Can someone help me out..
#1 Why is this statement false?
If the sequence of the coding strand of a gene is known, then the Amino Acid sequence of its cognate proteins can be determined.
Is it because of Introns?
Yes. The DNA sequence has all the general information for the subsequent synthesis of the protein, however it is missing the critical details. The introns will need to be spliced out and the 5' cap (in eukaryotes) will need to be put on the mRNA molecule transcribed from that piece of DNA. Taking out the introns leaves the necessary and sufficient sequence for correct protein synthesis, while the 5' cap (in eukaryotes) tells the ribosomal machinery where to start looking for the start codon, which provides the correct reading frame.
#2 Suppose the coding (non-template) strand of a DNA molecule has the sequence 5' TCATGTAAGA3'. This sequence could possibly code for
A) The amino terminal end of a protein but not the carboxyl end of a protein.
B) The carboxy end of a protein but not the Amino terminal end
C) either the amino terminal end or the Carboxy terminal end
D) neither the Carboxy nor the Amino terminal end
Please explain
First you have to transcribe this into RNA, which will have the sequence 5'UCAUGUAAGA3'. The directionality is the same and the sequence is the same, not complementary, since the question gives you the coding strand sequence (if they gave you the template strand then the 5' and 3' ends would be reversed and we would need the complementary sequence).
There are a few important things we have to remember about protein synthesis
1. always moves in the 5' to 3' direction along the mRNA molecule
3. the start codon is always AUG and codes for methionine
2. starts from the amino terminal end of the protein
4. terminates with the UAA, UAG or UGA codons
This sequence has both a AUG start or methionine codon and a UAA stop codon. If we assume that this is a chunk that won't be spliced and the sequence will be part of the mature mRNA as is, then it can't be the N-terminal end, because it would mean you start with AUG and immediately stop with UAA. However, it could possibly be the carboxy-terminal end with methionine being the last amino acid added to the polypeptide chain. If we assume that this is just one portion of the larger DNA sequence/RNA molecule before post-transcriptional modification then either one of those codons could possibly be spliced out (although there might be some rules why that can't happen, but that's more detail than I remember from my biochem class). This means that if the AUG is left in the mRNA, it could be the site where protein synthesis would be initiated and would give rise to the N-terminal end of the protein. This assumes that there is either a Shine-Delgarno sequence (if this is a prokaryote) or a methyl-G cap (in eukaryotes) preceding that 5' end they have given. If UAA is left in, then it would be the carboxy terminal end.
Biochem was last year, but this was a good memory exercise and I hope I remember things correctly. Any biochem gurus out there, please correct me if I have made any mistakes.
