Expert: Walter Hintz - 1/22/2009

Question
Hello Mr Hintz,

I have been taking more interest in science lately than I did when I was at school and I wonder if I can ask you something that has been bothering me for a while.  You see I understand that the through the work of the Human Genome Project scientists have been able to map the human genome but what I don't understand is what this means exactly.  Let me explain exactly what confuses me.  From my very rudimentary understanding of DNA I know that the base A only bonds with T and C only bonds with G and that therefore DNA is huge list of such pairs joined in double helix shape.  Now here is my problem, from my basic understanding dogs have a different DNA structure from grass and grass has a different DNA structure to humans but what does this mean exactly?  Does this mean that humans have (for the sake of argument) 10 million such pairs and grass has 1 million such pairs and dogs have 5 million such pairs?  If so this would mean that all the Human Genome Project has been able to do is say: "humans have 10 million base pairs".  If I am wrong about different species having different numbers of base pairs then to my mind the alternative is as follows.  The alternative is that all creatures have the same number of base pairs but in humans the first base pair is (for the sake of argument) A-T, the second is A-T, the third is T-A the fourth is G-C etc and in grass the first is G-C, the second C-G, the third T-A, the fourth A-T etc.  And therefore, what the Human Genome Project was able to do was find the entire sequence for the 10 million base pairs for humans.  However, this does not make sense to me either because this would mean that all humans have the same DNA to each other and all dogs have the same DNA to each other but from crime programs and such I know that one good way to identify a killer is to take a DNA sample.  So, I guess, another way to ask the question is what is the difference between inter-species DNA difference and intra-species DNA difference and also what role do chromosomes and genes play in all this?  I would also like to point out that I am not too sure about the definitions of all the technical terms I've used here.

Many thanks

Sachin

Answer
DNA represents a code made up of of 5 symbols A T G C and U (Found in RNA in place of T. These are symbols for nucleotides.  The secret of the code is not the number of bases but the sequence of the codons whch are made up of RNA.  The sequence of the DNA directs the sequence of the RNA.  There are 64 different combinations of 3 nucleotides making up RNA codons and each codon translates into a specific amino acid and every protein has a different sequence of amino acids.
 Example
DNA strand   ATAGCCGTACTGCCA
RNA STRAND   UAU CGG CAU GAC GGU  ( THESE ARE CODONS)
amino acids  val ser glu asp lys  These amino acids are bonded together as a protein possibly an enzyme
 The sequence of DNA determines the sequence of the RNA which determines the sequence of amino acids.  If we change one nucleotide in the DNA we change the sequence in the RNA and adifferent protein is translated.  All humans and all dogs do not have the same DNA.
The sequence of the DNA nucleotides is different in you snd me. A gene is a packet of DNA in a particular sequence and chromosomes are made up of genes.
 If you do not understand something I have said here let me know