Let's try to decode a protein. Using the genetic code table, What is the mRNA sequence that codes for the peptide MET-ARG-ILE-LYS-ILE-SER?
5' ATG CGG ATA AAA ATA TCC 3' (No, this is a DNA sequence.)
5' AUG CGG AUG AAA AUA UCC 3' (No, this mRNA sequence codes for MET-ARG-MET-LYS-ILE-SER)
5' AUG CGG AUU AAA AUA UCC 3' (That is correct.)
Using the genetic code table you can work backwards to deduce the mRNA sequence if you know the peptide sequence.
Is this the only possible mRNA sequence?
YES (No, more than one codon codes for an amino acid.)
NO (That is correct.)
There are redundancies to the code and there are amino acids that are coded by more than one codon.
In total, how many codons code for the amino acid ARG?
6 (That is correct.)
5 (No, there are more than 5 codons that code for ARG.)
3 (No, there are more than 3 codons that code for ARG.)
2 (No, there are more than 2 codons that code for ARG.)
There are six codons that code for the amino acid ARG.
The other amino acids in the sequence, with the exception of MET, are also coded by more than one codon.
With all these possible codons, there could be many different mRNA sequences that will code for the same peptide. Let's just use one of these possible mRNA sequences:
What happens if there is a mutation in the DNA sequence and the mRNA is now missing the 3rd G?
Peptide sequence stays the same. tRNAs can recognize changes in the mRNA and work around it. (No, tRNAs read and recognize whatever codons are presented. tRNAs cannot recognize mutations.)
The mRNA is translated but the peptide sequence will change. (That is correct.)
There is no translation of this mRNA sequence. (No, unless the first codon is a stop, the mRNA will be translated.)
The mRNA is translated and the peptide sequence doesn't change. (No, in this case, the peptide sequence will change when the mRNA is translated.)
A single base deletion in the mRNA sequence causes a frame shift. The codons after the mutation change and, depending on the change, the peptide sequence also changes.
What does this mRNA sequence code for now?
MET ARG STOP (That is correct.)
MET LYS STOP (No, CGA does not code for LYS.)
MET ARG LYS TYR (No, after a stop codon, no more amino acids are added to the peptide chain.)
The deletion in the mRNA sequence has caused a frame shift and now there is a STOP codon. Translation of the mRNA stops at the stop codon.
What is the peptide sequence if another G (the 2nd G) is deleted from the mRNA sequence?
VAL HIS LYS ASN ILE (That is incorrect, please try again.)
MET HIS LYS ASN ILE (That is correct.)
MET GIN LYS ASN ILE (That is incorrect, try again.)
VAL GIN LYS ASN ILE (That is incorrect, try again.)
If another nucleotide is deleted, there is another frame shift. The peptide sequence changes accordingly. In this case, the new mRNA sequence does not contain a STOP codon.
Let's delete one more nucleotide. What happens now?
The original peptide sequence is made but missing the 2nd amino acid. (This is true, but not the only correct answer.)
There has been another frame shift. (This is true, but not the only correct answer.)
The peptide sequence is MET ILE LYS ILE SER (This is true, but not the only correct answer.)
A total of 3 nucleotides, one codon, has been deleted. (This is true, but not the only correct answer.)
All of the above. (That is correct.)
None of the above. (No, there are correct answers.)
A total of three nucleotides, or one codon, have been deleted from the original mRNA. In this case, ARG is missing from the original peptide sequence.
Deletions in the DNA sequence are mirrored in the mRNA sequence. The peptide sequence changes depending on the "new" codons created by the frame shifted. An altered peptide sequence may cause a visible phenotype of the gene. Insertions of nucleotides can also cause frame shifts.
YOU'RE SO SMART!