DNA is a molecule that carries genetic instructions for growth, development, functioning and reproduction of living organisms. It consists of two strands coiled around each other in a double helix structure. Each strand is made up of repeating nucleotide units containing a sugar, phosphate, and one of four nitrogenous bases (adenine, thymine, cytosine, guanine). The bases bond together between the strands in specific base pairs (A-T and C-G) to form the rungs of the DNA ladder. Watson and Crick discovered that the structure of DNA is a twisted ladder with the bases pairing in the middle and the sugar and phosphate molecules forming the sides.

1. OPIC-D.N.A. And its Structure

2. Defination
Deoxyribonucleic acid is a molecule that carries the genetic instructions used in the
growth, development, functioning and reproduction of all known
living organisms and many viruses. DNA and RNA are nucleic acids;
alongside proteins, lipids and complex carbohydrates (polysaccharides), they are
one of the four major types of macromolecules that are essential for all known
forms of life. Most DNA molecules consist of two biopolymer strands coiled
around each other to form a double helix.

4. Structure of D.N.A.Structure of
D.N.A

5. Structure of D.N.A.
 DNA is made up of six smaller molecules -- a five carbon sugar
called deoxyribose, a phosphate molecule and four different
nitrogenous bases (adenine, thymine, cytosine and guanine).
Using research from many sources, including chemically
accurate models, Watson and Crick discovered how these six
subunits were arranged to make the structure of DNA. The model
is called a double helix because two long strands twist around
each other like a twisted ladder. The rails of the ladder are made
of alternating sugar and phosphate molecules. The steps of the
ladder are made of two bases joined together with either two or
three weak hydrogen bonds.


6.  The basic building block of DNA is called a NUCLEOTIDE. A
nucleotide is made up of one sugar molecule, one phosphate
molecule and one of the four bases. Here is the structural formula
for the four nucleotides of DNA. Note that the purine bases
(adenine and guanine) have a double ring structure while the
pyrimidine bases (thymine and cytosine) have only a single ring.
This was important to Watson and Crick because it helped them
figure out how the double helix was formed.

7.  These pictures show a ball and stick model of two DNA nucleotides.
Gray balls are carbon atoms, blue balls are nitrogen, red balls are
oxygen and the pink ball is phosphorous. The hydrogen atoms are not
shown.
Adenine Nucleotide (purine) Cytosine Nucleotide (pyrimidine)

8. Base Pairs
The nucleotides of DNA line up so that the sugar and phosphate
molecules make two long backbones like the handrails of a ladder. To
make the rungs of the ladder, two bases join together, between the sugar
molecules on the two handrails. The phosphate molecules do not have any
"rungs" between them. THERE IS ONLY ONE WAY THE BASES CAN
PAIR UP ON THE RUNGS OF THE DNA LADDER. An adenine
molecule only pairs with a thymine. A cytosine only pairs with a guanine.
They can pair in either order on a rung, giving four possible combinations
of bases --
A-T or T-A and C-G or G-C

9.  Believe it or not, it is this chain of base pairs that makes up the
code that controls what everything looks like. Below is a
picture showing how the bases pair. You will see that a purine
with two rings always pairs with a pyrimidine with one ring. In
this way the width of the DNA molecule stays the same. The
dotted lines represent weak hydrogen bonds. These form
between parts of the molecules that have weak positive and
negative charges. Because the hydrogen bonds are weak, they
are able to break apart more easily than the rest of the DNA
molecule. This is important when DNA reproduces itself and
when it does its main work of controlling traits that determine
what an organism looks like.
Adenine and Thymine pairing ***** Guanine and Cytosine pairing

10. The Double Helix Model
 In this model of a very short section of DNA
you can see how the A-T and C-G base pairs
make up the rungs of the ladder and the sugars
and phosphates make up the two long strands.
In this picture the DNA is not twisted. The DNA
in one chromosome would actually be hundreds
of thousands of bases long

12.  These two models shows how all the atoms of the
sugars, phosphates and nitrogenous bases fit
together to make the "spiral staircase" or
"twisted ladder" shape first suggested by the x-
ray diffraction pictures of DNA taken by
Rosalind Franklin and Maurice Wilkins.


13. THANK YOU

14.  Lingaraj Pradhan