Question 3 (8 points) In the dalmatians, there is a single gene that encodes for

Question

Question 3 (8 points)

In the dalmatians, there is a single gene that encodes for the protein that makes the coat pigments for the black fur spots. You are studying how this gene is regulated and isolated the DNA that encodes the gene as well as the processed mRNA molecule transcribed from the gene.

With the isolated DNA and mRNA molecules, you mix them in a test tube to hybridize them to one another, meaning that the DNA and mRNA molecules pair and align with one another based on sequence complementarity. Then, using a high-powered specialized microscope you take a picture of the mRNA paired with the DNA, which is shown below. In the image, the DNA is black and the mRNA is blue.

(A) You notice that there are loops in the DNA strand that are not paired with the mRNA molecule. Based on what you know about the similarities and differences between the DNA sequence and processed mRNA sequence for a given gene, explain what the loops in the DNA strand could be.

(B) Use what you know about transcription and mRNA processing to show on the diagram (label) which side of the mRNA is its 5´ and which is its 3´ end and explain how you can identify which end is which.

(C) A mutant spot gene was isolated that does not make pigment, and thus the animals have no spots. This mutant gene does make mRNA encoding the protein at similar levels as the wild type gene, but the mRNA molecules are slightly longer than usual. You repeat an experiment mixing and hybridizing DNA of the mutant gene with mRNA transcribed from the mutant gene and take a picture of it, shown below.

Based on your answers above and changes in the pattern of pairing between the wildtype DNA/mRNA molecules and the mutant DNA/mRNA molecule, suggest a possible explanation for (1) the nature of the mutation, (2) the effect it would have on the mRNA sequence, and (3) the effect it likely will have on the protein product.

Explanation / Answer

Hi

I could not open the images.

But it is clear that the concept behind the question is transcription and post-transcriptional modifications. mRNA molecules are transcribed using DNA as template by the enzyme RNA polymerase. But still you find that there lies a size difference between DNA template and mRNA transcribed. This is because mRNA does not contain intron sequences. Transcription includes following steps-

DNA template ----------------------> hnRNA (pre-mature RNA)------------------------------> mRNA

hnRNA is heterogeneous nuclear RNA which is exactly similar in length to the DNA template. This hnRNA undergoes a phenomenon known as splicing which is performed by spilsosome. In this step the introns are spliced out and mature RNA is formed. So when you hybridize mRNA and template DNA you will notice the difference in the length, in the form of loops. These loops are nothing but the intron sequences which were removed during splicing.

B) The steps in the formation of mature mRNA include modifications at the 5' and 3' ends of pre-mRNA. These include a 5' cap of 7-methylguanosine and a poly-A tail at 3' end.

C) There must be a splice site mutation, which may be insertions, deletions, point mutation or frameshift mutation, that leads to change in a number of nucleotides in the specific site at which splicing takes place during the processing of precursor messenger RNA into mature messenger RNA. This may cause a presence of introns in mRNA. All these mutations lead to the formation of an abnormal protein that could not perform its normal biological function. Same has occurred in the case described in the question. Introns are not spliced out completely, hence, mRNA molecules observed are longer than usual.

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