Reproduction in Bacteria

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Bacteria Reproduction

Just like any other organism, bacteria also reproduce to continue their species. Since they are unicellular and do not have a well-organised cell, bacteria have been grouped under prokaryotes. However, they do show both sexual and asexual means of reproduction. In this topic, we will have a brief overview of all types of means of reproduction in bacteria.

In asexual reproduction in bacteria, 5 methods are observed. These are:

  1. Binary fission

  2. Reproduction through conidia

  3. Budding

  4. Reproduction through cyst formation

  5. Reproduction through endospore formation

However, in asexual reproduction, genetic recombination is not observed and that is why sexual reproduction has high significance in the continuation of a bacterial species. This is because, in sexual reproduction, genetic material is exchanged between two cells and which facilitates genetic recombination and creates a genetic drift in the species of a bacteria. There are 3 ways bacteria reproduce sexually, these are:

  1. Transformation

  2. Transduction

  3. Conjugation

How Do Bacteria Reproduce?

As we have already discussed, bacteria reproduce through both asexual and sexual means. IN this section we will learn about these different modes of reproduction in bacteria.

Asexual Reproduction in Bacteria

Binary Fission: In binary fission, a single bacterial cell divides into two daughter cells. At first, the bacterial cell reaches critical mass in its form and cell components. The circular double-stranded DNA of the bacteria undergoes replication and new complementary strands are formed. These two strands of DNA are then moved to the different poles of the cell and a transverse septum then takes place and develops in the middle region of the cell which separates the two new daughter cells and thus binary fission I completed. It is a rapid process and takes minutes to complete.

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Conidia Formation: The formation of conidia takes place in filamentous bacteria such as Streptomyces through the formation of a transverse septum at the apex of the filament. The part bearing the conidia is called the conidiophore and after it is detached from the mother cell, in a suitable substratum it germinates giving rise to new mycelium. This type of asexual reproduction is also called fragmentation.

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Budding: In this method of reproduction, the bacterial cell develops a small swelling at one side which continuously increases in size. At the same time, the nucleus also undergoes division where one part with some cytoplasm enters the swelling and the other part remains with the mother cell. The outgrowth is called the bud and it eventually gets separated from the mother cell by a partition wall. This method of reproduction also comes under vegetative reproduction in bacteria. Example: Rhodomicrobium vannielii

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Cysts: Cysts are formed by the deposition of additional layers around the mother cell and are the resting structure during unfavourable conditions. When conditions are favourable again, the mother cell behaves like its normal self again. Example: Azotobacter.

Reproduction through endospore formation: Endospores in a bacterial cell are formed during stressful conditions such as desiccation and starvation. They contain a central protoplast, and a core consisting of DNA, ribosomes, enzymes and the t-RNA, everything necessary for the formation of a new cell. Only one endospore is formed in one bacterial cell and on germination, it gives rise to a new bacterial cell.

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Sexual Reproduction in Bacteria

Transformation:  In transformation, a bacterium takes up DNA from its environment and often DNA that’s been shed by another bacteria. The phenomenon was first discovered by Griffith in 1928 and the mechanism was worked out by Avery in 1944. In this process, the DNA of a capsulated bacteria is transferred into a non-capsulated bacteria. If the DNA is circular it is called a plasmid. The plasmid can be copied in the receiving cell and passed on to its descendants.

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Transduction: In this type of sexual reproduction of bacteria, foreign genes are transferred into a bacterial cell with the help of a virus. These viruses are called bacteriophage and they are not virulent. The virus acts as a carrier vehicle and passes over genes from one host to another. Transducing bacteriophages may carry the same genes in which the reproduction method would be known as restricted transduction. They can also carry different genes at different times in which the reproduction process would be known as generalised transduction.

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Conjugation: This process was first discovered in Escherichia coli by Tatum and Lederberg in 1946. They found that two different types of nutritional mutants grown together on minimal medium produced an occasional wild type.

Bacteria that show conjugation are dimorphic, meaning that they have two types of cells, one male (F+) or donor cell and a female (F-) or recipient cell.

The male or donor cell possesses 1 to 4 sex pili on the surface and fertility factor (transfer factor, sex factor) in its plasmid. It contains genes for producing sex pili and other characters needed for gene transfer. Sex pili are 1to 4 narrow protoplasmic outgrowths. The sex pili and fertility factor are absent from the female or recipient cells.

If these two types of cells happen to come nearer, a pilus of a male cell establishes a protoplasmic bridge or conjugation tube with the female cell. It takes 6-8 minutes for the process to complete.

These were the three types of sexual reproduction in bacteria and it introduces genetic variation in a bacterial species which is important for the survival of any species and allows groups to adapt to environmental changes.

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FAQ (Frequently Asked Questions)

1. What Are Transposable Elements?

Transposable elements are also important in bacterial genetics. These are parts of DNA which "jump" from one place to another within a genome, and in doing so they cut and paste themselves or insert copies of themselves in new spots. 

In bacteria, transposable elements sometimes carry genes for antibiotic resistance and pathogenicity (genes that make bacteria disease). If one of these elements "jumps" from the chromosome into a plasmid, the genes can be easily passed on to other bacteria by conjugation or transformation. That means the genes can spread quickly through the population

2. Do Bacteria Reproduce Sexually or Asexually?

Bacteria can produce through both sexual and asexual means.  Sexual reproduction takes place through:

  • Conjugation

  • Transformation

  • Transduction

Through genetic recombination or exchange of genetic material, they introduce new characteristics in the new daughter cell. Hence there is genetic variation within the species.

Asexually, bacteria reproduce through

  • Budding

  • Fragmentation

  • Binary fission

  • Cyst formation 

  • Endospore formation

They assist a bacterial cell to proliferate at a rapid rate as well as to survive in stressful environmental conditions.