Mutations are spontaneous or induced inheritable changes in
the base sequence of genomic nucleic acid, which change the
genotype (genetic properties) and leading to a change in
the phenotype (physical properties) of the organism
Molecular basis of mutation
Base-pair substitution
Frameshift mutations are caused by deletions,
insertions or translocations of DNA
Reversion - same-site vs. second-site (suppressor)
reversion to the original phenotype
Mutation rates
Missense mutations occur at frequencies of about
10-6 per generation
Nonsense mutations occur at 10-6 to
10-8 per generation
Transpositions occur at frequencies of about
10-4 per generation
Mutagens - incuce mutations; some examples of mutagens
are:
Base analogs
UV or X-irradiation
DNA repair errors
Transposons
Ames test - a test for carcinogenicity based on
assessment of mutagenic activity of chemical compounds
Genetic Recombination
Genetic Recombination is genetic exchange between homologous
nucleic acid sequences from different sources; accomplished by
bacteria in several ways:
Transformation
Competent bacterial cells take up DNA from other
sources
DNA fragments are integrated into their genome or
simply allow it to reside in their cytoplasm (plasmids)
Transduction
DNA is moved from one bacterial cell to the next
within the capsid of a bacteriophage
Types of transduction
Specialized transduction occurs if the
bacteriophage DNA becomes integrated into the host
chromosome at a specific site
Generalized transduction occurs if the
bacteriophage DNA is integrated at multiple sites within the
host chromosome
Conjugation
Genetic transfer that involves direct interaction of
donor and recipient bacterial cells
Mediated by sex pili
Requires the presence of plasmids and concomitant
DNA replication
Plasmids may mediate only their own transfer (e.g., F
plasmids)
Plasmids may mediate the transfer of donor chromosomal
DNA (e.g., Hfr plasmids)
Plasmids may carry genes for antibiotic resistance,
virulence factors, etc. in addition to the genes that allow
them to replicate
Transposition
Transposable elements such as insertion
sequences, transposons and some viruses are
capable of moving around the genome
Gene function is disrupted when these elements insert
within genes
Other genes may also be carried and inserted along with
transposons or viruses
Transposition occurs at frequencies of 10-5 to
10-7 per generation
