I understand that AUG is the "start codon", and, because of this, most proteins begin with methionine as their first amino acid.
However, this ORF problem on Rosalind.info states that "AUGCUGAC... can be translated as ...AUGCUG..., ...UGCUGA..., and ...GCUGAC". That is, something (the Ribosome? The GTP cap mentioned here? Something else?) must cause the translation to be shifted by one or two nucleotides in the sequence.
My first question is : What mechanism(s) cause(s) this shift in translation to another Open Reading Frame (ORF)?
Specifically, I want to know how the start codon, AUG, can have one ("A") or two ("AU") of its bases ignored in the Ribosome during translation.
My second question is : How often does this happen? That is, what is the percentage of (known) proteins that start with methionine vs. the percentage that start with something else due to this "ignoring" of the bases of the start codon?
I am interested in the answers for both prokaryotes and eukaryotes.