Mechanisms of genetic exchange

1. Competence:

It is the ability of a cell to take up extracellular (naked) DNA from its environment. It results from changes in cell wall of bacteria. It usually arises at the late log phase. On the basis of the conditions under which the bacteria uptakes the DNA, it can be differentiated into natural competence and induced/ artificial competence.

  2. Lytic phage:

The phage which causes lysis of the host is called a lytic or virulent phage. An example of a lytic bacteriophage is T4, which infects E.coli found in the human intestinal tract.

3. Temperate phage:

Temperate phage are basically bacteriophages that can choose between a lytic and lysogenic pathway. They can either lyse the cell or behave as a prophage (viral DNA integrated into the bacterial chromosome).

4. F' plasmid:

F plasmid is an episome so once it integrated  into host chromosomal DNA, it can also disintegrate itself back from the host chromosome. F factor frequently carries several adjacent bacterial genes next to its disintegration sites along with it. This condition is called F' plasmid. They are the derivatives of Hfr plasmid.

5. Merozygote:

When a bacterium with F' conjugates with F- (-ve) cells, the F factor containing the chromosomal genes, is transferred to the F- cells. Thus, the chromosomal genes that are part of the F factor are now present as duplicate in the recipient cell. This creates a partially diploid cell called a merozygote.

6. Prototroph:

A bacterium must be able to synthesize all essential organic compounds. A bacterium that can accomplish this remarkable biosynthetic feat- one that the human body cannot duplicate- is a prototroph. It is said to be wild type for all growth requirements.

7. Auxotroph:

If a bacterium loses, through mutations the ability to synthesize one or more organic compounds, it is an auxotroph. It is mutant organism that requires a particular additional nutrient which the normal strain does not.

8. Abortive transduction:

It is an event of transduction in which the transducing DNA (genetic fragment from the donor bacterium) fails to be incorporated into the recipient chromosome, and when the latter divides, is transmitted to only one of the daughter cells.

9. Co-transduction frequency:

The co-transduction frequency is the ratio of transductants that co-inherit both markers divided by the total number of transductants. Basically, only a small amount of chromosome, a few genes, can be transferred by transduction. The closer two genes are to each other, the more likely they are to be transduced by the same phage. Thus, "co-transduction frequency" is the key parameter used in mapping genes by transduction.

The Wu formula can be used to estimate the correlation between co-transduction frequency and the physical distance between two genetic markers.

          co-transduction frequency= (1-d/L)^3

where, d= distance between two genes in minutes

L= the size of the transduced DNA (in minutes)