Both asexual and sexual reproduction occurs in bacteria.
Asexual reproduction in bacteria occurs by binary fission. It occurs during favourable conditions. The size of bacteria becomes double. Bacteria absorb nutrients. It synthesizes its DNA. Bacteria also develop other material for cell division. This material includes enzymes, cell wall material and cell membrane materials. The DNA replicates and new DNAs move to the opposite poles. The membrane grows inward at the middle of the cell. It forms a mesosome. Mesosome form materipl for cell membrane. The inward growth of membrane continues. Finally both part of membrane joins in the middle. It divides the bacteria in two.
Bacteria also develop single endospore. Endospores are used to pass unfavourable conditions. The endospore germinates in favourable condition and develops new single bacterium. So endospore formation is not a mean of reproduction.
True sexual reproduction is absent in bacteria. But genetic recombination takes place in bacteria. There are three type of genetic recombination in bacteria. These are conjugation, transformation and transduction.
The genetic recombination in which bacteria exchange genetic material through cytoplasmic bridge is called conjugation. Conjugation was discovered by Joshua Lederberg and Edward Tatum. Plasmid F (Fertility) is involved in the transfer of genetic material. F plasmids contain 25 genes. These genes are required for the production of sex pili. The bacteria which contain F plasmids are called F+ bacteria (male). The bacteria which lack F plasmids are called F bacteria (female). F+ cells are donor bacteria and F cells are recipient bacteria. Thus mating occurs between F+ and F.
Sometimes, F plasmid incorporates into main bacterial DNA. Such bacteria are called Hfr cells (High frequency of recombination). The DNA of Hfr bacteria replicates. F plasmid gets some part of this DNA. Now Hfr and F develop cytoplasmic bridge. F plasmids pass through it and transfer the genes of donor bacteria into the DNA of recipient bacteria.
The genetic recombination in which bacteria absorbs naked DNA is called transformation. Frederick Griffith discovered transformation. He performed experiments on pathogenic bacteria Streptococcus pneumoniae. There are two forms of Streptococcus pneumoniae bacteria. Pathogenic form of this bacterium forms smooth colonies on a culture dish. So it is called S form. They are virulent. The mutant forms lack an enzyme which manufactures the polysaccharide coat. They form rough colonies. So they are called R form. They are non-virulent. Griffith performed following experiments:
- He infected the mice with S strain of Streptococcus pneumoniae bacteria. The mice died of blood poisoning.
- He infected similar mice with R strain of S. pneumoniae. The mouse remains alive.
- He injected dead S strain of bacteria into the mice. The mice
remained healthy. Now he injected mice with a mixture containing dead S bacteria and live coatless R bacteria. The mice developed the disease symptoms and many of them died. The blood of the dead mice contained a large number of live virulent S bacteria. These bacteria have surface proteins of the live (previously R) strain. Thus the information of polysaccharide coat has passed from the dead S bacteria to the live R bacteria in the mixture. Griffith called it transformation.
Experiments of Oswald Avery and coworkers
Oswald Avery and his colleagues performed series of experiment and called it Transforming principle. They destroyed different compounds one by one and checked the transforming activity. When they destroyed DNA, transformation activity stopped. Thus l3NA causes transforming activity.
The genetic recombination in which genetic material is transferred by phage virus between two bacteria is called transduction. It has two forms:
(a)Generalized transduction: It occurs in lytic cycle of phage
virus. DNA of phages virus enter into E.coli bacteria. This DNA replicates and develops many new DNA and capsids. The DNA of bacteria is broken. Some pieces of DNA also enter into capsid of virus. Bacteria burst and release new phage viruses. Now this phage enters into recipient bacteria and transfer DNA of donor bacteria into the DNA of recipient bacteria. Bacterial endonucleases enzymes destroy the phage virus. Now these bacteria incorporate genes of donor bacteria and replicates.
(b)Specialized transduction: It occurs in Lysogenic cycle of phage virus. In this cycle viral DNA incorporate into bacterial DNA as prophage. It remains peacefully there. But sometime, it becomes lytic. It comes out of bacterial DNA. Some part of bacterial DNA remain attach with it. Viral DNA with a piece of bacterial DNA replicates and develops new capsids. Bacteria burst. Virus infects other bacteria and transfer genes of donor bacteria to recipient bacteria.