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Name: Lloyd
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: N/A 

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.


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

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

have the feeling your question is 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.


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