(a)

i) How many types of RNA polymerases are there in an eukaryote cell ? Mention which one of them transcribes hnRNA.

ii) Write the changes that hnRNA undergoes before it leaves the nucleus as mRNA.

or

(b) The length of DNA in any cell is far greater than the dimension of its nucleus. Explain how this enormous DNA is packaged in a eukaryotic cell.

Correct Answer:

Either (a)
i) There are 3 types of RNA polymerases in a eukaryotic cell. RNA Polymerase II transcribes hnRNA.
ii) The hnRNA undergoes three main modifications before leaving the nucleus as functional mRNA: Splicing (removal of non-coding introns and joining of coding exons in a defined sequence), Capping (addition of an unusual nucleotide, methyl guanosine triphosphate, at the 5' end), and Tailing (addition of 200–300 adenylate residues at the 3' end to form a poly-A tail).

Or (b)
Eukaryotic DNA packaging is achieved using a set of positively charged, basic proteins called histones (which are rich in basic amino acid residues like lysine and arginine). The negatively charged DNA is wrapped around a histone octamer (a unit of eight histone molecules) in a coiled manner to form a nucleosome. A typical nucleosome contains approximately 200 base pairs (bp) of DNA helix. These nucleosomes constitute the repeating units of a structure in the nucleus called chromatin, which appears under an electron microscope as a "beads-on-a-string" structure.

Step-by-Step Educational Explanation:

Part (a) i) Types of RNA Polymerases and hnRNA Transcription:
In eukaryotic cells, division of labor exists for transcription. There are three distinct types of RNA polymerases in the nucleus (excluding those found in organelles like mitochondria and chloroplasts):
1. RNA Polymerase I: Transcribes ribosomal RNAs (rRNAs) such as 28S, 18S, and 5.8S rRNAs.
2. RNA Polymerase II: Transcribes the precursor of messenger RNA, which is called heterogeneous nuclear RNA (hnRNA), as well as most small nuclear RNAs (snRNAs).
3. RNA Polymerase III: Transcribes transfer RNA (tRNA), 5S rRNA, and some snRNAs.
Therefore, there are 3 types of RNA polymerases, and RNA Polymerase II is specifically responsible for transcribing hnRNA.

Part (a) ii) Post-Transcriptional Modifications of hnRNA:
The primary transcript (hnRNA) contains both non-coding regions (introns) and coding regions (exons). It is non-functional and unstable. To become mature mRNA that can safely exit the nucleus for translation, it undergoes three critical steps:
1. Splicing: The non-functional introns are excised (removed), and the exons are joined together in a defined sequence by complex machinery called spliceosomes.
2. Capping: At the 5' end of the hnRNA, an unusual nucleotide called methyl guanosine triphosphate (⁷methylguanosine cap) is added. This cap protects the mRNA from degradation and helps in ribosomal recognition during translation initiation.
3. Tailing (Polyadenylation): At the 3' end, a stretch of 200 to 300 adenylate residues is added in a template-independent manner. This is called the poly-A tail, which helps in the export of mRNA, translation efficiency, and stability.

Part (b) DNA Packaging in Eukaryotes:
If we stretch the DNA of a single human diploid cell, it measures approximately 2.2 meters in length. This must fit into a nucleus that is only about 10^-6 meters in diameter. Eukaryotes solve this scale problem through a highly organized packaging hierarchy:
1. Charge-Based Binding: DNA is negatively charged due to its phosphate backbone. Cells synthesize basic, positively charged proteins called histones. Histones derive their positive charge from being rich in the basic amino acids lysine and arginine.
2. Histone Octamer: Eight histone molecules (two each of H2A, H2B, H3, and H4) organize to form a core unit called a histone octamer.
3. Nucleosome Formation: The negatively charged DNA wraps around the positively charged histone octamer to form a unit called a nucleosome. A typical nucleosome contains about 200 base pairs of DNA.
4. Chromatin Structure: The repeating units of nucleosomes form a thread-like structure in the nucleus known as chromatin. Under electron microscopy, chromatin resembles "beads-on-a-string," where the beads represent individual nucleosomes and the string represents linker DNA.