Question Video: Converting a DNA Sequence to mRNA Biology

A single strand of DNA undergoing transcription reads 3′-AATCCGATCG-5′. Reading 5′-3′, what will the sequence on the complementary strand of mRNA be? [A] TTCGGATCGA [B] GGAUUCGAUC [C] UUAGGCUAGC [D] AATCCGATCG [E] TTAGGCTAGC

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Video Transcript

A single strand of DNA undergoing transcription reads three prime to five prime AATCCGATCG. Reading five prime to three prime, what will the sequence on the complementary strand of mRNA be? (A) TTCGGATCGA, (B) GGAUUCGAUC, (C) UUAGGCUAGC, (D) AATCCGATCG, or (E) TTAGGCTAGC.

This question is asking us to transcribe a sequence of DNA into mRNA. You’ll recall that when a gene needs to be expressed as a protein, it first needs to be transcribed or copied into mRNA. This process is called transcription. This mRNA transcript can then be converted into a sequence of amino acids in the polypeptide. This is called translation, and once the polypeptide is formed, it can go on to fold into a protein with a specific function. The enzyme that converts DNA into mRNA is called RNA polymerase, which attaches to the DNA double helix as shown here. Once attached, RNA polymerase can unwind the helix and begin copying one of the DNA strands to form an mRNA transcript of the gene.

RNA polymerase moves along the DNA until it reaches the end of the gene and the mRNA transcript is released. Let’s look at this process of transcription in a bit more detail to see how this looks in the DNA sequence. The sequence we’ll use is the sequence in the question. Here you can see the two strands of DNA. You’ll recall that DNA has directionality. So, one strand is in the five prime to three prime direction, while the opposing strand is in the three prime to five prime direction. The sequence in this question is on the three prime to five prime strand. The three prime to five prime strand is actually what’s used as a template during transcription. So, once RNA polymerase binds and unwinds the helix, which is now represented here, RNA polymerase can start adding nucleotides to build the mRNA molecule.

Since the three prime to five prime strand is used as a template, the corresponding mRNA, shown here as this green arrow, will be assembled in the five prime to three prime direction. mRNA is synthesized using the same complementary base-pairing rules as in DNA. In DNA, guanine or G pairs with cytosine by forming hydrogen bonds indicated here as these black dots, and adenine pairs with thymine. There is one exception. In RNA, there is no thymine, and thymine is actually replaced by another nucleotide called uracil or U for short.

Now, let’s start filling in the mRNA sequence by adding the complementary bases. Adenine normally base-pairs with thymine, but since we’re forming mRNA and there is no thymine, uracil is used instead. Thymine in DNA pairs with adenine in mRNA, cytosine in DNA pairs with guanine in mRNA, and guanine pairs with cytosine. Why don’t you pause the video and see if you can work out the rest of the sequence?

Alright, now let’s fill it in. Therefore, the sequence of mRNA read in the five prime to three prime direction is UUAGGCUAGC.

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