Question Video: Identifying the DNA Template Which Turns into a Provided mRNA Sequence | Nagwa Question Video: Identifying the DNA Template Which Turns into a Provided mRNA Sequence | Nagwa

Question Video: Identifying the DNA Template Which Turns into a Provided mRNA Sequence Biology • Third Year of Secondary School

An mRNA sequence is provided below: 5′-AUG GGG CCC UAG GAC UAG-3′. What is the sequence of the template DNA strand that was used to create the mRNA strand?

04:52

Video Transcript

An mRNA sequence is provided below: five prime AUG GGG CCC UAG GAC UAG three prime. What is the sequence of the template DNA strand that was used to create the mRNA strand? (A) Three prime TAC CCC GGG ATC CTG ATC five prime. (B) Five prime TAC CCC GGG ATC CTG ATC three prime. (C) Five prime AUG GGG CCC UAG GAC UAG three prime. Or (D) three prime AUG GGG CCC UAG GAC UAG five prime.

DNA is a double-stranded helix, which is made up of a combination of four nucleotides: adenine, or A for short; cytosine, C; guanine, G; and thymine, T. Nucleotides on one strand can base-pair with nucleotides on the opposing strand of DNA in a very specific way. Adenine can base-pair or form hydrogen bonds with its complementary base thymine. And guanine can base-pair with cytosine. You can remember which nucleotide can bind with which by looking at the shape of the beginning letter. The nucleotides with the rounded letters G and C bind together. And the nucleotides with the letters with edges A and T can bind together.

Before getting into translation, let′s unwind a short piece of DNA to have a closer look at it. Note that DNA has directionality. And its upper sequence is written in the five prime to three prime direction, while the lower sequence is written in the three prime to five prime direction. During translation, RNA polymerase will attach to the DNA strand and unwind the DNA helix. Once the two strands are separated, RNA polymerase can use one of the strands as a template to start forming the mRNA transcript. The three prime to five prime strand is used as a template. And bases are added following complementary base-pairing rules.

It is important to know that RNA is made up of nucleotides but does not contain thymine but uses the nucleotide uracil instead. So, while in DNA normally thymine would base-pair with adenine, in RNA adenine pairs with uracil. So G pairs with C, T pairs with A, and A pairs with U. To make things clearer, let′s create an RNA sequence pairing with our example. RNA polymerase would aid to build the following RNA strand: GUC AAG. This is because G binds with C, U binds with A, C binds with G, and A binds with T. To distinguish DNA from RNA, the RNA backbone in this image is represented in blue.

Note that the produced mRNA strand is again directional. And as the three prime to five prime DNA strand was its template, its direction is five prime to three prime. With this information, we can now find out what the sequence of the template DNA strand was that was used to create the given mRNA strand.

The given mRNA strand starts at five prime, so the DNA strand has to start at the three prime end. This information allows us to exclude the two answer options (B) and (C). Because we know that thymine is found in DNA strands and uracil is found in RNA strands, we can exclude answer option (D) as well. This would indicate that the correct answer option is option (A). However, with what we learned in this video, we can check, so let′s do this.

Remember, we backtrack an RNA strand to a DNA strand. So the adenine in the RNA strand binds to a thymine on a DNA strand, not to uracil. The uracil on a RNA strand couples with an adenine on a DNA strand. Guanine and cytosine couple with each other no matter if we deal with RNA or DNA strands.

By filling in the remaining nucleotides, we can confirm that the correct answer is given by answer option (A): three prime TAC CCC GGG ATC CTG ATC five prime.

Join Nagwa Classes

Attend live sessions on Nagwa Classes to boost your learning with guidance and advice from an expert teacher!

  • Interactive Sessions
  • Chat & Messaging
  • Realistic Exam Questions

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy