Packaging of DNA Helix in Eukaryotes MCQ - Practice Questions & Answers

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Packaging of DNA Helix in Eukaryotes

The distance between two consecutive base pairs as 0.34 nm (0.34×10–9 m).
The length of DNA double helix in a typical mammalian cell can be calculated by multiplying the total number of bp with the distance between two consecutive bp, that is, 6.6 × 109 bp × 0.34 × 10-9m/bp), it comes out to be approximately 2.2 metres.
This length is far greater than the dimension of a typical nucleus (approximately 10-6m).
Therefore, such a huge molecule of DNA needs to be packaged in a small cell.
The eukaryotic chromosomes consist of the linear molecule of DNA.
There is a set of positively charged, basic proteins called histones.
A protein acquires charge depending upon the abundance of amino acids residues with charged side chains.
Histones are rich in the basic amino acid residues lysines and arginines.
Both the amino acid residues carry positive charges in their side chains.
Histones are organised to form a unit of eight molecules called histone octamer.
The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called a nucleosome.
A typical nucleosome contains 200 bp of DNA helix.
Nucleosomes constitute the repeating unit of a structure in nucleus called chromatin, thread-like stained (coloured) bodies seen in nucleus. The nucleosomes in chromatin are seen as ‘beads-on-string’ structure when viewed under an electron microscope (EM).
Histone Proteins: Histones are divided into two groups:
- Core histones
- Linker histones
Core histones are H2A, H2B, H3, and H4, where two H3/H4 dimers (H3 and H4 bound together) and two H2A/H2B dimers (these two bound together) form the histone octamer.
Linker histone H1 basically locks the DNA in place onto the nucleosome.
Approximately 147 base pairs (bp) of DNA are wrapped around the histone octamer.
Linker DNA length ranges between ~20–90 bp and varies among different species, tissues, and even fluctuates within a single cellular genome.
The beads-on-string structure in chromatin is packaged to form chromatin fibres that are further coiled and condensed at the metaphase stage of cell division to form chromosomes.
The packaging of chromatin at a higher level requires an additional set of proteins that collectively are referred to as Non-histone Chromosomal (NHC) proteins.
In a typical nucleus, some regions of chromatin are loosely packed (and stains light) and are referred to as euchromatin.
The chromatin that is more densely packed and stains dark are called Heterochromatin.
Euchromatin is said to be transcriptionally active chromatin, whereas heterochromatin is inactive.