D2.2.2 Regulation of transcription by proteins Notes

Why Transcription Needs To Be Regulated

1. Every cell in an organism contains the same genome, but different cells express different genes.
2. A muscle cell produces muscle proteins; a nerve cell produces neurotransmitters.
3. This specificity is achieved by regulating transcription, controlling which genes are transcribed into mRNA.
4. Proteins bind to specific DNA sequences to turn genes on or off.

Key DNA Sequences That Control Transcription

1. Promoters Are The Starting Point

1. The promoter marks where transcription begins.
2. RNA polymerase binds to the promoter to start transcribing the gene.
3. A common promoter sequence in eukaryotes is the TATA box (TATAAA), which helps position RNA polymerase accurately.

2. Enhancers Are Sequences That Increase Transcription

1. Enhancers can be located far away from the gene they regulate, sometimes thousands of base pairs upstream or downstream.
2. They work by DNA looping, bringing activator proteins bound to the enhancer close to the promoter.
3. This increases RNA polymerase activity and boosts transcription.

3. Silencers Are Sequences That Decrease Transcription

1. Silencers work similarly to enhancers but have the opposite effect, they reduce or block transcription.
2. Repressor proteins bind to silencers and prevent RNA polymerase from effectively transcribing the gene.

Key Proteins That Regulate Transcription

1. Transcription factors can be:
   1. Activators:
      1. Bind to enhancers.
      2. Increase the rate of transcription by helping RNA polymerase bind to the promoter or work more efficiently.
   2. Repressors:
      1. Bind to silencers.
      2. Decrease or block transcription by preventing RNA polymerase from accessing the promoter or functioning properly.