If one looks inside the nucleus of every human cell, there are 46 tiny rod like structure called as Chromosomes. These chromosomes are made of tightly coiled strands of Deoxyribonucleic acid – DNA. It is one of the two main nucleic acids found in our cells. DNA happens to be the genetic material in all living organisms, ranging from single-celled bacteria to multicellular mammals. They carry the cell’s genetic blueprint and responsible for its transfer to the next generation. DNA contains all the information required for making new cells and also for efficient functioning of the body. These instructions are in the form a sequence of A, C, G and T nitrogenous bases, which are present in long sequences throughout the length of the DNA molecules. The order, or sequence, of these bases form the instructions for the synthesis of all the proteins and other molecules required for development and growth of our body.
A chromosome may contain tens of thousands of segments of DNA called as Gene, which is the basic unit of heredity represented by a specific DNA segment that codes for a functional molecule. In short we can that DNA is a long molecule that provides all the individuals with a unique genetic code.
DNA- The molecule:
- DNA is made of monomers called as Nucleotides. Several nucleotides combine to form a polynucleotide, DNA. When Nitrogenous bases bound to sugar molecule-they are called Nucleoside and with addition of one or more Phosphate groups, nucleotides are formed.
- A nucleotides is made up of three components – a nitrogenous base, a pentose (five-carbon) sugar ( -2 deoxyribose sugar) in DNA, and one or more phosphate groups. The ribose sugar molecule is connected to both Nitrogenous base and Phosphate molecule.
- The nitrogenous bases are so named because they contain carbon and nitrogen. They are called as bases as they contain an amino group.
- DNA gets its name – a nucleic “acid” even though it also has a basic component (nitrogenous bases) because of the the phosphate group. The phosphate molecule with its negative charge and exposure to the outside environment contribute significantly to acidic nature of DNA. As we know that DNA also contains nitrogenous bases, however their basic properties are masked to a great extent due to their hydrogen bonding to form base pairs and also due to the position they occupy – inside of helix. Hence it’s the acidic part of the molecule that dominates, and that is why call DNA as an acid.
- Nitrogenous bases are divided into two two groups : Purines -adenine and guanine as they have two carbon-nitrogen rings as primary structure. pyrimidines – cytosine, thymine and Uracil ( Present in RNA – other important nucleic acid which will be discussed next post) which have a single carbon-nitrogen ring as their primary structure.
- The symbols of the nitrogenous bases present in DNA are – Cytosine (C) , Thymine (T), adenine (A) and Guanine (G).
- The phosphate residue attaches to the hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide to form a 5′–3′ phosphodiester linkage.
DNA -Double helix
- DNA is a double stranded molecule and it exhibits a unique double helix resembling a twisted ladder. The two thin strands of the ladder runs anti parallel and connected by rungs.
- The sugar and phosphate molecule lie on the outside of the helix, forming the DNA’s backbone (two anti parallel strands of ladder).
- The nitrogenous bases are paired with each other in the interior of the helix, like a pair of staircase steps.
- The DNA sequence is made up long sequences of the four nitrogenous bases, Adenine- A, Cytosine-C, Guanine-G and Thymine- T or nucleotides and the order of the nucleotide sequence determines genetic information.
- The bases pairing always occurs in the same way- A pairs with T, C with G by means of a Hydrogen bond
- DNA strands have a starting and a end called as 5′ ( read as 5 prime) and 3′ ends respectively. These two strand runs in a anti-parallel manner. The DNA strands are complementary to each other. If the sequence of one strand in 5′ to 3’direction is AATTGGCC, the complementary strand would have the sequence – 5′ GGCCAATT 3′.
The double helix structure of DNA was discovered by Nobel laureates Francis Crick and James Watson with significant help from Rosalind Franklin and Maurice Wilkins.
Image Source : DNA double helix – Genome Research Limited , yourgenome.org
Purine and pyrimidines . Nucleotide structure – Openstax
Phosphase – sugar backbone image : Madeleine Price Ball – Wikipedia