Perfect!Before mRNA leaves nucleus, the DNA should have replicated and an mRNA strand should have synthesized, i.e. transcription. However, the DNA being replicated is not part of the question because the very first statement says that the events being discussed are from transcription. I made the mistake of ignoring this part (thanks for reminding me with this quesiton btw, tomorrow we have to read the stem of the questions VERY carefully, each word is relevant). First make a mental note of transcription:
- DNA breaks into two strands by bonds between complementary bases breaking, 1 occurs the first time.
- One strand acts as template strand and free RNA nucleotides pair with this template strand's complementary nucleotides, 4 occurs the first time.
- Bonds form between complementary bases, the free RNA nucleotides and the complementary nucleotides to hold the RNA nucleotides in place, 2 occurs the first time.
- RNA polymerase I believe then joins up the RNA nucleotide backbone by forming sugar-phosphate bonds while they are held in place by the previous step, thus, 3 occurs the first time.
- Once this RNA strans is complete (the mRNA has formed), complementary bonds between its nucleotides and the template DNA's nucleotides break, 2 occurs for the second time
- The DNA forms bonds with the complementary bases of the other strand which was broken off in the very first step to make the DNA molecule again, 1 occurs for the second time.
- DNA has been formed again, mRNA has formed, and thus, mRNA is ready to leave and it does so.
So only 1 and 2 occur twice. Hope I helped.