February 25, 2019 Monday
Bedtime Story
"The Origin and Behavior of Multiple Loci in Maze" 1950 PNAS
Barbara McClintock, after a series of
careful micro analysis, came to the conclusion that there were two dominant and
interacting genetic loci on the 9th chromosome that were interacting
and manifesting phenotypically in a very strange manner.
When there was no Activator or Ac element
present in the chromosome then the Ds inhibits the synthesis of anthocyanins
and the seed is colorless.
When one copy or the allele of the Ac is
present then the suppression effect of the Ds is taken off from the
aleurone-color gene which is then able to produce pigments.
What she further observed was that the
presence of Ac gene did not generate a fixed color pattern on the maize seeds
but a very variable one.
Her genetic analysis of these variable
colored or mosaic patterned seeds shows that the genetic element of Ds was
mobile on the chromosome 9.
Not only was the Ds gene mobile on the
chromosome but its mobility was controlled at least to some extent by the Ac
gene.
She found that the movement or the
transposition (this is the formal genetic term for such mobile genes) of the
element Ds depended on the number of elements or copies of Ac gene present.
The different locations of Ds gene that in
turn depended on the number of copies of Ac gene resulted in the mosaic color
pattern of maize seeds.
She further generalized the basic idea from
this experiment and extrapolated to a much larger scale, in fact, to the entire
biology.
She proposed that this genetic
transposition and its control was a general method of gene regulation through
transposition of genetic elements that allowed multicellular organisms having
cells of identical genomes to function differently.
This idea challenged the established
orthodoxy of a genome being a static set of instructions that was passed from
one generation to other.
She published this study in 1950 in PNAS in
the paper titled “The origin and behavior of multiple loci in maize”.
I shall quote to you the just the first
paragraph of the entire paper as it gives a general idea what she was implying.
I also find it interesting to read through
the style of writing of the original author as that speaks something about the
personality.
It is of course impossible to read all the
original papers of all the scientists that we discuss in these bedtime stories
but whenever time permits it is worth it as that gives you a more realistic
idea how scientific knowledge gradually accrues over time through the tiny
contributions of unknown and lost apes.
“In the course of an experiment designed to
reveal the genetic composition of the short arm of chromosome 9, a phenomenon
of rare occurrence (or recognition) in maize began to appear with remarkably
high frequencies in the cultures.”
Stay tuned to the voice of an
average story storytelling chimpanzee or login at http://panarrans.blogspot.com
Good night Mon Ami and my fellow cousin ape.
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Another great educator and a teacher that I am aware of is
Professor Subhashish Chattopadhyay in Bangalore, India.
While I narrate stories, Professor Subhashish an electronic
engineer and a former professor at BARC, does and teaches real mathematics and
physics.
He started the participation of Indian students at the
International Physics Olympiad.
Do visit him here:
All his books can be downloaded for free through this link:
For edutainment and English education of your children, I
recommend this large collection of Halloween Songs for Kids:
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