I am reviewing an example on the basics of the genetic code; this example is listed at the bottom of the following webpage: https://www.atdbio.com/content/14/Transcription-Translation-and-Replication.
I have produced the example below and added Roman numbers to better indicate the parts that I am questioning (V and VII).
One strand of genomic DNA (strand A, coding strand) contains the following sequence reading from 5′- to 3′-:
This strand will form the following duplex:
The sequence of bases in the other strand of DNA (strand B) written 5′- to 3′- is therefore
The sequence of bases in the mRNA transcribed from strand A of DNA written 5′- to 3′- is
The amino acid sequence coded by the above mRNA is
However, if DNA strand B is the coding strand the mRNA sequence will be:
and the amino-acid sequence will be:
(Well, I suppose Chargaff's rules apply for DNA.)
The Attempt at a Solution
I understand that Strand A (5' -> 3') is this:
However, if the above is (or designated) the coding strand, then why is the mRNA sequence (following transcription) not this (see below)?
5'-UCGUCGACGAUGAUCAUCGGCUACUCGA-3' (Thought One)
I thought that the defining trait of the DNA coding strand is that its base sequence is almost the same as the resulting mRNA sequence, with the exception that uracil (U) substitutes for thymine (T).
I have the same question for Strand B (see IV):
Why is the mRNA sequence of Strand B, if it is coding, not:
5'-UCGAGUAGCCGAUGAUCAUCGUCGACGA-3' (Thought 2)?
(Although Strand B is the template strand in III, it clearly states later to assume that it is coding.)
As you may notice, my Thoughts 1 and 2 (for Strands A and B, respectively) are the switched answers for V and VII. That is, what I thought was the correct mRNA sequence corresponding to Strand A (see Thought 1) is actually the listed sequence that corresponds to Strand B (see VII).
I really appreciate your help on where I went wrong.