by Microbiology Doctor-dr
REPRODUCTIONSexual reproduction is the most prevalent means of reproduction in bacteria, but they can also reproduce sexually through genetic recombinations, which is a primitive kind of sexual reproduction.
Asexual Reproduction
The usual methods of asexual reproduction is binary fission and endospore formation.
Binary Fission
In bacteria, it is the most common method of asexual reproduction. It occurs when the conditions are favourable for growth. A cell doubles in size before splitting into two identical daughter cells. Each daughter cell increases in size until it reaches the size of the parent cell. The procedure is also known as the life cycle.)
Figure: Binary fission in a bacterium
Mechanism of Binary Fission
Before fission, the bacterial cell absorbs resources from its surroundings and produces RNA, DNA, proteins, enzymes, and other macromolecules. New cell wall material is generated, and cell mass and size increase. The DNA replicates, and the cytoplasmic membrane at the cell's centre expands inward. A mesosome is often linked to the cytoplasmic membrane near the site of inward expansion and is thought to have a function in membrane production. During replication, the DNA is connected to the mesosome, which keeps it in place. The creation of a septum between the two daughter cells is caused by the inward expansion of the cell wall.
Endospore Formation
Before fission, the bacterial cell absorbs resources from its surroundings and produces RNA, DNA, proteins, enzymes, and other macromolecules. New cell wall material is generated, and cell mass and size increase. The DNA replicates, and the cytoplasmic membrane at the cell's centre expands inward. A mesosome is often linked to the cytoplasmic membrane near the site of inward expansion and is thought to have a function in membrane production. During replication, the DNA is connected to the mesosome, which keeps it in place. The creation of a septum between the two daughter cells is caused by the inward expansion of the cell wall.
Mechanism of Endospore Formation
When a cell's growth is completed and the Forespone is fully formed, endospore creation occurs. At one end of the cell, the cell collects DNA components in order to manufacture axial filament. A two-layered spore septum is formed when the cytoplasmic membrane invaginates. It forms forespore by surrounding the axial filament and cytoplasm. Between the layers of spore septum, the spore wall is built. Finally, the parent cell's cell wall ruptures, releasing the endospore.
When favourable conditions for growth return, the endospore germinates. Water is ingested, cytoplasmic enzymes are triggered, protoplast vegetative growth resumes, and the endospore wall breaches. Proteins are synthesised by the protoplast, which develops to the size of a vegetative cell.
Sexual Reproduction
In bacteria, sexual reproduction is accomplished through genetic recombination. Recombination of genes occurs when genetic material from homologous chromosomes is exchanged, resulting in the development of a new genotype. It's said to be a primitive method of sexual reproduction. In the absence of gamete synthesis and fertilisation, it differs from eukaryotic sexual reproduction.
Bacterial Recombination
The cells do not merge in bacterial recombination, and only a fraction of the donor cell's DNA is normally transferred to the receiving cell. The donor DNA fragment is placed beside the recipient DNA in such a way that homologous genes are next to one other inside the recipient cell. Enzymes nick the recipient DNA, causing a segment to be removed. In place of the removed DNA, the donor DNA is incorporated into the recipient DNA. Because its DNA contains DNA from both the donor and the recipient cell, the recipient cell becomes a recombinant cell. Recombinant DNA is the term for this type of DNA. Specific enzymes are likely to break down the extracted DNA fragments from the receiving DNA.
Methods of Bacterial Recombination
In bacteria, genetic recombinations result from three types of gene transfer:
i. Transformation: Transfer of cell-free DNA from one cell to another.
ii. Conjugation: Transfer of genes between cells that are in physical contact with one another.
iii. Transduction: Transfer of genes from one celi to another by a bacteriophage.
i. Transformation
Griffith discovered the first evidence of genetic recombination or the exchange of hereditary material in bacteria during his transformation experiment in 1928.
Transforming Experiment
Griffith discovered two strains of Streptococcus pneumoniae, the bacterium that causes pneumonia fever, while working with it. When grown on nutrient agar, one was capsulated, causing disease and forming smooth colonies. S-type was the name given to it. When cultivated on nutrient agar, the other was non-capsulated, did not induce illness, and formed rough colonies. R-type was the moniker given to it. Griffith inserted live R-cells and heat-killed S-cells into a mouse. The mouse died only a few days after becoming infected. Griffith was able to isolate viable S-cells from the blood of a deceased mouse. Griffith came to the conclusion that heat-killed S-cells released a component that allowed R-cells to form capsules and become pathogenic. When it was discovered that the change was heritable, Griffith coined the phrase "transforming principle."
Griffith's transformation experiment
Mechanism of transformation
Identification of Transforming Principle
In 1944, Avery, Macleod, and McCarty isolated and identified DNA as the transforming principle. It was defined as bacterial genetic material and a transforming agent. A short piece of DNA is released by the donor during transformation and actively taken up by the recipient, where it replaces a similar piece of DNA. Since 1940, transformation has also been demonstrated in Bacillus, Haemophilus and Azotobacter species.
ii. Bacterial Conjugation
Conjugation involves transfer of DNA between cells in direct contact. The process was first demonstrated experimentally by Joshua Lederberg and Edward Tatum in 1946, in Escherichia coli. They observed that normally E. coli can synthesise all the amino acids required by it, if given a supply of glucose and mineral salts. Lederberg and Tatum induced mutations by exposing the bacteria to radiations. They obtained two mutants, one mutant was unable to synthesise biotin (a vitamin) and amino acid [methionine and the other could not synthesise amino aci threonine and leucine. Both mutants were combined and cultured on a medium that was devoid of all four factors. Despite the fact that none of the cells should have grown, a few hundred colonies grew from a single bacterium. This suggests that genetic information is being exchanged. No chemical was isolated, so the transformation was ruled out. In E. coli, an electron microscope later revealed that direct cell contact or conjugation occurs.
Conjugation between Hfr and F cells.
Sex Factors
Francois Jacob and Elie L. Wollman's findings demonstrating that multiple mating types exist in E. coli led to a better knowledge of conjugation in bacteria. Sex factor, or F-factor, is an extrachromosomal fragment of DNA found in some bacteria (fertility factor). These cells are donor cells and were given the names male or F. Female or F cells are recipient cells because they lack the F factor. Hair-like pilli coat E. coli, but the F contains one to three additional pilli, known as F pilli or sex pilli, which are responsible for physical contact between cells. Later, new strains of F cells were discovered that have a higher rate of sexual reproduction with F cells. These strains are known as Hfr strains, or high frequency recombination strains. This breakthrough contributed to a better understanding of conjugation.
Mechanism of Conjugation
During conjugation, the F and F cells make physical contact and create a tube termed the conjugation tube. The F factor replicates and unwinds. Through the sex pilus, a single-stranded F factor passes to the recipient cell. DNA replication takes place in both the donor and recipient cells, restoring the double-stranded form of the DNA in both.
iii. Transduction
The transmission of a double-stranded fragment of DNA from a donor cell to a recipient cell via a third party, generally a bacteriophage, is known as transduction. Zinder and Lederberg found the phenomena in 1952 while looking for sexual conjugation in Salmonella typhimurium, a bacterium that causes typhoid in mice. They paired mutants that couldn't synthesis specific nutrients with mutants that couldn't synthesise these nutrients and cultivated them on nutrient-deficient media. They were able to isolate recombinants that could produce all of the nutrients. This suggests that genetic material is exchanged by conjugation.
Transduction in bacteria
They then conducted a U-tube experiment. They put nutritional mutants in each tube arm and separated them with a tiny glass filter that prevents bacteria from passing through. Even so, the recombinants were discovered. This eliminates conjugation. According to Zinder and Lederberg, the exchange of genetic material is mediated by a filterable agent. It was also discovered that Dnase does not eliminate this agent (deoxyribonuclease). This indicates a lack of metamorphosis. This filterable agent was quickly identified as a phage virus or bacteriophage. The behaviour was later discovered in E. coli.
Mechanism of Transduction
The bacteriophage or phage virus attaches itself to the bacterial cell's surface and injects its DNA into the cell during infection. The viral DNA drives the creation of viral proteins, and the cell assembles new phage particles. The phages are released when the bacterial cell wall bursts. This is referred to as the lytic cycle. After infection, certain viruses, however, integrate their DNA into the bacterial chromosome. Temperate viruses are identified by their recombinant DNA, which is referred to as prophage. Bacteria that cause lysosomal disease are known as lysogenic bacteria.
Patterns of Transduction
Transduction occurs in two patterns:
i. Generalized Transduction
If all fragments of bacterial DNA have a chance to enter a transducing phage, the process is called generalized transduction.
Generalized Transduction
During phage assembly, viral enzymes hydrolyze the bacterial chromosome into numerous little fragments, and any section of the bacterial chromosome can be integrated into the phage head. It isn't usually linked to any viral DNA. A huge number of transduced phages containing various bacterial chromosomal segments are created. Only a small percentage of phages carry bacterial DNA. The donor DNA gets incorporated into the genome of the recipient cell when these aberrant phage particles infect a bacteria.
ii. Specialized Transduction
Temperate phages can only transmit a few restricted genes from the bacterial chromosome that are near to the prophage in the bacterial chromosome in this sort of transduction. As a result, the procedure is also known as limited transduction. It happens when a bacteriophage genome, after becoming integrated as a prophage in the DNA of the host bacterium, becomes free upon induction and takes a little portion of the bacterial chromosome with it into the phage head. When a phage infects a cell, it carries the bacterial genes that have become a part of it with it. Such genes can recombine with the infected cell's homologous DNA. The phage lambda of E. coli is the best model for studying specialised transduction.
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