the following snippet of mRNA is translated, what sequence of amino acids would be produced? (Hint: Use a codon table to help find the answer.) mRNA: 5′-GCGACU-3′ a. Ala-Thr b. Ser-Ala c. Arg d. Ser-Arg 2. The figure shown below is part of a gene, including the promoter and transcription start site. Which strand of DNA is the template strand, and what will be the first base in the RNA molecule that is produced when this gene is transcribed? 5’C G3 promoter sequence URUGEREKELURUHOUKUHOCHKKUUU OHUSHCHERHEUCHUCUUHULUHCHHUUU transcription start site 3C G5 Use the dropdowns below to complete your answer. The template strand is on the and the first RNA base will be Reset 3. Suppose you inject a molecule of mRNA consisting only of several hundred “A” (adenosine) nucleotides into a cell. Would this mRNA be translated? Why or why not? a. No, because translating it would not produce a useful polypeptide. b. No, because it lacks a ribosome binding site and AUG start codon. c. Yes, because cells have many molecules of tRNA with UUU anticodons. d. Yes, because each of the AAA codons can be translated to an amino acid.
The Correct Answer and Explanation is:
Let’s address each of the questions in your prompt:
1. What amino acid sequence is produced from mRNA 5′-GCGACU-3′?
To translate this mRNA sequence, we divide it into codons (triplets):
- GCG
- ACU
Using the codon table:
- GCG codes for Alanine (Ala)
- ACU codes for Threonine (Thr)
Correct answer: a. Ala-Thr
2. Which strand is the template strand, and what is the first base in the RNA transcript?
To determine the template strand and the first RNA base:
- Transcription always uses the template strand, which is complementary to the RNA.
- The RNA is synthesized 5′ to 3′, using the DNA strand that runs 3′ to 5′ as the template.
- The first RNA base is complementary to the first DNA base after the promoter.
Assuming the sequence is:
- Top strand (5′ to 3′): 5′-C G…
- Bottom strand (3′ to 5′): 3′-G C…
If the top strand is 5′ to 3′, then the bottom strand (3′ to 5′) is the template strand.
Thus:
- Template strand: Bottom
- First RNA base: C (because it’s complementary to the G on the template strand)
Answer: The template strand is on the bottom and the first RNA base will be C.
3. Would an mRNA strand made only of adenosines be translated?
Correct answer: a. No, because translating it would not produce a useful polypeptide.
Explanation:
mRNA translation into proteins requires specific sequences to guide the ribosome. A viable mRNA must contain:
- A start codon (usually AUG, which codes for methionine).
- A coding sequence made of codons that correspond to amino acids.
- A stop codon to terminate translation.
An mRNA made only of “A” nucleotides would consist of repeating AAA codons. While AAA does code for the amino acid lysine, the mRNA lacks an AUG start codon, which is essential for initiating translation. Without this start signal, the ribosome cannot begin translating the mRNA.
Even if some translation occurred by chance, the polypeptide would consist only of lysine residues, which would not fold into a functional protein. Proteins need a specific sequence of various amino acids to achieve a three-dimensional structure necessary for biological activity.
Additionally, such an unnatural mRNA is unlikely to be processed correctly in the cell, as it would lack important regulatory elements like a 5′ cap and 3′ poly-A tail (which ironically consists of adenosines but serves a different function).
Therefore, injecting mRNA made entirely of “A”s would not lead to meaningful protein production.