Elongation

Start of elongation

After initiation

1. the initiation complex forms

2. RNA polymerase joins

3. a transcription bubble is created

This sets up elongation.

Elongation definition

Elongation is the extension of the RNA transcript by RNA polymerase as it moves along the DNA template strand.

RNA polymerase reading direction

RNA polymerase reads the DNA template strand in the 3′ → 5′ direction.

RNA synthesis direction

RNA polymerase synthesizes the RNA transcript in the 5′ → 3′ direction. (“Read up, write down.”)

Sense strand (plus strand)

The sense (or +) strand is not read by RNA polymerase. It protects the exposed region of DNA from exonucleases and resembles the RNA transcript sequence.

Antisense strand (minus strand, template strand)

The antisense (–) strand is the actual template used by RNA polymerase to build the RNA transcript. It is complementary to the RNA.

Why RNA resembles the sense strand

The RNA transcript is complementary to the template strand, so it ends up matching the sense (coding) strand sequence, except RNA uses uracil (U) instead of thymine (T).

Base pairing in elongation

During elongation, RNA polymerase inserts complementary ribonucleotides: G pairs with C, A pairs with T, and U (uracil) pairs with A.

Primary transcript

The first RNA product of elongation (also called hnRNA or pre-mRNA in eukaryotes). It closely resembles the coding strand.

Role of transcription bubble

The transcription bubble advances with RNA polymerase, unwinding DNA ahead and rewinding DNA behind, allowing continuous RNA synthesis.

Termination trigger

Elongation continues until RNA polymerase reaches a termination sequence, signaling the end of the transcript.

Functions of the two strands

The sense (+) strand protects and mirrors the transcript, while the antisense (–) strand provides the actual template for RNA polymerase to copy.

Elongation summary

Key points: transcription bubble, template strand (read 3′ → 5′), RNA synthesized 5′ → 3′, RNA resembles coding strand, and elongation ends at termination signal.

Polymerase motif

Polymerases follow the general rule: read up (3′ → 5′), write down (5′ → 3′).