The Hunt for DNA
With Mendel’s chapter gone (see first part...), the hunt for what caused heredity ended. But, the end was not an end, it was a beginning of hunt for what the gene actually is.
In the time-period from publication of Mendel’s work to
complete acceptance of his work, the biology had advanced manifold and that caused
the seekers to dare to get into the field with a search for actual (chemical)
nature and structure of gene.
Those advancements included betterment of microscope, clear study cells, new discoveries like sex-linked traits by T.H. Morgan, linkage by Bateson and Punnett and advent
By now, the search for the actual structure and nature of
genetic material was concentrated to a cellular organelle called nucleus or
more specifically to chromatin or chromosome. But their nature and structure (chemical)
were unknown at that time.
In 1928 a British-Medical officer conducted a series of experiments with pneumonia causing bacteria (Streptococcus pneumoniae) and some mouse by infecting and sometime killing them and those death
3 years after Griffith died, in 1944 his “transforming
principles” were finally chemically identified by Oswald Avery, Colin MacLoed,
Maclyn McCarty as DNA- a molecule identified long ago (contemporary of Mendel) in 1969 by
Friedrich Miescher while he was searching the origin of WBCs from pus-coated
bandages. Miescher named this ‘mysterious’ substance ‘nuclein’, and he
unknowingly had discovered the chemical basis of all life – DNA.
About after another decade in 1952, Alfred Hershey and
Martha Chase experimented with radiolabelled bacteriophage and E. coli and
established strongly that DNA is the genetic material.
At this point chemical nature of genetic material is
unveiled, but one thing was still left- it’s chemical structure.
During the dormancy period of work of Mendel, there was some
significant discoveries that includes – 1.Discovery of 5 nitrogenous bases
namely adenine, guanine, cytosine, thymine and uracil from nucleic acid (nuclein
of Miescher) as its components by Albercht Kossel which were later classified
into purines and pyrimidines, and also changed nuclein into DNA 2. Discovery of
sugar, phosphate and mentioned bases as major components of a single nucleotide
(a monomer of the polyneucleotide called DNA or RNA), presence of deoxyribose
sugar as carbohydrate in DNA and presence of ribose sugar as carbohydrate in
RNA and many more all by one person named Phoebus Levene, he also proposed the
most accurate model of DNA of that time and for next some decades with the
double helical model still a half century away. And some experimental results
like after Mendel was done like- 1. DNA of any species is constituted of purine
and pyrimidine in 1 : 1 ratio (i.e. (A+T)=(C+G))- known as “Chargaff’s rule”
honoring it’s creator Erwin Chargaff (late 1940s). 2. Double helical form of
DNA as captured in X-ray diffraction photographs of DNA by Rosalind Franklin in
1951.
Those significant discoveries and experimental evidences along with chemical and physical studies of DNA paved the most famous article of ‘Nature’ ever, titled "Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid" by Francis Crick and Jemes Dewey Watson in 25th of April, 1953.
With the structure and nature of the DNA revealed it was now
the time to have its manifestation pave the future.
