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Name: Robert S.
Status: Educator
Age: Old
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Question:
The DNA code consists of A,C,G.T, four letters. this allows 24 permutations (4 items taken 3 at a time). Why are the references the number of amino acids limited to 20, rather than 24? Also, how long is a "typical" double helix string?



Replies:
Robert,

In nature there are many more variations amino acids than the simple 20 found in humans. However, when analyzing the human genome sequence, there is a code for all 64 permutations (4^3), only some of them share amino acids. This is a safe-guard against mutations of one or two nucleotides. For example, the amino acid Alanine is coded with four different nucleotide sequences: GCA, GCC, GCG, GCU.

I am not exactly sure how long one strand of double helix is, but I can tell you that it is very long!

Saundra Sample


Actually when you take 3 letters at a time, it allows for 64 combinations (4 to the 3rd power). There are only 20 different kinds of amino acids found in the proteins of living organisms but most amino acids have more than one 3 letter code. There are a few that have only one, and a few that have as many as 6.

Most have at least two. This allows for a change in the letters (a mutation) without changing the amino acid order and therefore the nature of the protein. In mammals, our DNA "strings" are arranged in a linear fashion called chromosomes, and each is a different length. But the estimate is probably in the millions of "letters" per chromosome.

vanhoeck


Your math is off...it allows for 64 permutations....4x4x4....4 letters in triplets. The code has synonyms. That is some triplets, such as UUU and UUC code for the same amino acid.

Peter Faletra Ph.D.
Office of Science
Department of Energy


Dear Robert:

The problem is even worse than you imagined. Each amino acid is specified by a codon, which is composed of three nucleotides. Since each position of the codon can be occupied by any one of the four bases (A,C,G,T), the number of potential codons is 4x4x4 = 64.

The answer to "why only 20 amino acids?" can be viewed as a manifestation of the general problem of how many letters there should be in an alphabet. Too few don't allow for enough complexity in the words that you can make, and too many are unwieldy to use. The astonishing diversity of life certainly testifies that a lot can be done with 20 of them. (In all fairness, some amino acids can be modified after they are incorporated into proteins. These changes aren't dictated by the sequence of the gene encoding the protein, but they may be very important for the function of the protein within the cell.)

So, how does a cell deal with all these different codons? Some amino acids are encoded by only one codon, others by as many as six different codons. Some of the transfer RNAs (tRNA), which are the devices that read the DNA codons one at a time, are not very choosy about which nucleotide is present in the third position of the codon. This seems to be a compromise between the need for precision and the need for speed and efficiency. All together, 61 of the 64 codons do encode amino acids. The other three are "stop codons" -- when the protein-synthesizing machinery comes to one of these, it terminates the growing chain of amino acids at that point.

The human genome, which is made up of about 3.2 billion base pairs of DNA, is divided up into 23 chromosomes. (Remember that most of our cells contain two such sets of chromosomes, one inherited from the father, the other from the mother.) The size of the DNA string in an average chromosome is therefore somewhere in the neighborhood of 150 million base pairs. For more information on this, I suggest that you visit the highly readable website for the Human Genome Project, which is located at

http://www.ornl.gov/hgmis/project/info.html

Tom Douglas



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