What is central dogma and describe the packing of $DNA$ helix ?
 

Francis Crick proposed the Central dogma in molecular biology, which states that the genetic information flows from $DNA \to RNA \to $ protein.

  In some viruses the flow of information is in reverse direction, that is, from $RNA$ to $DNA$ .

Taken the distance between two consecutive base pairs as $0.34\,nm\,(0.34 \times {10^{ - 9}}\,m)$, if the length of $DNA$ double helix in a typical mammalian cell is calculated (simply by multiplying the total number of $bp$ with distance between two consecutive bp, that is, $6.6 \times {10^9}\,bp \times 0.34 \times {10^{ - 9}}\,m/bp$ ), it comes out to be approximately $2.2\, metres$ .

A length that is far greater than the dimension of a typical nucleus (approximately ${10^{ - 6}}\,m$). How is such a long polymer packaged in a cell?
          If the length of $E.$ coli $DNA$ is $1.36\, mm$ , 

          In prokaryotes, such as, $E.$ coli, though they do not have a defined nucleus, the $DNA$ is not scattered throughout the cell. $DNA$ (being negatively charged) is held with some proteins (that have positive charges) in a region termed as ‘nucleoid’. The $DNA$ in nucleoid is organised in large loops held by proteins.

          In eukaryotes, this organisation is much more complex. 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.