►   Protein – Protein interactions :

Protein-protein interaction is present in both prokaryotes and eukaryotes. The external signals affect the gene expression with the help of such interaction. Proteins interact with each other to form a homo or hetero-dimers before binding to DNA molecules. Eg. transcription initiation factors interacts with TATA box binding proteins (TBP) to activate transcription of a gene.

Fig 1-1.6.2 : Interaction between various protein factors

►  RNA Polymerase:

• A single RNA polymerase is responsible for transcribing all types of RNA in prokaryotic system.
  
  
• However, eukaryotes have three different RNA polymerases, which have been found to specialize in the synthesis of various types of RNA:
  

  •  RNA polymerase I (Pol I) - transcribes rRNA (ribosomal RNA) genes.

  •  RNA polymerase II (Pol II) - transcribes protein-coding genes or mRNA (messenger RNA).

  •  RNA polymerase III (Pol III) - transcribes other functional RNA genes (e.g., tRNA).

• In eukaryotes, transcription occurs inside the nucleus. All the enzymes responsible for translation are present in the cytosol therefore the transcripts formed then move out of the nucleus through nuclear pores into the cytosol (the liquid phase of the cytoplasm), where translation occurs. Organelle genomes (like the mitochondrial genome) are transcribed within the organelle (the mitochondria) and translation is also within the organelle (the mitochondria).
  
  
• Since prokaryotes have no nucleus, the step involving the movement of transcripts from nucleus to cytoplasm does not take place, and translation can take place immediately in the cytoplasm, directly on the growing transcript.