Structure of DNA
Ø Deoxyribonucleic acid (DNA) is the heredity material
found in humans and all living organisms. It is a double-stranded molecule and
has a unique twisted helical structure.
Ø DNA is made up of nucleotides, each nucleotide has
three components:
2.
phosphate group
3.
a
nitrogen-containing base attached to the sugar.
Ø Each base pairs are bonded through Hydrogen bonding.
These nitrogenous bases are Adenine (A), Guanine (G), Cytosine
(C), and Thymine (T) .
- Ø There are two types of nitrogenous bases – Purines
(Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine).
- Ø Cytosine is common for both DNA and RNA and Thymine is present in DNA. Uracil is present in RNA at the place of Thymine.
- Ø A nitrogenous base is linked to the OH of 1'C pentose
sugar through a N-glycosidic linkage to form a nucleoside
Ø Most DNA is located at the cell nucleus so is called nuclear DNA, however, a small amount of DNA is also located in mitochondria, and so is referred to as mitochondrial DNA.
Ø DNA as an acidic substance present in nucleus was
first identified by Friedrich Meischer in 1869. He named it as ‘Nuclein’.
Ø During 1953 James
Watson and Francis Crick, based on the X-ray diffraction data
produced by Maurice Wilkins and Rosalind Franklin, proposed a very simple but
famous Double Helix model for the structure of DNA.
Features
of the Double-helix structure of DNA are as follows:
1.
DNA is made of
two helical chains that intertwine with each other to form a double helix. The
most widely accepted structure of DNA is right-handed helix DNA.
2.
The two chains have anti-parallel polarity. It means, if one chain
has the polarity 5' 3', the other has 3' 5'.
3.
These chains are connected to each other via nitrogen bases
through hydrogen bonding. Adenine pairs with Thymine through 2 hydrogen
bonds. Similarly, Cytosine pairs with Guanine through 3 hydrogen bonds.
4.
The base pairs A = T and G ≡ C are known as complementary
base pairs. Hence, the amount of Adenine is equal to the amount of Thymine, and
the amount of Guanine is equal to the amount of Cytosine. (Chargaff's rule)
5.
There is a presence of major groove and minor groove These
groves act on base-pair recognition and binding sites for protein.
6. The distance between each turn is 3.4 nm.The major groove is 2.2 nm wide and the minor groove is 1.1 nm wide.
Function of DNA
Ø DNA stores complete genetic information that
requires to specify an organism.
Ø
It is the source of information that is needed in order to
synthesize cellular proteins, and other macromolecules required by an organism.
Ø
The sequence of its nucleotides (Adenine, Thymine,
Cytosine, Guanine) forms a code that specifies how proteins are made.
Ø
It can replicate to give rise to two daughter cells and transfer
one copy to the daughter cells during cell division. Thus, maintaining the
genetic materials from generation to generation.
Ø
DNA can undergo mutations
(changes in sequence).
Ø
Some
mutations lead to beneficial variations, which can drive evolution.
Ø
DNA doesn’t just store information—it also regulates
when and how genes are expressed.
Ø
Certain genes are turned on or off depending on
the cell’s needs and environmental conditions.
