November 29, 2017 Wednesday
Bedtime Story
CRISPR/Cas Immune System
I will continue with the abstract of Horvath’s
2007 paper:
“Clustered regularly interspaced short
palindromic repeats (CRISPR) are a distinct feature of the genomes of most
Bacteria and Archaea and are thought to be involved in resistance to
bacteriophages.
We found that, after viral challenge,
bacteria integrated new spacers derived from phage genomic sequences.
Removal or additional of particular spacers
modified the phage-resistance phenotype of the cell.
Thus, CRISPR, together with the associated
cas genes, provided resistance against phages, and resistance specificity is
determined by spacer-phage sequence similarity.”
This was a top quality genetic twiddling
exercise that proved beyond doubt that the strain of bacteria that had become
resistant to phage infections (remember that to begin with they were all
susceptible) did so because of the acquired phage-derived sequences into their
spacers.
The CRISPR story does not end here but I
shall for now stop here because I think this much is enough to make the point
that I wish to put forward.
We shall briefly summarize the working of
the CRISPR/Cas immune system and see how closely it resembles digital
technology and software programming.
Any bacteria or archaea with a CRISPR
system (so far CRISPRs have been found in 50% of the sequenced bacterial
genomes and in 90% of the sequenced archaea) represents a genetic code that was
either itself or its ancestor once attacked by a foreign genetic code.
Moreover, not only the attacked genetic
code managed to survive and fend off the attack but also managed to snip in a
little bit of that alien parasitic code (viral DNA) and add it into its own
genome.
It along with its progenies are then far
better prepared by any future attacks by the same virus.
The Archaea or the bacteria would match the
sequence of the attacking virus with the template in its CRISPR/Cas spacers.
If the sequence happens to be a match, the
system generates or transcribes CRISPR-RNA or crRNA.
This crRNA associates with Cas proteins to
form ribonucleotide complex that can recognize foreign nucleic acids.
Cas proteins are RNA-guided DNA
endonuclease enzyme that memorizes, and then later interrogates and if found
alien, then cleaves the foreign DNA.
The Cas9 protein can cleave any sequence of
invading DNA that is complimentary to the guide RNA for after all it is an endonuclease.
All this is so fantastically close to the
way the digital technology works that one can even write an algorithm for
CRISPR immunity which would read something like this.
We shall consider the algorithm in the
nights to come.
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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