Directionality of Transcription and Translation
- Imagine trying to read a book backwards, it wouldn’t make sense.
- Similarly, transcription and translation follow a specific direction: 5′ to 3′.
- This directionality is crucial for ensuring that genetic information is accurately copied and translated into proteins.
What Does 5′ to 3′ Mean?
- Nucleic acids like DNA and RNA have a direction because their structure is asymmetric.
- Each nucleotide has:
- A phosphate group attached to the 5′ carbon of the sugar.
- A hydroxyl group ( –OH) attached to the 3′ carbon.
- Think of a train track: one end is the starting point (5′), and the other is the destination (3′).
- The train (enzymes) can only move in one direction.
Transcription: Synthesizing RNA from DNA
- During transcription, RNA polymerase reads the DNA template strand and builds a complementary RNA strand.
- The enzyme adds RNA nucleotides to the 3′ end of the growing RNA strand, meaning it synthesizes RNA in the 5′ to 3′ direction.
A common mistake is thinking that RNA polymerase moves in the 5′ to 3′ direction along the DNA. In reality, it moves in the 3′ to 5′ direction on the DNA template strand to create an RNA strand in the 5′ to 3′ direction.
Why 5′ to 3′?
- RNA polymerase can only add nucleotides to the 3′ end of the RNA strand.
- This is because the energy for the reaction comes from breaking the phosphate bonds of the incoming nucleotide, which must attach to the 3′ hydroxyl group.
Always remember: RNA synthesis is antiparallelto the DNA template strand. If the DNA template is 3′ to 5′, the RNA is synthesized 5′ to 3′.
