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TLB’S Biology Classes, , MOLECULAR BASIS OF INHERITANCE, PART 3, Central dogma in molecular biology, , In some viruses the flow of information is in reverse direction, that, is,from RNA to DNA. It is known as Reverse transcription., , Length of DNA

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TLB’S Biology Classes, , Packaging of DNA in Prokaryotes, In prokaryotes, such as, E. coli, DNA is held with some proteins in a, region termed as ‘nucleoid’., , Negatively charged DNA is held with positive charged proteins., Packaging of DNA in Eukaryotes, In eukaryotes, DNA is packed with the help of a set of positively, charged, basic proteins called histones., • Histones are organised to form a unit of eight molecules called, as histone octamer., • The negatively charged DNA is wrapped around the positively, charged histone octamer to form a structure called nucleosome, , •, , A typical nucleosome contains 200 bp of DNA helix, • Nucleosomes constitute the repeating unit of a structure in, nucleus called chromatin, • The nucleosomes in chromatin are seen as ‘beads-on-string’, structure when viewed under electron microscope (EM)., , •

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TLB’S Biology Classes, , Non-histone Chromosomal (NHC) proteins., , The packaging of chromatin at higher level requires additional set of, proteins that collectively are referred to as Non-histone Chromosomal, (NHC) proteins., , Euchromatin, In a typical nucleus, some region of chromatin are, • loosely packed, • Lightly stained and, • transcriptionally active, chromatin are referred to as euchromatin., , Heterochromatin, •, , The chromatin that is more, • densely packed, • darkly stained, • transcriptionally inactive, Chromatin are called heterochromatin .