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DNA is the hereditary material in humans. It’s made-up of chemical building blocks known as nucleotides .It’s seen in all prokaryotic and eukaryotic cells .Deoxyribonucleic acid is the full form of DNA. It is a molecule that contains the instructions an organism needs to develop, live and reproduce. These instructions are found inside every cell and it is passed down from parents to their children. It is a nucleic acid. And have four major types of macromolecules which are known to be essential for all forms of life.

Structure of DNA


Franklin first had pointed out that the DNA is made two spirals. The works of Rosalind Franklin lead to Watson  and Crick’s discovery. It was in 1953, James Watson and Francis Crick discovered the structure of DNA. DNA has a double helix structure, because it looks like a twisted ladder. The sides of the ladder are made of alternating sugar (deoxyribose) and phosphate molecules while the steps of the ladder are made up of a pair of nitrogen bases.

There are 4 types of nitrogen bases Adenine (A) Thymine (T) Guanine (G) Cytosine (C) . The nitrogen bases have a specific pairing pattern.This pairing pattern occurs because the amount of adenine equals the amount of thymine; the amount of guanine equals the amount of cytosine A=T . The pairs are held together by hydrogen bond.


In Eukaryotic organisms they store the majority of DNA inside the cell nucleus, some DNA in organelles such as mitochondria .Based on location: Nuclear DNA Located within the nucleus of eukaryotic cells.The structure is linear with open ends.It has two copies per cell. Includes 46 chromosomes containing 3 billion nucleotides.Nuclear DNA is diploid. Mitochondrial DNA Located in the mitochondria.100-1,000 copies per cell.Closed, circular structures.Contain for example 16,569 nucleotides in humans.Mitochondrial DNA is haploid. FORMS OF DNA A-DNA: It is a right-handed double helix . Dehydrated DNA takes an A form that protects the DNA during extreme conditions such as desiccation. Protein binding also removes the solvent from DNA.The DNA takes an A form. B-DNA: It’s a right-handed helix. The majority of DNA has a B type conformation under normal physiological conditions.

Z-DNA: It’s left-handed DNA. With  a zig-zag pattern. Andres Wang and Alexander Rich discovered Z-DNA.It’s found at the start site of gene.Plays role in gene regulation.

C-DNA: It’s the complementary DNA. It only contains the coding sequences. They are the exons of gene, which codes for a functional protein.

Watson and Crick described the DNA molecule which was in  B form. In several other forms DNA  can exist. The primary difference between the forms is the direction that the helix spirals. A,B and C are right-handed helix Z is left-handed . The major form that is found in the cell is B.


  •   DNA helices can be right handed or left handed.
  •   The two strands of DNA separate from each other On heating and these again hybridize on cooling.
  •   Melting temperature (Tm)  is at which the two strands separate completely .Tm is specific for each specific sequence.
  •   The B sample of DNA having higher melting point must have more C-G content because the C-G pair has 3 hydrogen bonds.

The sequence of bases along the DNA molecule encodes for the sequence of amino acids in every protein in all organisms.


  • Replication
  • Mutation
  • Genetic engineering
  • Immunological
  • Transcription
  • Translation
  • Cellular Metabolism
  • DNA Fingerprinting
  • Gene Therapy
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Deoxyribonucleic acid


James Watson and Francis Crick

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