Question

A gene for microtubule proteins is being made in a cell.

How can
mRNA
represent an exact copy of that gene?


A section is cut out of
DNA
and is inserted into the
mRNA
.

It is put together using base-pairing rules.

Helicase splits the
DNA
into two
mRNA
strands.

Answer 1

Correct answer: "It is put together using base-pairing rules."

Why: During transcription an RNA polymerase reads one DNA strand (the template strand) and synthesizes an mRNA strand by adding ribonucleotides that are complementary to the DNA bases (A pairs with U in RNA, T with A, G with C, C with G). The mRNA therefore carries the same information as the gene (it is complementary to the template strand and matches the coding strand except that U replaces T).

Why the others are wrong:
- A section is not cut out of DNA and inserted into mRNA. RNA is synthesized de novo from nucleotides; in eukaryotes introns may be removed from the pre-mRNA, but DNA is not cut out and pasted into the RNA.
- Helicase does not split DNA into mRNA strands. Helicase unwinds DNA during replication; transcription is carried out by RNA polymerase which locally opens the DNA and builds an RNA copy.