bacteria: reproduction and gene transfer
- Binary fission
Reproduction and growth:
- In unicellular organism cell growth and reproduction are two tightly linked processes unlike multicellular organism.
- After gaining a fixed size bacteria reproduce through binary fission, budding and fragmentation.
- Bacteria in optimum condition grow and divide rapidly and double its population in every 9.8 minutes.
- It is the most common mode of cell division and growth cycle of bacterial population.
- In binary fission single cell divides into two identical cells with development of transverse septum (cross wall).
- Two daughter cells contains nucleus of its own which is identical to the parent cell.
- Cytoplasm divides leads to production of two equal sized cells.
Process of binary fission:
- Before the division DNA in the bacterial cell is tightly coiled
- DNA is then uncoiled and duplicated.
- Each copy of the DNA is pulled to the separate poles.
- Synthesis of new cell wall begins
- Once the new cell wall is synthesised fully it results in complete split of bacterium.
- New daughter cells now have tightly coiled DNA, plasmids and ribosomes.
|Types of binary fission||Example|
- Gene transfer means movement of genetic information in organisms.
- There are two types of gene transfer method one is vertical in which gene is transferred from parents to offspring and another one is horizontal in which gene is transferred in between two organisms.
- In prokaryotes vertical gene transfer is by the means of binary fission and horizontal gene transfer method consist of three process i.e. transformation, transduction and conjugation.
- In 1928 Fred Griffith discovered this method of horizontal gene transfer.
- In this process naked DNA molecule or fragment from surrounding environment is uptake by the recipient and incorporated in its chromosome.
- It is of two types natural and artificial, natural transformation is very rare event and observed in both gram negative and gram positive bacteria.
- Ability of bacteria to uptake DNA fragment and get transformed is known as competence.
Process of transformation:
- Competent bacteria naturally pull DNA fragment into their cell from the environment.
- These DNA fragment naturally released in the environment after a bacterial cell die.
- Ds DNA once crosses the membrane in cytoplasm the 3’ end is leading.
- The translocated strand interested in the chromosome of recipient bacteria by homologous recombination.
- Now the recipient bacteria undergoes replication and the cells acquired new phenotype are said to be transformed.
- In transduction, DNA is transfer from donor bacteria to recipient bacteria by bacteriophage (functions as vector).
- It was discovered by Lederberg and Zinder in 1951.
- Bacteriophage due to high specificity of surface receptors has narrowest host range.
- Transduction has one advantage over conjugation is that it doesn’t require physical contact of donor to recipient cell.
- Transduction process is resistant to the DNase enzyme.
- The phage infects the host and inserts its phage DNA into the cytoplasm of the host.
- During lytic cycle the phage DNA along with the bacterial chromosome is broken down into pieces
- Bacterial chromosome packed into the viral capsid is released by the lysis of the bacterium.
- Now the transducing phage with bacterial chromosome is ready to infect another bacterium in this way donor’s DNA enters into the cytoplasm of second bacterium.
- Host recombinase recA is present in the cell due to which donor DNA recombines with homologous bacterial DNA and produces transductants.
- The process of transfer of plasmid or other transmissible DNA element from donor to recipient via sex pilus or conjugation tube.
- Recipient of conjugation is known as transconjugants.
- Is can transfer DNA regions of hundreds to thousands of kilobases and has board host range fro DNA transfer.
- Occur in between many species of gram negative and gram positive bacteria even occurs between plants and bacteria.
- Conjugation involves F plasmid is most common.
- F+ structure contains tra locus which has pilin gene with some regulatory proteins responsible for the formation of pili on surface.
- Proteins present on pili attach to the F- cell surface and responsible for making contact between them but doesn’t transfer plasmid.
- The traD enzyme on the base of the pili makes the membrane to fuse.
- After the conjugation initiated the enzyme relaxes attached to the conjugative plasmid and make a nick at oriT.
- The nicked strand is now transferred to the recipient cell
- F+ cell carry such integrated F element is known as Hfr cell.
- The F element of Hfr cell is replicated along with the bacterial chromosome and in this way transmitted from one to next generation.