The DNA sequence below gives the first 12 base pairs of the transcribed region of a gene, and the template and nontemplate strands of DNA are identified. The transcription start is the thymine nucleotide at the end of the sequence given. Use the diagram to answer the list of questions. Make a copy of the diagram before you begin answering the questions, or have one group member diagram the answers for bacteria and another group member diagram the answers for eukaryotes.
Nontemplate strand ___________ TTGCTACGGTCA___________
Template strand ___________ AACGATGCCAGT___________
Write the polarity of the two DNA strands shown.

Question

Nontemplate strand ___________ TTGCTACGGTCA___________

Template strand ___________ AACGATGCCAGT___________

Write the polarity of the two DNA strands shown.

Accepted Answer

Hi, everyone. Welcome back. Here's our next problem. It says, what is the polarity of the two DNA strands in the following DNA sequence that gives the 1st 14 base pairs of the transcribed region of a gene. And the templates and non template strands of DNA are identified. The transcription start is the thymine nucleotide at the end of the sequence given. And we're given a non template strand that starts and end with thymine and a template strand that starts and ends with adenine. Our answer choices are choice. A non template strand is five prime to three prime and the template strand is three prime to five prime choice B nonn template strand is three prime to five prime and the template strand is five prime to three prime. Choice C, both strands are five prime to three prime or choice D, both strands are three prime to five prime. Well, even before we think this whole problem through, we can eliminate choices C and D because they have both strands being written in the same direction. But we know that our two strands must be complimentary to each other. And when we look at the way they're written starting from the left. They are complimentary. We have t in the non template strand with a, in the template strand AC G in the non template T G C in the template and so forth. Since they are properly complementary to each other, they must be written in the standard format of opposite directions for the two strands. So choice C and D can be eliminated because if they were written both five prime to three prime or both three prime to five prime, you wouldn't see complementary base pairs. So now we just have to figure out which is written and which orientation. So to do that, we want to think about what is the direction of transcription. Well, we can recall that R N A polymerase synthesizes synthesizes R N A in the five prime to three prime direction. Well, since it's synthesizing it from working from a DNA template, that means that it moves along the DNA template in the three prime to five prime direction, since we know our new R N A molecule is being synthesized complementary to the DNA template strand. So with that in mind, let's go back to our question, which tells us the transcription start is the thymine nucleotide at the end of the sequence given. So our end, since we usually read left to right would be the right side. However, here's where it gets a little complicated the strand that has a thymine nucleotide at the end is the non template strand. So we just need to think it through a little bit more. So if transcription starts at that timing, at the end of the nonn template strand, it must be starting at the A nucleotide on the right side of the template strand. And since we know our template strand, the R N A polymerase is moving along in a three prime to five prime direction, the start of transcription must represent the three prime end of the template strand of DNA. Therefore, the left side must be the five prime end. And since our non template strand is complementary, we know that the non template strand must be written in the opposite direction three prime on the left five prime on the right. So let's match that to our answer choice. And that would be choice B, the non template strand is three prime to five prime and the template strand is five prime to three prime. So that choice B is our correct answer. And we'll see you in the next video.

Verified video answer for a similar problem:

Key Concepts

DNA Strands and Polarity

Transcription Process

Base Pairing Rules