Transcription

Where does transcription occur in eukaryotic cells?

In the nucleus, where DNA is contained and protected by the nuclear envelope.

Why does transcription occur instead of DNA leaving the nucleus?

DNA is too large to leave the nucleus, so a shorter RNA copy (mRNA) is made to carry the genetic code to the ribosomes in the cytoplasm.

What is transcription?

The process of copying a section of DNA (a gene) into a complementary strand of RNA.

What enzyme controls the unwinding and unzipping of DNA during transcription?

DNA helicase.

What happens when DNA helicase acts during transcription?

It breaks hydrogen bonds between base pairs, separating the two strands.

Which DNA strand codes for the protein?

The sense strand (5′ to 3′).

Which strand acts as the template during transcription?

The antisense strand (3′ to 5′), which is complementary to the sense strand.

Why does the antisense strand act as the template strand?

So that the complementary mRNA formed has the same base sequence as the sense strand (except U replaces T).

What base replaces thymine in RNA?

Uracil (U) replaces thymine (T) in RNA.

What enzyme catalyses the formation of phosphodiester bonds in the RNA strand?

RNA polymerase.

What happens when RNA polymerase reaches the end of the gene?

Transcription stops, and the mRNA strand detaches from the DNA template.

What is the product of transcription called?

Messenger RNA (mRNA).

How does the base sequence of mRNA compare to the gene sequence?

It is complementary to the antisense strand and identical to the sense strand, except uracil replaces thymine.

Where does the mRNA go after it is formed?

It leaves the nucleus through a nuclear pore and travels to a ribosome in the cytoplasm for translation.

Why is mRNA shorter than DNA?

It only copies the base sequence of a single gene rather than the whole chromosome.

Why is transcription essential for protein synthesis?

It produces mRNA carrying the genetic code needed for translation at the ribosome.