Transcribing DNA to RNA is the same principle as transcribing DNA to DNA. There is one key difference however; when transcribing DNA to DNA the A (adenine) in the template pairs to a T (tyrosine) in the copy. In the DNA to RNA trasnscription process changes s bit that whenever we have an A it will now pair to a U (uracel).
For example, in your first DNA base codon (a codon is a three letter code for an amino acid) triplet you have TAC which transcribes to AUG or the amino acid met which is the start codon for transcription to begin. If you look at your last codon you have the triplet ACT which translates to UGA which is the codon for transcription to stop. As you look through the transcription sequence you will see that it continues to be series of codons which each codon translatiing to a specific amino acid, so your DNA sequence to be transcribed should be divisible evenly by three. As this amino acid chain builds then, presto, we have a protein. I assume you have with a translation matrix in order to be able to translate the condons into an amino acid. Remember that the genetic code is redundant in that for the 20 amino acids coded there can be more than one codon. There actualy are 64 different codons for 20 amino acids, this is to protect against mutations.
Now, for your mutations look for a removal or deletion of a specific necleotide, this mutation will shift "the reading frame" by one nucleotide giving you a completely different set of codons and thus a completely different protein, maybe even one that cannot be translated. This is called a frameshift mutation and it looks as if that may be what you have in this case.