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Question: What is nucleosome? Explain the packaging of DNA helix. Draw a well labelled diagram of nucleosome. Solution: Hint: Chromosomal DNA is packaged inside microscopic nuclei with the help of histones. Complete Solution: A nucleosome is the system used for DNA packaging in eukaryotes. The system is made by winding the DNA around eight histone protein cores. There are five types of histone proteins, H1, H2A, H2B, H3 and H4. Four of them (H2A, H2B, H3 and H4) occur in pairs to produce histone octamer, called nu body or core of nucleosome. H1 protein is responsible for linking two octamers. This nucleosome combines to form a solenoid. The solenoid defines the packing of DNA as a 30 nm fibre of chromatin and results from the helical winding of at least 5 nucleosome strands. This solenoid is coiled and supercoiled to form a compact structure which can be packaged inside the nucleus. Today, researchers know that nucleosomes are structured as follows: Two each of the histones H2A, H2B, H3, and H4 come together to form a histone octamer, which binds and wraps approximately 1.7 turns of DNA, or about 146 base pairs. The addition of one H1 protein wraps another 20 base pairs, resulting in two full turns around the octamer, and forming a structure called a chromatosome. The resulting 166 base pairs is not very long, considering that each chromosome contains over 100 million base pairs of DNA on average. Therefore, every chromosome contains hundreds of thousands of nucleosomes, and these nucleosomes are joined by the DNA that runs between them (an average of about 20 base pairs). This joining DNA is referred to as linker DNA. Each chromosome is thus a long chain of nucleosomes, which gives the appearance of a string of beads when viewed using an electron microscope.

Note: There are positively-charged proteins that strongly adhere to negatively-charged DNA and form complexes called nucleosomes. Each nuclesome is composed of DNA wound 1.65 times around eight histone proteins....