DNA: Deoxiribonucleic acid

DNA is a thread like polymer of deoxyribonucleotides. The length of DNA is defined as number of nucleotides present in it. Haploid content of human DNA is 3.3 × 109 bp. A double elix structure for DNA was given by Watsen and Crick. James Watson and Francis Crick proposed the double helix model of DNA

Features of  double helix structure of DNA:

(i) DNA is made up of two polynucleotide chains, where the backbone is made up of sugar and phosphate groups and the nitrogenous bases projects toward centre.

(ii) There is complementary base pairing between the two strands of DNA.

(iii) The two strands are coiled in right-handed fashion and are anti-parallel in orientation. One chain has a 5’ ⟶ 3’ polarity while the other has 3’ ⟶ 5’ polarity.

(iv) The diameter of the strand is always constant due to pairing of purine and pyrimidine, i.e., adenine is complementary to thymine while guanine is complementary to cytosine.

(v) The distance between two base pairs in a helix is 0.34 nm and the two strands are right-handed coiled.

Packaging of DNA

(i) Packing of DNA in prokaryotes

 

  • In prokaryotes, well-defined nucleus is absent so DNA is present at some places called nucleoid. The negatively charged DNA is coiled with some positively charged non-histone basic proteins.
  • DNA in nucleoid is organized in large loops held by positively charged proteins.

(ii) Packaging of DNA in eukaryotes

  • It was discovered that chromosome is made up of DNA and protein.
  • Later, it also comes in discovery that chromatin fibres look like beads on the string, where beads are repeated units of proteins.
  • The proteins associated with DNA is of two types-histon and non-histone proteins. Histones forms a unit of 8 molecules called Histone Octamer.
  • The negatively charged DNA molecule wraps around the positively charged histone proteins to form a structure called Nucleosome.
  • The nucleosome core is made up of four types of histone proteins— H2A, H2B, H3 and H4—occurring in pairs (total 8).
  • 200 bp of DNA helix wraps around the nucleosome by 1¾ turns.
  • Repeating units of nucleosomes form a thread-like structure called chromatin fibre. 
  • These chromatin fibres further coil and condense at metaphase stage of cell division to form chromosomes.
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