[thedoc (OP)]

Neil Denham  asked the Naked Scientists:
   
Hi there, really love the new genetics podcast, as I do with all your shows!

My question is...

When I went to the Wellcome Collection recently I saw their copy of the Human Genome printed out, it took up a whole bookshelf. When I opened one of the volumes I noticed that it was made up of the letters A C G and T. Why these letters (and not say A B C and D?).

Also, I know that chromosomes have the letters X and Y, what is the difference, are chromosomes not DNA?

Hope this question is not to stupid, I have lots to learn about the subject of genetics!

Neil Denham

What do you think?

[evan_au]

The Human Genome in its natural environment is stored in a spiraling ladder structure called DNA.
The rungs of the ladder are made up of matching pairs of amino acids:

• Adenine pairs with Thymine (A and T)
• Guanine pairs with Cytosine (G and C)
• Thus A, C, G and T make up the binary code of the genome
• You can duplicate DNA by splitting it in half, and each half can then recreate the other half
• See http://en.wikipedia.org/wiki/DNA#Base_pairing
  

A normal human genome consists of 46 strands of DNA, called chromosomes; these strands come in pairs, with one chromosome coming from your father and the other coming from your mother.

The chromosome pairs were arranged from largest to smallest, and numbered 1 to 22.

The sex-determining chromosomes are unusual -  for example, they don't look like a pair, and they don't exactly act like a pair. The sex chromosomes in humans are called X and Y - women have two X chromosomes, while men have one each of X and Y. http://en.wikipedia.org/wiki/Human_genome

The larger sex chromosome was called X (in mathematics, often used for "the unknown"). The smaller Y chromosome was discovered later, and called by the next letter of the alphabet, according to http://en.wikipedia.org/wiki/X_chromosome

To summarise: the 46 strands of human DNA are each made up of many A C G T base pairs.
Two of these strands are called X and Y.