Expert: Adam Ross - 10/13/2009

Question
1.why adenine always pair with thymine and guanine with cytosine?
2.what characters are needed for genetic code to act as a start and stop codon?

Answer
1. A pairs with T and C with G because the enzymes that replicate DNA using the leading strand to "read" the sequence, and place the corresponding base in the newly synthesized strand. Also, the molecular structures of A-T and C-G allow them to form proper bonds that are the correct size and orientation withing the double stranded DNA helix.

2.ATG usually initiates (start codon)

There are three stop codons: UAG (in RNA) / TAG (in DNA), UAA / TAA, and UGA / TGA

Hope that helps,
Adam