A portion of a polypeptide contains the amino acids Leu-Arg-Gln-Glu-Gly. Write t

Question

A portion of a polypeptide contains the amino acids Leu-Arg-Gln-Glu-Gly.

Write the possible mRNA sequences. (Hint: Use A/G and U/C to indicate that either adenine/guanine or uracil/cytosine could occur in a particular position, and use N to indicate that any RNA nucleotide could appear.)


Write the possible DNA template sequences. (Hint: Use A/G and T/C to indicate that either adenine/guanine or thymine/cytosine could occur in a particular position, and use N to indicate that any DNA nucleotide could appear.)

Explanation / Answer

Write the sequence you want to complement on a piece of paper. Turn the paper 180 degrees and write the complementary bases below the ones you just wrote before you turned the paper. In this case, remember to substitue U's for T's since its RNA. knowing that every G comlements with C.. and every T complements with A.. AND that every T is substituted by U in the mRNA version.. original sequence: 5' GATCAC 3' mRNA comlement 3' CUAGUG 5' for a sequencing reaction, researchers use a mix of nucleotides where the majority are normal but a small fraction lack the hydroxyl group. Now, most of the time, the polymerase adds a normal nucleotide, and the reaction continues. But, at a certain probability, a terminator will be put in place, and the reaction stops. If you perform this reaction with lots of identical DNA molecules, you'll wind up with a distribution of lengths that slowly tails off as fewer and fewer unterminated molecules are left. The point at which this tailing off takes place is dictated by the fraction of terminator nucleotides in the reaction mix. Now we just need to know what base is present when the reaction stops. This is possible by making sure that only one of the four nucleotides given to the polymerase can terminate the reaction. If all the C's, T's and G's are normal, but some fraction of the A's are terminators, then that reaction will produce a population of DNA molecules that all end at A. By setting up four reactions, one for each base, it's possible to identify the base at every position. There are only two more secrets to DNA sequencing. First, you need to make sure every polymerase starts copying in the same place, otherwise you'll have a collection of molecules with two randomly located ends. This part is easy, since DNA polymerases can only add nucleotides to an existing strand. So, researchers can "prime" the polymerase by seeding the reaction with a short DNA molecule that base pairs with a known sequences that's next to the one you want to determine. The other trick is that you need to figure out how long each DNA molecule is in the large mix of reaction products that you're left with. The negative charge on phosphates makes this easy, since it ensures that DNA molecules will move when placed in an electric field. So, if you start the reaction mix on one side of an aqueous polymer mesh (called a gel) and run a current through the solution, the DNA will worm its way through the mesh. Shorter molecules move faster, longer ones slower, allowing the population of molecules to be separated based on their sizes. By running the four reactions down neighboring lanes on a gel, you'll get a pattern that looks like the one below, which can be read off to determine the sequence of the DNA molecule.

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