Which one of the following is the correct sequence of the coding strand of the given gene?
Show Hint
Always look at the promoter/terminator arrow first!
If the arrow points left, transcription goes right-to-left.
The coding strand must have its $5'$ end at the promoter side (right) and $3'$ end at the terminator side (left).
Read that strand from right to left to get the $5' \rightarrow 3'$ answer!
Step 1: Understanding the Question:
This question requires us to identify the correct nucleotide sequence of the coding (non-template) strand of a double-stranded DNA gene in its standard $5' \rightarrow 3'$ orientation. Step 2: Key Formula or Approach:
To solve transcription-related strand problems, recall these fundamental rules:
1. Transcription always starts at the promoter and moves towards the terminator.
2. RNA polymerase synthesizes RNA in the $5' \rightarrow 3'$ direction, meaning it must read the template strand in the $3' \rightarrow 5'$ direction.
3. The coding strand runs parallel to the RNA transcript (in the $5' \rightarrow 3'$ direction of transcription) and has the same sequence as the RNA (with T instead of U). Step 3: Detailed Explanation:
Let's analyze the given diagram:
• Direction of Transcription:
- The promoter is on the right, and the terminator is on the left.
- The "Transcription start site" arrow points to the left.
- Therefore, transcription proceeds from right to left.
• Identifying the Template Strand:
- Since transcription goes right-to-left, the template strand must run $3' \rightarrow 5'$ from right to left.
- Looking at the top strand: it has $3'$ on the right (promoter side) and $5'$ on the left (terminator side).
- Therefore, the top strand ($5'$ GTGAAGCCGTTACAGCAC $3'$) is the template strand, as reading it right-to-left goes from $3'$ to $5'$.
• Identifying the Coding Strand:
- The coding strand must run $5' \rightarrow 3'$ in the direction of transcription (right-to-left).
- The bottom strand has $5'$ on the right and $3'$ on the left.
- Looking at the bottom strand: left-to-right it is written as:
$3'$ CACTTCGGCAATGTCGTG $5'$
- Since the coding strand's $5'$ end is on the right, we must read this strand from right to left to write it in the standard $5' \rightarrow 3'$ sequence format:
Rightmost nucleotide: G (at the $5'$ end)
Next: T
Next: G, and so on.
- Reading the bottom strand from right to left gives:
$5'$ GTGCTGTAACGGCTTCAC $3'$ Step 4: Final Answer:
The sequence of the coding strand in the standard $5' \rightarrow 3'$ direction is $5'$ GTGCTGTAACGGCTTCAC $3'$, which matches option (A).
