Molecular Genetics

Hi, I was hoping to get help with this. We just started this chapter and was given a practice problem. I was hoping you could walk me through it so i can understand...THANKS

Using the following amino acid sequence, provide the tRNA molecules (anitcodons) that were used for its translation and the anti-sense strand of the DNA for this gene. Include polarity of all items

COOH-methionine-proline-isoleucine-aspartate-glycine-glutamine-methinone-leucine-proline-valine-arganine-NH2

First of all, this amino acid sequence is written backwards. Proteins/peptides are synthesized from N-terminal to C-terminal. So, what you need to do is look up genetic code and write a sequence of the mRNA: 5’-CGU-GUU-CCU-etc 3’. (Arg-Val-Pro-etc.). Note that you will have 2-4 codons coding for the same amino acid (I just chose the first one of the bunch...).

Now, you convert mRNA sequence into coding strand (or sense strand) of DNA by changing U to T in the mRNA sequence: 5’-CGT-GTT-CCT-etc 3’. Once you done that, you can write the sequence of the complementary non-coding (or anti-sense) strand of DNA: 3’-GCA-CAA-GGA-etc 5’. Then you will just need to rewrite that sequence properly 5’-3’: 5’...-AGG-AAC-ACG-3’.

For the anti-codons of tRNA you simply need to take each codon of mRNA and write a complement:
For translation of 5’-CGU-GUU-CCU-etc 3’ you will need tRNAs with anticodons GCA, CAA, and GGA. Note that since we are dealing with tRNA anticodons will contain U instead of T.

I am not sure what “polarity of all items” refers to. Does your texbook talk about polarity? Let me know.

Good luck.

Thank you so much. Its amazing how much I learn from your little post when I sit through 1 1/2 hrs, of lecture and walk out not knowing anymore than when I walked in.

I couldn't find anything in my book (for this chapter) about polarity. Its only mentioned when talking about chemistry of water. The hydrogen bonds and so forth. But, I do "kinda" remember something about it from his lecture. He was talking about the sequencing from 5-3. Which I am totally confused on because it explains it in my book in "1" sentence. So, could you explain it to me? Does that explain polarity. How their ends are opposite??

Also another question I had.... how do I know whether or not the sequence is written back wards?
THANKS a million for all your help!!

wannabnurse

You are welcome. I am glad to help.

First, polarity refers to partial charges of a molecule that is usually uncharged (like water, where O has partial -ve charge and H has partial +ve charge). Each base pair of DNA has partial charges that play a role in base pairing (look up schematic of A-T and G-C pairs). Complementary bases or codons have geometrically opposite partial charges that complement each other and form hydrogen bonds. I checked my books on the subject and they talk about opposite polarity of 2 DNA strands and tRNA being anti-polar to RNA template, but that does not really help answering question in your problem.

Now, your sequence was given as:

"COOH-methionine-proline-isoleucine-aspartate-glycine-glutamine-methinone-leucine-proline-valine-arganine-NH2"

While met is usually 1st amino acid transcribed in the peptide/protein, it is the last one in this case because it is the C-terminal amino acid (as indicated by COOH written prior to it). Arg, on the other hand, is the 1st amino acid because of NH2 written after it, making it N-terminus amino acid.
All proteins/peptides always made from N-terminus to C-terminus. It can be written as H2N-CHR-COO-(NH-CHR')n-COOH. Similar thing happens with DNA and mRNA. They are always synthesized from 5' to 3' of the backbone (look up chemical formula).