On Tue, 1 Jun 1999, Bruce Fleming wrote:
> I'd like to know if anyone has encountered a similar case.
> Is it really possible for 24 Codons to be on in contrast to the normal
> which is 20? A person with this characteristic can be said to acquire
> super immunity against all disease including AIDS. Comments welcome.
well, you're asking a question based on false assumptions. in humans, and
as far as we know in all organisms, all 64 possible codons code for
something: in your terminology, all 64 are 'on'.
here's a (very brief) explanation of the primary structure of DNA, just in
case.
DNA is made up of a sequence of nucleotide residues, usually called bases.
there are four different bases which can be used in DNA, which are often
known by their code letters C, T, A and G. these can be incorporated into
DNA in any order, so you could have a DNA molecule ATGGAGTCGACGTAG. in
this respect, a DNA sequence is just like a book, made up of a sequence of
characters, and, like a book, DNA contains information; the information in
DNA specifies how and when to make proteins and some other molecules which
are the building blocks of cells and, thus, of living organisms.
protein, like DNA, are made up of a sequence of building blocks, but where
DNA has bases, proteins have amino acids. where there are 4 bases, there
are 20 amino acids that can be be built into proteins. so, how does a DNA
sequence, with its four building blocks, specify a protein sequence, with
20 possible building blocks? the answer is that the DNA is not read as a
sequence of single bases, but as a sequence of triplets of bases: these
triplets are called codons. this is a bit like splitting a book up into
words, except that in DNA, all the words are 3 letters long and there are
no spaces between them!
thus, when a DNA sequence is translated into protein, each codon is
translated into a particular amino acid in the protein sequence. the rules
for translating are now well-known, and are the same for almost all
organisms. the system also allows a 'stop' signal to be included, so that
when that part of the sequence is reached, translation is stopped and the
protein is finished. proteins are started a bit differently: the start is
marked by a codon for the amino acid methionine.
if there are four bases in DNA, and words contain three bases, then there
are 4 * 4 * 4 = 64 different possible words in DNA. since there are only
20 amino acids which need to be specified, plus a stop signal, this is
excessive.
the obvious thing to do would be to use only 21 of the 64 codons, and have
the other 43 do nothing (that is, be 'null codons'). however, it turns out
that this is a bit of a bad idea, because of mutations. if a mutation
changes one base of a DNA sequence so that a codon which was previously
useful becomes a null codon, and so the information is ruined.
what nature does instead is assign a meaning to every codon, so that if
there is a mutation, the new sequence is at least meaningful, if wrong.
furthermore, things are arranged so that many mutations do not affect the
result of translation: for instance, since GTT, GTC, GTA and GTG all code
for valine, a mutation in the third position of a valine codon will have
no effect.
the rules, known as the genetic code, can be written out as a table, and
you can see such a table at:
http://ftp.hgu.mrc.ac.uk/Softdata/Misc/gencode.htm
note that this table is for RNA rather than DNA, so where a table for DNA
would have a T, this table has a U. looking at this table, the entry "CCG
Pro (P)" shows that the codon CCG (an C followed by a C followed by a G)
is translated into a proline in the protein (pro and P are abbreviations
for proline).
thus, we can translate the sequence i wrote out earlier:
ATGGAGTCGACGTAG
consists of the codons:
ATG GAG TCG ACG TAG
which code for:
met glu ser thr STOP
M E S T
so, the DNA sequence ATGGAGTCGACGTAG codes for the protein
methionine-glutamate-serine-threonine.
so, in response to your question: no. if a person had 24 codons turned on,
they would almost certainly be dead, as they wouldn't be able to translate
their DNA very effectively.
> I saw the following posted elsewhere.
>> >About 10 or 11 years ago in the US, there was a baby born with AIDS.
> >They tested him at birth and at 6 months and he tested positive for
> >AIDS. They tested him a year later and he still tested positive. Then
> they didn't test him again until
> >he was 6, and what was amazing is that this test showed that he was
> >completely AIDS free! In fact, there was no trace that he ever had
> AIDS
> >or HIV whatsoever!
> >
> >He was taken to UCLA to see what was going on and those tests showed
> that
> >he didn't have normal human DNA. In the human DNA we have 4 nucleic
> acids
> >that combine in sets of 3 producing 64 different patterns that are
> called
> >codons. Human DNA all over the world always has 20 of these codons
> turned
> >on and the rest of them are turned off, except for 3 which are the
> stop
> >and start codes, much like a computer. Science always assumed that the
> >ones that were turned off were old programs from our past. I've always
> seen
> >them like application programs in a computer. Anyway...this boy had 24
> codons
> >turned on - 4 more than any other human being. Then they tested this
> kid
> >tosee how strong his immune system was. They took a very lethal dose of
> >AIDS in a petri dish and mixed it with some of his cells and his cells
> >remained completely unaffected. They kept raising the lethalness of the
> >composition
> >- and finally went up to 3,000 times more than what was necessary to
> >infect a human being and his cells stayed completely disease free. Then
> they
> >started testing his blood with other things like cancer and discovered
> >that this kid was immune to everything! Then they found another kid
> with
> these
> >codons turned on - then another one - then another one - then 10,000,
> >then 100,000, then a million of them - and at this point, UCLA, by
> watching
> >world-wide DNA testing, estimates that 1% of the world has this new
> DNA.
> >That breaks down to approximately 60 million people who are not human
> by
> >the old criteria.
what is written here is wrong. it might be that the author was confused,
and that what he really meant was that this child had 24 amino acids
rather than the normal 20. this strikes me as very unlikely: it took the
present system of translation many millions of years to evolve, and it has
remained stable for over a billion years. the jump from 20 amino acids to
24 would require new amino-acids synthesis and degradation pathways, new
transporters, new regulatory networks, new amino-acyl tRNA synthetases
and, last but by no means least, the alteration of almost every other gene
in the genome to fit the new genetic code. in short, it's not going to
happen.
furthermore, it's not clear how adding new amino acids would improve the
immune system. if the genetic code was massively altered, that would give
immunity to viruses, but cancer and parasitic cells like bacteria, fungi,
malaria, etc, should be unaffected.
all in all, it sounds like someone's been reading too much Greg Egan :).
oh, and well done for asking! if you are ever suspicious of something
someone says, don't be afraid to check it out. this is a central principle
of science!
hope this helps,
tom
