Reproduction in Bacteria: Meaning, Diagrams, and Types

Reproduction is the biological process by which new individual organisms or offspring are produced from their parents. Microorganisms like bacteria also continue their existence by replicating themselves by the process of reproduction. Bacteria are unicellular organisms, and the Reproduction in Bacteria takes place by both asexual and parasexual methods. Bacteria are single-celled prokaryotic organisms. They don’t have well-organized cells organelles but can reproduce at a very fast rate. A single bacteria can easily grow into a colony of bacteria within a few hours. To know the reason and process behind the interesting methods of reproduction in bacteria, scroll down.

Types of Reproduction in Bacteria

Bacteria mainly reproduce through two methods asexual and parasexual. These are given below:

A. Asexual Reproduction in Bacteria

A genetically identical copy of the offspring of bacteria is produced during asexual reproduction. Five different methods of asexual reproduction are observed in different bacteria. They are:

1. Binary Fission

i. Binary fission is the simplest asexual reproduction process in which a single bacterial cell divides into two.
ii. In this process, the bacterium first copies its DNA by replication enzymes that begin at a point of origin on the chromosome then continue separating the strand in two.
iii. After duplication of the chromosome, the bacterial cell grows in size and prepares itself for binary fission. In this stage, the cytoplasmic content of the cell and the cell organelles increases. The two chromosome strands move away to the opposite poles of the cell.
iv. The plasma membrane invaginates and forms a transverse septum in the middle of the cell, which divides the parent cell into two new identical daughter cells.
v. Each of the daughter cells contains the genetic material and all required cell organelles.
vi. Binary fission is a rapid process, and it divides into daughter cells within 30 minutes. In this process, single bacteria can grow a bacteria colony within 4-5 hours.

2. Conidia Formation

i. In some types of filamentous bacteria like Streptomyces, conidia formation can be seen.
ii. Conidia are small, chain-like, spherical, spore-like structures formed at the tips of the filaments by a transverse wall.
iii. The part of the filament which has conidia is known as conidiophore.
iv. After detachment of each conidium from the mother in a suitable substratum, it germinates, giving rise to a new mycelium.

3. Budding

i. Budding is a type of asexual reproduction in which bacterial cell develops small outgrowth or bud due to cell division at one particular site.
ii. These buds develop into tiny individuals; simultaneously, the nucleus also undergoes division. One nucleus with some cytoplasm enters the bud.
iii. Bud will detach from the parent cell by a partition wall when it gets fully mature. 

Example: Hyphomicrobium, Caulobacter, Rhodopeudomonas, Hyphomicrobeales, etc.

4. Cysts

i. In some bacteria, cyst formation occurs.
ii. Cysts are the resting or dormant stage of the mother bacteria cell.
iii. Cysts are formed by the deposition of an additional layer around the mother wall.
iv. In this stage, the metabolic process of the cell is slowed.
v. When bacteria get a favourable environment, they break down the cyst’s wall by the process called excystation and germinate to form a new bacterium.
vi. The main function of cysts is to protect against adverse changes in the environment.
vii. Examples found in certain species of Azotobacter.

5. Endospore

i. Spores are found to form inside the vegetative cell. Therefore, they are also known as endospores.
ii. Endospores are perennial, tough, resistant, dormant, and specialised spores formed to overcome unfavourable conditions.
iii. The sporangium is the endospore-producing mother cell.
iv. A single spore is established inside a bacteria cell.
v. In endospore, the bacteria can withstand exceedingly high and low temperatures, acidic and basic conditions, and large amounts of radiation.
vi. For example: In Clostridium and Bacillus, the endospore has many wall layers containing heat-resistant chemicals such as sialic acid and dipicolinic acid.
vii. These can easily be dispersed through wind, water, and the gut of animals.
viii. On obtaining favourable climatic conditions, endospores break to release a bacteria cell.

B. Parasexual Methods in Bacteria

Parasexual methods in bacteria occur in three different methods that are discussed as follows:

1. Transformation

i. This phenomenon was discovered by Griffith in 1928, and the mechanism was worked on in 1944 by Avery.
ii. In this process of horizontal gene transfer, bacteria take up foreign naked DNA from the extracellular environment, and also it can incorporate this DNA into their own genome.
iii. Sometimes the exogenous genetic material may exist together as a plasmid with chromosomal DNA.
iv. All the bacteria are not able to take DNA from the surrounding environment. Such bacteria are made artificially competent in the laboratory by using chemicals such as calcium phosphate or by electroporation, or by using heat shock. Competent cells are those cells that have the capability to uptake the DNA from the extracellular environment.
v. The receptivity for transformation is present for a shorter period.
vi. DNA from capsulated bacteria is transferred into non-capsulated bacteria.

2. Transduction

i. Transduction is a method of gene transfer in bacteria where a bacterial gene is transferred by a virus. Some specific viruses that infect bacteria are called bacteriophages.
ii. This was first discovered by Zinder and his teacher Lederberg (1952) in Salmonella typhimurium.
iii. The transductions are mainly two types: Generalized and specialized transduction.
iv. In generalized transduction, only the transfer of pure DNA content or genetic content takes place from the bacteria to the other, whereas in specialized transduction, the DNA content or genetic content which is transferred from one bacteria to the other is not packed as a pure bacterial DNA.

Generalised Transduction

i. This type of transduction occurs in the lytic cycle of the phage virus.
ii. DNA of phages virus enters into E. coli bacteria at first.
iii. When the virus enters into the bacterial cell, the virus takes over the host machinery and synthesizes virus components such as the genome, enzymes, capsid, head-tail, and tail fibres. Then the viral enzyme chops up the host cell DNA into small fragments.
iv. During the assembly phase of a virus, components form progeny viruses, sometimes by mistakes, the donor DNA gets packed into the virus capsid (bacteriophage head). Such abnormal bacteriophages when infects a new cell and can transfer this donor DNA into new bacteria. Since this donor DNA is not viral DNA, it does not replicate inside recipient bacteria but undergoes homologous recombination with the recipient cell’s chromosomal DNA forming a recombinant cell.

Specialised Transduction

i. This type of transduction occurs in the lysogenic cycle of the phage virus.
ii. This type of transduction is carried out only by temperate bacteriophage.
iii. At first temperate bacteriophage enter into donor bacteria, and then its genome gets integrated with the host cells DNA at a certain location and remains dormant and passes generation to generation.
iv. When such a lysogenic cell is open to certain stimuli such as some chemicals or UV lights, it causes induction of the virus genome from the host cell genome and begins the lytic cycle.
v. Upon induction from donor DNA, this phage genome sometimes carries a part of bacterial DNA with it.
vi. The bacterial DNA lies on the sides of integrated phage DNA and is only carried during induction.
vii. When such a bacteriophage carries a part of donor bacterial DNA infects a new bacteria, it can transfer that donor DNA fragments into a new recipient cell.
viii. So, in this type of transduction, only those restricted genes situated on the side of the integrated viral genome have a chance to enter into the recipient cell.

3. Conjugation

i. The exchange of genetic material directly from one bacterial cell to a different bacterial cell is thought of as conjugation.
ii. Tatum and Lederberg first discovered conjugation in E. coli in the year 1946.
iii. The bacteria that show conjugation contains two types of cells depending on the presence or absence of F-plasmid.
iv. A male cell (F⁺) or donor cell has F-plasmid, and a female cell (F^–) or recipient cell lacks F-plasmid.
v. The male or donor cell (F⁺) forms a thin, tubelike sex pilus and establishes contact with an F^– recipient cell.
vi. The pilus then draws the two bacteria together and forms a conjugation tube in which the donor bacterium transfers the genetic material to the recipient bacterium.
vii. The genetic material is present in both the donor and recipient strand as a single strand of F-plasmid.
viii. Thus a complementary strand is synthesized in both the male and female cells.
ix. The recipient of the female cell contains a copy of the F-plasmid and becomes a donor cell.
x. Within 6-8 minutes, the conjugation method gets completed.
xi. This type of genetic recombination in bacteria introduces genetic variation in a bacteria species which is important for the survival of species and allows groups to adapt to the environmental changes.

Summary

Bacteria are unicellular and reproduce by both asexual and parasexual methods. Asexual reproductions of bacteria are of five types. They are binary fission, conidia formation, budding, cysts, and endospores formation. Binary fission is a simple cell division and rapid process to divide into two daughter cells. Parasexual methods are accomplished through transformation, transduction, and conjugation.

FAQs

Q.1. What are the asexual reproduction methods of bacteria?
Ans: Binary fission, conidia formation, budding, cysts formation, and endospores formation are asexual reproduction methods of bacteria.

Q.2. What is the most common method of reproduction found in bacteria?
Ans: Binary fission is the most common method of bacterial reproduction.

Q.3. In which type of bacterial reproduction are viruses carriers of DNA?
Ans: In transduction, virus (bacteriophages) carries DNA of one bacterium to another bacteria.

Q.4. Who first discovered the transformation process of recombination?
Ans: In 1928, Griffith first discovered the transformation process.

Q.5. Mention different parasexual methods in bacteria.
Ans: Transformation, transduction, and conjugation are three different processes of parasexual methods in bacteria.

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