Question

RNA polymerases in eukaryotes show division labour. Substantiate.

Answer 1

RNA polymerase is an enzyme, which synthesizes RNA from a DNA template. It is essential to life, and is found in all living organisms and many viruses. Bacteria and archaea only have one RNA polymerase while eukaryotes have multiple types of nuclear RNAP (RNA polymerase), each responsible for synthesis of a different subset of RNA.

Complete Answer:
An RNA polymerase (RNAP), catalyzes the process of transcription. The sequence of the RNA polymer is complementary to that of the template DNA and is synthesized in the 5’→ 3′ direction. This RNA strand is called the primary transcript and needs to be processed before it can be functional inside the cell.

Eukaryotes have multiple types of nuclear RNAP, each one of then responsible for the synthesis of a different subset of RNA:
1. RNA polymerase I- synthesizes a pre-rRNA 45S (35S in yeast). It matures and will form the major RNA sections of the ribosome.
2. RNA polymerase II- synthesizes precursors of mRNAs and most sRNA and microRNAs.
3. RNA polymerase III- synthesizes tRNAs, rRNA 5S and other small RNAs found in the nucleus and cytosol.
4. RNA polymerase IV and V- found in plants and are less understood; they make siRNA. In addition to the ssRNAPs, chloroplasts also encode and use a bacteria-like RNAP.

- RNAP can initiate transcription at specific sequences of DNA known as promoters. Then, it produces an RNA chain that is complementary to the template DNA strand. Nucleotides are added into the RNA strands and this process is known as elongation; in eukaryotes, RNAP can build chains as long as 2.4 million nucleotides.
- RNAP, then, releases its RNA transcript at specific DNA sequences encoded at the end of genes, which are known as terminators.
- The products of RNAP are: transfer RNA (tRNA) that transfers specific amino acids to growing polypeptide chains at the ribosomal site of protein synthesis during translation; ribosomal RNA (rRNA) that incorporates into ribosomes; micro RNA (miRNA), which regulates gene activity; messenger RNA that acts as a template for the synthesis of proteins by ribosomes; and catalytic RNA (ribozyme), which functions as an enzymatically active RNA molecule.
- Hence, RNA polymerases in eukaryotes show division of labour by getting involved in the transcription of different types of RNA.

Note: RNA polymerases interact with a lot of proteins to complete their task. These proteins help in improving the binding specificity of the enzyme and help in unwinding the double helical structure of DNA, modulate the enzyme activity based on the requirements of the cell and change the speed of transcription.