Ask A Scientist© Molecular Biology Archive

name       Lloyd
status     student
age        20s

Question - My questions are: Of the DNA codes
ATGC, which comes first, the TA, AT,
or the GC, CG? What is the correct order?


Dear Loyd,
Suppose a string of beads, could you tell which bead
came first? The nucleotides which are abbreviated with
A, G, T, and C form a string that goes on and on, in
every possible order. So it is impossible to say what
is the correct order.

The string that nucleotides form is more complicated
than the example of beads, because actually there are
two strings that lay antiparalel and intertwined.
Every nucleotide pairs with another, and you've got it
right: A pairs with T, and G pairs with C (and,
ofcourse, T with A, and C with G). Every pair is kept
together by weak physical forces, and the two strands
that are thus closely together are spiralled due to
the specific shape of the nucleotides.

In the recently published 'book of life', the human
genome, there would be millions of these four letters
in every possible combination. But in fact, the book
would be double the size if we would write down both
strands, giving all nucleotides present in the
chromosome. For convinience, only one strand is given
and there is no convention which of the two.

I can't answer the question 'what nucleotide comes
first'. We don't know which one evolved first, though
we think that RNA existed before DNA (RNA consists of
similar nucleotides, but now the codes are A, G, C and
U). Scientists usually use the alphabetic order ACGT
for arbitrary reasons, or the order GATC which is
palindromic: if you read the nucleotides on the
opposite strand that pair with these four, you read
GATC again because the strands pair antiparallel. The
same would be true for AGCT, and several other
combinations.

In conclusion, there is no answer to your question but
I hope I gave some clarity on the subject.

Trudy
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A, C, G, and T are abbreviations for the 4 building blocks (bases) that
make up DNA. There isn't one particular order that's correct, because the
DNA in a single human cell contains 1,000,000,000 or so of these blocks.
 From your question, you seem to understand that these blocks come in pairs
of A:T (or T:A) and C:G (or G:C), and you're correct on that. Those pairs
appear in every possible order somewhere in the genome -- all the DNA that
makes up an organism.
---------
Christine Ticknor
Ph.D. Student
Yale University
New Haven, Connecticut
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That's like saying what's the first letter in a word or sentence. What word?
  What sentence? That's the definition of DNA. It is made up of A, T, C, G
but its the combination and length of the message that makes each stretch of
DNA different from another. Since DNA is double stranded and since the
nucleotides pair specifically, once you know the order of one strand of DNA,
you automatically know the sequence of the other strand. So to answer your
question, the order depends on the section of DNA. If one letter is A the
opposite letter will be T. If the letter is T the opposite letter will be A.

Van Hoeck
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 have the feeling your question is wrong....in that none "come first".
A,T,C,&G are each abbreviations for the four base nucleotide found in DNA.
Depending on their order they will in series of three's often code for a
certain amino acid.

pf
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