MICROBIOLOGY

Microbial   Recombination  and Plasmids: Bacterial Conjugation:  The initial evidence for bacterial conjugation,the transfer of genetic information by direct cell to cell contact, came from an elegant experiment performed by Joshua Lederberg and Edward L. Tatum in 1946. They mixed two auxotrophic strains, incubated the culture for several hours in nutrient medium, and then plated it  on  minimal  medium. To  reduce  the  chance  that  their  results were due to simple reversion, they used double and triple auxotrophs on the assumption that two or three reversions would not often occur simultaneously. Lederberg and Tatum did not directly prove that physical contact of the cells was necessary for gene transfer. This evidence was provided by Bernard Davis (1950), who constructed a U tube consisting of two pieces of curved glass tubing fused at the base to form a U shape with a fritted glass filter between the halves...

Microbial   Recombination  and Plasmids: Transposable Elements :  The chromosomes of bacteria, viruses, and eucaryotic cells contain pieces of DNA that move around the genome. Such movement is called transposition. DNA segments that carry the genes required  for  this  process  and  consequently  move  about  chromosomes are transposable elements or transposons.Unlike other processes that reorganize DNA, transposition does not require extensive areas of homology between the transposon and its destination site. Transposons behave somewhat like lysogenic prophages  except that they originate in one chromosomal location and can move to a different location in the same chromosome. Transposable elements differ from phages in lacking a virus life  cycle  and  from  plasmids  in  being  unable  to  reproduce  autonomously and to exist apart from the chromosome. They were firs...

MICROBIOLOGY : MICROBIAL MOLECULAR BIOLOGY AND GENETICS

MICROBIOLOGY : MICROBIAL MOLECULAR BIOLOGY AND GENETICS

MICROBIAL RECOMBINATION AND PLASMIDS: Bacterial Plasmids:  Conjugation, the transfer of DNA between bacteria involving direct contact, depends on the presence of an “extra” piece of circular DNA known as a plasmid. Plasmids play many important roles in the lives of bacteria. They also have proved invaluable to microbiologists  and  molecular  geneticists  in  constructing  and  transferring new genetic combinations and in cloning genes. Plasmids are small double-stranded DNA molecules, usually circular, that can exist independently of host chromosomes and are present in many bacteria (they are also present in some yeasts and other fungi). They have their own replication origins and are autonomously replicating and stably inherited. A replicon is a DNA molecule or sequence that has a replication origin and is capable of being replicated. Plasmids and bacterial chromosomes are separate replicons. Plasmids have relatively few genes, gen...

MICROBIAL RECOMBINATION AND PLASMIDS: In a general sense, recombination is the process in which one or more nucleic acids molecules are rearranged or combined to produce a new nucleotide sequence. Usually genetic material from two parents is combined to produce a recombinant chromosome with a new, different genotype. Recombination results in a new arrangement of genes or parts of genes and normally is accompanied by a phenotypic change. Most eucaryotes exhibit a complete sexual life cycle, including meiosis, a process of extreme importance in generating new combinations of alleles (alternate forms of a particular gene) through recombination. These chromosome exchanges during meiosis result from crossing-over between homologous chromosomes, chromosomes containing identical sequences of genes . Until about 1945 the primary focus in genetic analysis was on the recombination of genes in plants and animals. The early work on recombination in higher eucaryotes laid the foun...

The Expressions of Mutation:  The expression of a mutation will only be readily noticed if it produces a detectable, altered phenotype. A mutation from the most prevalent gene form, the wild type, to a mutant form is called a forward mutation. Later, a second mutation may make the mutant appear to be a wild-type organism again. Such a mutation is called a reversion mutation because the organism seems to have reverted back to its original phenotype. A true back mutation converts the mutant nucleotide sequence back to the wild-type sequence. The wild-type phenotype also can be regained by a second mutation in a different gene, a suppressor mutation, which overcomes the effect of the first mutation.  If the second mutation is within the same gene, the change may be called a second site reversion or intragenic suppression. Thus, although revertant phenotypes appear to be wild types, the original DNA sequence may not be restored. In practice, a mutation is visibly...