DNA as Code of Life

What is DNA?

DNA can be looked at in two ways.

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.

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.

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.

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

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.

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.

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