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
- 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.