Question

If you knew a particular amino acid sequence could you figure out the dna for that sequence

Answer 1

Yes. Knowing which nucleotide base codon the amino acids is made of, you can "decode" amino acids to the original DNA sequence. 

Answer 2

Yes if we know an amino acid sequence present in an mRNA we can figure out the DNA for that sequence. As the amino acid sequence are triplet codon present on an mRNA strand so the sequence that is complementary of the triplet codon would be present in the DNA.

For example, one amino acid sequence is AUG so the complementary sequence in the DNA for this amino acid sequence would be TAC. So in place of uracil adenine come in DNA.

Therefore by complementary base pairing of the amino acid sequence, we can find out the DNA for that sequence.