Frederick Griffith (1879 - 1941) was a British medical officer. In 1928, in what is today known as
Griffith's experiment, he discovered a transforming principle, which is today known as DNA.
Griffith was trying to make a vaccine to prevent pneumonia infections in the epidemics after World
War I by using two strains of the Streptococcus pneumoniae bacterium. The rough strain (R strain)
did not cause pneumonia when injected into mice and was not covered with a polysaccharide
capsule. The smooth strain (S strain) did have a polysaccharide capsule and was deadly when
injected, causing pneumonia and killing the mice in a day or two. When the S strain was heated to
inactivate it and then injected into mice, it produced no ill effects in the subjects. However, when
dead S coupled with live R was injected into the mouse, the mouse died. After isolating bacteria from
the blood of the mice, Griffith discovered that the normally nonpathogenic R bacteria had acquired
polysaccharide capsules. The bacteria isolated from the mice infected with the mixture of live R and
heat inactived S were all of the S strain, and maintained this phenotype over many generations.
Griffith hypothesized that some "transforming principle" from the heat inactivated S strain converted
the R strain to the virulent S strain. It wasn't until several years later that Griffith's "transforming
principle" was identified as DNA.
Griffith was killed at work along with longtime friend and bacteriologist William M. Scott in London as
a result of an air raid. He died holding a page that included formulas that seemed to be a
breakthrough, however they were too random to be interpreted. Today the paper has remained in a
preservation lab so that one day somebody can make sense of it and hopefully discover something
that Griffith wasn't able to complete.
He was the uncle of John Stanley Griffith, a winner of the Royal Society's Faraday Medal.
Virulence
Virulence refers to the degree of pathogenicity of a microbe, or in other words the relative ability of a
microbe to cause disease. The word virulent, which is the adjective for virulence, derives from the
Latin word virulentus, which means "full of poison." From an ecological point of view, virulence can
be defined as the host's parasite induced loss of fitness.
Genetic engineering
Modern biology and biochemistry make intensive use of recombinant DNA technology. Recombinant
DNA is a man-made DNA sequence that has been assembled from other DNA sequences. They can
be transformed into organisms in the form of plasmids or in the appropriate format, by using a viral
vector.[109] The genetically modified organisms produced can be used to produce products such as
recombinant proteins, used in medical research,[110] or be grown in agriculture.[111][112]
Episomes
An episome is a plasmid that can integrate itself into the chromosomal DNA of the host organism
(Fig. 3). For this reason, it can stay intact for a long time, be duplicated with every cell division of the
host, and become a basic part of its genetic makeup. This term is no longer commonly used for
plasmids, since it is now clear that a region of homology with the chromosome such as a transposon
makes a plasmid into an episome. In mammalian systems, the term episome refers to a circular DNA
(such as a viral genome) that is maintained by noncovalent tethering to the host cell chromosome.
Genetic recombination
Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used
in the development and functioning of all known living organisms. The main role of DNA is the
long-term storage of information and it is often compared to a set of blueprints, since DNA contains
the instructions needed to construct other components of cells, such as proteins and RNA
molecules. The DNA segments that carry this genetic information are called genes, but other DNA
sequences have structural purposes, or are involved in regulating the use of this genetic information.
A DNA helix does not usually interact with other segments of DNA, and in human cells the different
chromosomes even occupy separate areas in the nucleus called "chromosome territories".[96] This
physical separation of different chromosomes is important for the ability of DNA to function as a
stable repository for information, as one of the few times chromosomes interact is during
chromosomal crossover when they recombine. Chromosomal crossover is when two DNA helices
break, swap a section and then rejoin.
Recombination allows chromosomes to exchange genetic information and produces new
combinations of genes, which increases the efficiency of natural selection and can be important in
the rapid evolution of new proteins.[97] Genetic recombination can also be involved in DNA repair,
particularly in the cell's response to double-strand breaks.[98]
Transformation (genetics)
In molecular biology, Transformation is the genetic alteration of a cell resulting from the uptake and
expression of foreign genetic material (DNA). Separate terms are used for genetic alterations
resulting from introduction of DNA by plasmid-encoded conjugation or by viruses (transduction).
Transformation of animal cells is usually called transfection.
The term transformation is also used more generally to describe mechanisms of DNA and RNA
transfer in molecular biology (i.e. not only the genetic consequences). For example the production of
transgenic plants such as transgenic maize requires the insertion of new genetic information into the
maize genome using an appropriate mechanism for DNA transfer; the process is commonly referred
to as transformation.
RNA molecules may also be transferred into cells using similar methods, but this does not normally
produce heritable change and so is not true transformation.
The effect was first demonstrated in 1928 by Frederick Griffith, an English bacteriologist searching
for a vaccine against bacterial pneumonia. Griffith discovered that a non virulent strain of
Streptococcus pneumoniae could be transformed into a virulent one by exposure to strains of
virulent Streptococcus pneumoniae that had been killed with heat. That the transforming factor was
genetic in nature was not demonstrated until 1944, when Oswald Avery, Colin MacLeod, and Maclyn
McCarty showed gene transfer in Streptococcus pneumoniae. Avery, Macleod and McCarty call the
uptake and incorporation of DNA by bacteria transformation.
1928 - Frederick Griffith transforms nonpathogenic pneumococcus bacteria into a virulent variety by
mixing them with heat-killed pathogenic bacteria.
1944 - Oswald Avery, Colin MacLeod, and Maclyn McCarty discover that the transforming factor is
pure DNA.
[edit] Mechanisms
[edit] Bacteria
In bacteria, transformation refers to a stable genetic change brought about by taking up naked DNA
(DNA without associated cells or proteins), and competence refers to the state of being able to take
up exogenous DNA from the environment. Two different forms of competence should be
distinguished, natural and artificial.