Showing posts from August, 2013


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
expressed when a protein that is in some …


Considerable information is embedded in the precise order of nucleotides
in DNA. For life to exist with stability, it is essential that
the nucleotide sequence of genes is not disturbed to any great extent.
However, sequence changes do occur and often result in altered
phenotypes. These changes are largely detrimental but are
important in generating new variability and contribute to the
process of evolution. Microbial mutation rates also can be increased,
and these genetic changes have been put to many important
uses in the laboratory and industry.
Mutations [Latin mutare, to change] were initially characterized
as altered phenotypes or phenotypic expressions. Long before
the existence of direct proof that a mutation is a stable, heritable
change in the nucleotide sequence of DNA, geneticists predicted that
several basic types of transmitted mutations could exist. They believed
that mutations could arise from the alteration of single pairs of
nucleotides and from the addition or deletion o…


M E D I C I N E:
 Epigenetics, Nucleosome Structure, and Histone Variants

Information that is passed from one generation to the
next— to daughter cells at cell division or from parent to
offspring—but is not encoded in DNA sequences is referred
to as epigenetic information. Much of it is in the
form of covalent modification of histones and/or the
placement of histone variants in chromosomes.
The chromatin regions where active gene expression
(transcription) is occurring tend to be partially
decondensed and are called euchromatin. In these regions,
histones H3 and H2A are often replaced by the

histone variants H3.3 and H2AZ, respectively.
Shown here are the core histones and a few of the known variants.
Sites of Lys /Arg residue methylation and Ser phosphorylation are indicated.
HFD denotes the histone-fold domain, a structural domain
common to all core histones chaperones, helping to ensure the proper assembly
and placement of nucleosomes. Histone H3.3
differs in sequence from H3 by only four amino aci…



The topological state of cellular DNA is intimately connected
with its function. Without topoisomerases, cells
cannot replicate or package their DNA, or express their
genes—and they die. Inhibitors of topoisomerases have
therefore become important pharmaceutical agents, targeted
at infectious agents and malignant cells.
Two classes of bacterial topoisomerase inhibitors
have been developed as antibiotics. The coumarins, including
novobiocin and coumermycin A1, are natural
products derived from Streptomyces species. They inhibit
the ATP binding of the bacterial type II topoisomerases,
DNA gyrase and topoisomerase IV. These
antibiotics are not often used to treat infections in

humans, but research continues to identify clinically effective variants.

The quinolone antibiotics, also inhibitors of bacterial
DNA gyrase and topoisomerase IV, first appeared in
1962 with the introduction of nalidixic acid. This compound
had limited effectiveness and is no longer used


Bacteria In cancer Therapy ( On Humble Request from Maria Alejandra Cano Cháves)


Resistance to conventional anticancer therapies in patients with advanced solid tumors has prompted the need of alternative cancer therapies. Moreover, the success of novel cancer therapies depends on their selectivity for cancer cells with limited toxicity to normal tissues. Several decades after Coley's work a variety of natural and genetically modified non-pathogenic bacterial species are being explored as potential antitumor agents, either to provide direct tumoricidal effects or to deliver tumoricidal molecules. Live, attenuated or genetically modified non-pathogenic bacterial species are capable of multiplying selectively in tumors and inhibiting their growth. Due to their selectivity for tumor tissues, these bacteria and their spores also serve as ideal vectors for delivering therapeutic proteins to tumors. Bacterial toxins too have emerg…