DNA Structure and Function

DNA Structure and Function

  • DNA is deoxyribonucleic acid.
  • DNA contains hereditary information in almost every organism.
  • Almost every cell in our body has similar DNA structure.
  • Most of the DNA present in our body located in the nucleus.
  • Information in the DNA stored as codes that are chemical bases.
  • Adenine (A), Guanine(G), Cytosine(c) and thymine(T)
  • Adenine and Guanine are purines.
  • Cytosine and Thymine are pyrimidines.
  • When a purine makes a bond with pyrimidine that is called a base pair, each base is attached with a sugar molecule.
  • A base, sugar, and phosphate together are called a nucleotide.
  • Human DNA consists of 3.4 billion bases.
  • The structure of  DNA is a double helix that is formed by the bonding of base pairs attached to a sugar-phosphate backbone.
  • Both strands of DNA contains the same biological information.
  • There are coding and non-coding regions present on DNA.
  • Only 2% of DNA is coding DNA.
  • There is phosphate attached to 5’ and sugar to 3’
  • DNA replicates and fulfills the requirement for a new cell.

The double-helical structure of DNA

  • double-helical structure of DNA was Given by James Watson and Francis Crick in 1953
  • Most DNA is right-handed.
  • Only Z-DNA is left-handed.
  • Always A=T, G≡C by hydrogen bonding
  • The pairing of base pair is complementary to each other
  • Both strand of DNA is antiparallel (5’-3’ 3’-5’)
  • There are 10 base pairs in each helical turn.
  • The helical structure of DNA contains two grooves
  • The narrow groove is known as Minor groove
  • The wide groove is known as Major groove

DNA as a genetic material

Griffith experiment:

  • By Frederick Griffith in1928
  •  He proved that DNA is a genetic material that carries all the genetic information.
  • Colonies of Pneumonia (Diplococcus pneumoniae)
  • Grow them in nutrient agar medium
  • Colonies smooth(S) and rough(R)

R Strain is the nonvirulent colonies (didn’t cause illness)

S Strain is the virulent colonies

Types of DNA

DNA are of three types

  1. B-DNA
  2. A-DNA
  3. Z-DNA


Helix rotation Right-handed Right-handed Left-handed
Rise/base-pair 0.34 nm 0.29 nm 0.37 nm
Distance per turn 3.4 nm 3.2 nm 4.5 nm
No. of base pairs 10 11 12
The topology of major groove Deep, Wide Narrow, Deep Flat
The topology of minor groove Shallow, Narrow Shallow, Broad Deep, Narrow
Helical diameter 2.37 nm 2.55 nm 1.84 nm


DNA packaging

Why DNA needs to be packed?

There are 3 million base pairs of DNA in our body and DNA is in nucleus inside the cell so DNA should be compacted to fit easily into the nucleus.

Chromatin: Chromatin is a complex structure of DNA and protein. The unit of Chromatin is known as Nucleosome.

When the process is DNA packaging starts, it coiled around histone protein that is known as a nucleosome.

In the case of DNA packaging histone forms octamer of H2A, H2B, H3, H4.

H1 is another histone that helps in the packaging process,  is attached to nucleosome for compaction.

The DNA which connects two nucleosomes is known as Linker DNA.

At 10nm we can observe the nucleosome and DNA looks like beads on a string.


A chromatid is the condensed form of chromatin having a scaffolding structure, which can observe in the metaphase chromosome.


When DNA get packed known as a chromosome.

The human cell contains 46 chromosomes.

Out of 46 22 are in autosome pair and 2 are sex chromosome (XY, XX)

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