Question

What is nucleosome ? Draw diagram of double stranded polynucleotide chain of DNA and explain its structure.

Answer 1

A nucleosome is a basic unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound in sequence around eight histone protein cores.

Structure of DNA:

DNA is usually a double helix and has two strands running in opposite directions. It is twisted around each other like the strands of a two-stranded rope with free ends. Each chain is a polymer of subunits called as nucleotides. Each strand has a backbone made up of deoxyribose sugar molecules linked together by phosphate groups. The 3' C of a sugar molecule is connected through a phosphate group to the 5' C of the next sugar. This linkage is also called as 3-5' phosphodiester linkage. All DNA strands are read from the 5' to the 3' end where the 5' end terminates in a phosphate group and the 3' end terminates in a sugar molecule. According to Watson and Crick model, DNA is a double helix with one turn of 34 A${}^o$ and 10 base pairs per turn. Thus, the distance between adjacent bases is 3.4 A${}^o$. The helix diameter is 20A${}^o$. The base pairing of opposite strands is stereochemically selective, Adenine always pairing with Thymine, and Guanine with Cytosine. Two and three hydrogen bonds are formed in A-T and G-C base pairs, respectively. The attachment of bases to the backbone sugars through glycosidic bonds is asymmetrical. This results in the formation of two different grooves on opposite sides of the base pairs, the major and minor grooves.