Transcription, Step by Step

Why cells need transcription

DNA is the long-term storage molecule, but cells cannot use most genes by reading directly from DNA every time they need a product. Transcription solves that problem by creating an RNA version of the needed instructions.

This makes transcription the first major step in gene expression.

Initiation: the start signal

Transcription begins when RNA polymerase binds to a region of DNA called a promoter.

The promoter is not part of the message that will be translated later. Instead, it is a control region that helps position the enzyme at the correct starting place.

Once the polymerase is in position, the DNA opens locally so one strand can be used as the template.

Only one strand is read

This is a common sticking point for beginners. DNA is double-stranded, but during transcription, one strand is used as the template for that transcript.

The other strand is still there, but it is not the one being copied in that event.

Elongation: building the RNA

During elongation, RNA polymerase moves along the DNA template and links RNA nucleotides together.

The base-pairing logic is similar to what students already know from DNA, but with one important difference:

• A on the DNA template pairs with U in RNA
• C pairs with G

Because RNA uses uracil instead of thymine, the output strand is chemically RNA, not DNA.

Termination: the stop signal

Transcription ends when RNA polymerase reaches a termination sequence.

At that point, the RNA transcript is released, the local DNA double helix reforms, and the enzyme detaches or moves on depending on the system.

The cell now has an RNA copy of the relevant instructions.

What happens in eukaryotes after transcription

In eukaryotic cells, the first RNA product is often not the final usable messenger RNA.

It may need processing, including:

• removal of introns
• joining of exons
• additional chemical modifications

That means "transcription is finished" does not always mean "the final mRNA is ready."

The idea to hold onto

Transcription is easiest to understand as controlled copying.

The cell does not copy its entire genome whenever it needs something. It copies selected regions, at selected times, into RNA forms that the rest of the cell can use.