DNA as Code of Life

What is DNA?

DNA can be looked at in two ways.

  • As code of life
  • As vehicle of heredity

DNA as code of life

  • DNA is called the code of life as it holds the complete instruction set for building life.
  • The code is present in the nucleus of every cell of every living organism (with few exceptions like RBC, neuron etc.).
  • The code, present in the nucleus, is read by a host of machineries in the cell to build proteins which are the building blocks of life. (more on this below)
  • Proteins constitutes 55% of the dry mass of cell. Everything from structure of the cell to enzymes are made of proteins.
  • Further if we zoom in, we find that the total DNA material in humans is arranged as smaller manageable pieces.

What is Chromosome?

  • The smaller manageable pieces of DNA have arranged themselves as linear strands each of which is called a chromosome.
  • There are 23 different chromosomes in human DNA. In addition, there are 2 version of 23 chromosomes. (one set of 23 inherited from each parent).
  • Thus, in total there are 46 chromosomes in the nucleus of every cell if our body.
  • Imprinting: Each chromosome holds the memory of which parent it is inherited from through a process called imprinting.

What is Genome?

The complete set of all DNA material or genetic material is called the genome of that organism.


  • Note that way we have used the term DNA is more as a generic name.
  • DNA is really a complex molecule that make up the instruction for building life.

Building blocks of DNA

  • DNA stands for Deoxyribose Nucleic Acid or in short nucleic acid or nucleotide which are the building blocks of genetic material.
  • Each nucleotide has 3 parts
  • A nitrogen base
  • A phosphate group
  • Sugar molecule

Shape of DNA

Shape of DNA
  • It looks like a twisted ladder. (called fancily as double-helix)
  • It is the molecules the genetic is made of and the way they are arranged that gives it its structure and consequent function.
  • If you think of DNA as a ladder, the phosphate group and sugar molecule make the sides of the ladder acting as its backbone. Thus, it is commonly called sugar-phosphate backbone.
  • As the name suggests it acts only as backbone. to hold the rungs of the ladder.
  • The nitrogen bases constitute the rungs of the ladder which is where the real code of life is written.
  • Further the ladder has 2 sides to it, each side made of a nitrogen base, sugar and phosphate.
  • Two rungs (nitrogen base) from each side come together to make the ladder as shown in the figure. Each side with a sugar-phosphate backbone and nitrogen base (rung) of the ladder is called one strand.
  • This makes human DNA a double-stranded structure. (there are single-stranded DNA also).
  • Further this ladder is spiraled and spiraled to form a twisted structure.

Base Pair

  • If the entire set of genetic material(genome) is considered to be a book, nitrogen bases are the letters.
  • There are 4 types of bases namely adenosine, guanine, cytosine and thymine. Simply, there are 4 letters to the book A, G, C and T.
  • Two strands come together due to joining of bases.
  • The bases join one another depending on their shapes. (You can’t put square boxes in round holes)
  • Accordingly, A always pairs with T and G always pairs with C. (So, if you know one strand you know what’s on the other)
  • Note: While in nature only 4 lettered-DNA is found, scientists have recently created a synthetic DNA with 8 letters.
  • The human genome is made of 3 billion base pairs. (So total 6 billion bases)
  • Simply there are 3 billion letters in the book of life for humans.

Mitochondrial DNA

  • In addition to DNA in the nucleus, some DNA is also present in the mitochondria.
  • This we inherit from only our mothers.
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