larger and more complex, with a variety of specialized cell types.


While there are fundamental differences in cell division between prokaryotes and eukaryotes, the basic purpose remains the same - the production of new cells for growth, development, and reproduction.


What are cell division  in prokaryotes ? 

In prokaryotes, cell division occurs through a process called binary fission. This involves the replication of the genetic material (usually a single circular DNA molecule) within the cell, followed by the elongation of the cell and division into two daughter cells. Prokaryotes do not have a nucleus or complex organelles, so the division process is relatively simple and rapid.

However, in eukaryotes, cell division is a more complex process that involves three main components: 

1. DNA replication: The cell's genetic material, located within the nucleus, is duplicated in preparation for cell division. This is necessary to ensure that each daughter cell receives a complete set of genetic information.

2. Mitosis or meiosis: Eukaryotic cell division can occur through two different processes, depending on the type of cell division required. In mitosis, a single cell divides into two identical daughter cells, each containing the same number of chromosomes as the parent cell. Mitosis is involved in growth, repair, and asexual reproduction in eukaryotes. On the other hand, meiosis is a specialized form of cell division that occurs in cells involved in sexual reproduction. It results in the production of four daughter cells, each containing half the number of chromosomes as the parent cell. Meiosis plays a crucial role in generating genetic diversity.

3. Cytokinesis: After the genetic material is divided, cytokinesis occurs, which involves the physical separation of the cytoplasm and the formation of two distinct daughter cells. In animals, cytokinesis typically involves the formation of a cleavage furrow and the subsequent pinching off of the two daughter cells. In plants, a cell plate forms at the center of the dividing cell, eventually developing into a new cell wall that separates the two daughter cells.



Do  prokaryotes divide by mitosis ? 

Prokaryotes do not divide by mitosis. Mitosis is a process of cell division specifically found in eukaryotes. Prokaryotes divide through a process called binary fission.

Binary fission is a form of asexual reproduction where the prokaryotic cell replicates its genetic material and then divides into two identical daughter cells. Here are some characteristics of prokaryotes in terms of their cell division:

1. Lack of nuclear membrane: Prokaryotes do not have a nucleus like eukaryotes. Their genetic material, which is a circular DNA molecule, is found in the cytoplasm rather than within a membrane-bound nucleus.

2. Single circular chromosome: Prokaryotes typically have a single circular chromosome, which contains most of their genetic material. This chromosome is replicated during binary fission.

3. Rapid division: Prokaryotes can replicate and divide very quickly. Under optimal conditions, some species can divide every 20 minutes, resulting in a rapid population growth.

4. No spindle apparatus: Unlike in eukaryotic cells, prokaryotes do not form a spindle apparatus to separate their duplicated genetic material. Instead, the replicated DNA molecule attaches to the cell membrane at opposite ends of the cell, and as the cell elongates, the DNA molecules are pulled apart.

5. Absence of membrane-bound organelles: Prokaryotes lack membrane-bound organelles, such as mitochondria or chloroplasts. This means that their division process does not involve organelle separation.

6. No cytokinesis: Prokaryotes do not undergo a process of cytokinesis like eukaryotes. Instead, the division occurs through a constriction of the cell membrane and cell wall, resulting in the physical separation of the two daughter cells.

Overall, prokaryotes divide through binary fission, which is a simpler and faster process compared to mitosis in eukaryotes.

Characteristics in prokaryotes 

Cell division in prokaryotes is best characterized as "binary fission".



POLYPLOIDY :

Colchicine an alkaloid obtained from the corms of the autumn crocus plant, Colchicum autumnale, acts as a poison for cell division.  It checks the formation of spindle by preventing  the assembly  of microtubules and so blocks mitosis and cell division. However,  colchicine allows chromosomal replication and separation of chromatids, thus doubling the number of chromosomes in the cell. The duplicated chromosomes remain in the same cell. Such an increase in the number  of full chromosome sets is called POLYPLOIDy .Polyploidy may produce 

(a) More vigorous offspring 

(b) Even new varieties and species. The plant breeders have used colchicine _ induced POLYPLOIDy for producing  new vigorous varieties of useful  crops, fruit trees and ornamental  plants.


  Colchicine is also used to stop cell division  at metaphase  so that condense chromosomes may be obtained  for studying their  morphology  and prepare a karyotype. Colchicine  is sometimes used to stop division of cancer  cells.

What is its other significance  ?

Colchicine, an alkaloid 
  
  Colchicine is also used to stop cell division at metaphase  so that condensed chromosomes may be obtained  for studying  their morphology  and prepare a karyotype. 

 Colchicine is obtained from the corms of the autumn crocus  plant, Colchicum autumnale. 


Why is mitosis called equational division?

It is so because mitosis is the  division of parent cell into two identical daughter cells, each with a nucleus having the same amount of DNA, the same number and kind of chromosomes, and the same hereditary instructions as the parent cell.

What happens during interphase? What is its duration. It is divisible into how many period ? Name them.


Interphase  is a period of intense synthesis  and growth. The cell produces biomolecules (e.g., carbohydrates, lipids, proteins, nucleic acids),  needed for its growth and carrying out its specific functions. Also, the cellular  molecules needed for cell division are stocked.

  Interphase lasts for 10_20 hours. It is further  divisible into three periods :   G1 phase, S phase and G2 phase.



What is the status of chromosomes  and DNA content in each daughter  cell ? 

Meiosis results in the formation of 4 daughter  cells, each having half the number of chromosomes and half of the nuclear DNA amount  present in the parent cell.

After cell division, each daughter cell will have the same number of chromosomes as the parent cell. The chromosomes are duplicated during the S phase of the cell cycle, so each daughter cell will have an identical copy of the parent cell's DNA content. This ensures that the genetic material is properly distributed to the daughter cells.








































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