James Watson And Francis Crick Model

They proposed a very simple but famous double helix model for the structure of DNA. This was based on the observation of Erwin chargaff.

Salient features of the double helix structure of DNA are :-

1. DNA consist of two polynucleotide chains. The backbone is constituted by sugar – phosphate and the bases project inside.
2. Two chains of DNA run in antiparallel fashion with 5′ 3′ polarity in other chain.

• Two chains of DNA run in Antiparallel fashion with 5`————3′ polarity in one and 3′——–5`polarity in chain.
• These bases in two strands are paired through hydrogen bonds.
• Adenine forms 2H- bonds with thymine and vice versa.
• Similarly, Guanine is bonded with cytosine with 3H – bonds.
• As a result always a purine(A,G) comes opposite to pyrimidine (C,T,U).

CENTRAL DOGMA

FRANCIS CRICK proposed the central dogma of molecular biology which explains the one way for the synthesis of RNA from DNA And the process is called transcription, while synthesis of protein from RNA called Translation.
Some viruses show reverse transcription the synthesis of DNA from an RNA template. A class of RNA viruses, called retroviruses, are characterized by the presence of an RNA-dependent DNA polymerase (reverse transcriptase).

Packaging of DNA helix

• The DNA inside of a cell is organized so that it fits well within the small size of a cell.
• Because if DNA wouldn’t be compact it wouldn’t fit in the cell. Human cells contains 2-3 meters long DNA.
• Imagine how effective compaction is that it makes a molecule 2-3 meters long fit in a cell that its million times tinier.

Packaging of DNA in Prokaryotes :-

1. In prokaryotes, DNA is not scattered throughout the cell although they do not have a defined nucleus.
2. DNA is organized into loops held by proteins which have positive charge.

DNA Packaging in Eukaryotes

• In eukaryotes, the DNA occurs inside nucleus. There are set of basic and positively charge proteins is called histones.
• Histones are rich in lysine and arginine, which carry positive charge.
• There are five types of histones proteins H1, H2 A, H2B, H3, H4 Four of them (H2A H2B H3 H4) occur in pair to form Histone Octamer or nobody.
• Due to all of those phosphate groups along DNA backbone results it has a negative charge and is attracted to positively charged molecules such as histones.
• The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called Nucleosome.
• The condensed DNA or the nucleosome constitute repeating unit of a structure in nucleus called chromatin.
• The condensed DNA or the nucleosome constitute repeating unit of a structure in nucleus called chromatin.
• Nucleosomes in chromatin are seen as ‘beads-on-strings’ under Electron Microscope.
• Nucleosome further coils to form Solenoid/ chromatin fibre.
• The packaging of chromatin at t higher level requires additional set of proteins that collectively a t Was the colect chromosomal (NHC) proteins.
• Chromatin is differentiated into two regions on the basis of staining behavior in a typical nucleus