Codons and Amino Acids, tricky question?

Question

Let's go over some basics.
DNA is made of base pairs, AT and GC pair up.
RNA forms and is fed into ribosomes, every three bases form a codon which in kind forms one of the 20 or acids that in turn form complex proteins.
I think that DNA splits into two RNA molecules and RNA gets fed into ribosomes.

Here is where things get confusing to me.  If the codon is AAA then at one point it the DNA molecule had TTT on the other side.   However AAA and TTT make different amino acids.  So both sides of the DNA molecule code for completely different amino acids and proteins, and yet the order of their bases are fundamentally linked and if you change one side you change the other.  That seems like a heck of a lot of dependency, alther one base pair with a mutation and you screw up two proteins. 

 In fact it seems like too much dependency.  So is what I said correct or did I make a mistake?  I may have made multiple mistakes and if so please try to provide corrections that help me understand the dependency problem.

Rob · Accepted Answer

You have made a mistake I am afraid.

DNA is split by DNA helicase into the two strands as you say. However, mRNA (the RNA that goes to the ribosome) is made from one strand of the DNA only, known as the template strand. The template strand is worked out by the enzyme that makes mRNA, RNA polymerase, by what is known as the 5 prime 3 prime orientation.

DNA is made up of nucleosides - the base bound to a phosphate and deoxyribose sugar. This molecule has two distinct ends - the 5' (five prime) and 3' end. 5' of one nucleoside will bind to the 3' of the next nucleoside. The RNA polymerase reads the DNA in the 5' to 3' direction - it is biochemically constrained to read it this way - it cannot read it in the 3' to 5' direction. This then gets around the problem as you said of reading AAA or TTT.

If we use a different codon as an example

A A G 
T T C

A ---> G is 5' to 3'. and so we could assume this is the template stand.

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Codons and Amino Acids, tricky question?

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