Describe types of recombination process and its importance in gene expression.
Answers
Explanation:
Genetic Recombination in Bacteria
Index to this page
Transformation
Conjugation
Transduction
Significance of genetic recombination in bacteria.
Reductionism
Bacteria have no sexual reproduction in the sense that eukaryotes do. The have
no alternation of diploid and haploid generations
no gametes
no meiosis
But the essence of sex is genetic recombination, and bacteria do have three mechanisms to accomplish that:
transformation
conjugation
transduction
Transformation
Many bacteria can acquire new genes by taking up DNA molecules (e.g., a plasmid) from their surroundings [View]. The ability to deliberately transform the bacterium E. coli has made possible the cloning of many genes — including human genes — and the development of the biotechnology industry.
Link to a discussion of cloning genes by transforming E. coli with recombinant DNA molecules.
The first demonstration of bacterial transformation was done with Streptococcus pneumoniae and led to the discovery that DNA is the substance of the genes. The path leading to this epoch-making discovery began in 1928 with the work of an English bacteriologist, Fred Griffith.
The cells of S. pneumoniae (also known as the pneumococcus) are usually surrounded by a gummy capsule made of a polysaccharide. When grown on the surface of a solid culture medium, the capsule causes the colonies to have a glistening, smooth appearance. These cells are called "S" cells.
Streptococcus pneumoniae (pneumococci) growing as colonies on the surface of a culture medium. Left: The presence of a capsule around the bacterial cells gives the colonies a glistening, smooth (S) appearance. Right: Pneumococci lacking capsules have produced these rough (R) colonies. (Courtesy of Robert Austrian, J. Exp. Med. 98:21, 1953.)
However, after prolonged cultivation on artificial medium, some cells lose the ability to form the capsule, and the surface of their colonies is wrinkled and rough ("R"). With the loss of their capsule, the bacteria also lose their virulence. Injection of a single S pneumococcus into a mouse will kill the mouse in 24 hours or so. But an injection of over 100 million (100 x 106) R cells is entirely harmless.
Encapsulated (left) and nonencapsulated (right) pneumococci. The encapsulated forms produce smooth colonies (above). (Courtesy of Robert Austrian, J. Exp. Med. 98:21, 1953.)
The reason? The capsule prevents the pneumococci from being engulfed and destroyed by scavenging cells — neutrophils and macrophages — in the body [View]. The R forms are completely at the mercy of phagocytes.
Pneumococci also occur in over 90 different types: I, II, III and so on. The types differ in the chemistry of their polysaccharide capsule.
Unlike the occasional shift of S -> R, the type of the organism is constant. Mice injected with a few S cells of, say, Type II pneumococci, will soon have their bodies teeming with descendant cells of the same type.
However, Griffith found that when living R cells (which should have been harmless) and dead S cells (which also should have been harmless) were injected together, the mouse became ill and living S cells could be recovered from its body. Furthermore, the type of the cells recovered from the mouse's body was determined by the type of the dead S cells. In the experiment shown, injection of
living R-I cells and
dead S-II cells
produced a dying mouse with its body filled with living S-II pneumococci.
The S-II cells remained true to their new type. Something in the dead S-II cells had made a permanent change in the phenotype of the R-I cells. The process was named transformation.