DEOXYRIBONUCLEIC ACID (DNA) - Dr. Tasneem Jahan

Lecturer, Dept. of Zoology St. Ann’s College for Women Hyderabad

DEOXYRIBONUCLEIC ACID (DNA):

The Watson and crick model of the double helical structure of DNA.

In 1953 two biochemists, james Watson of America and Francis crick of Britain, proposed a model for the double stranded DNA molecule to explain its structure. For this epic contribution Watson and crick were honoured with the 1962 Noble prize in medicine or physiology along with Maurice Wilkins who provided the x-ray crystallographic proof for the proposed structure.

Important postulates of Watson and crick model are:

1) DNA is composed of two helical polynucleotide chains

which are coiled around a common axis. The chains run

in a opposite directions. 2) The chains consists of deoxyribose residues joined by 3’,5’

phosphodiester bridges with the nitrogen bases projecting

perpendicularly from the chain into the central axis. 3) The 2 strands of this double-stranded molecule are held

together by hydrogen bonds between the purine and

pyrimidine bases. The pairing between the purine and

pyrimidine nucleotides on the opposite strands are very

specific and are dependent upon hydrogen bonding of

adenine with thymine (A-T) and guanine with cytosine (G-

C).

Complementary,

antiparallel

nucleotide

chains in a DNA

molecule.

• Three hydrogen bonds hold the guanosine to the

cytosine (G=C), whereas the other pair, adenine-

thymine, is held together by two hydrogen

bonds (A=T). • The chains are not identical but are

complementary in terms of the appropriate base-

pairing i.e., A to T and G to C as shown for a

hypothetical fragment of two chains of DNA. • The chains do not run in the same direction with

respect to their internucleotide linkages but rather

are antiparallel.

• If, for example, two adjacent deoxyribosides A and G in

the same chain are linked 3’-5’, the complementary

deoxyribosides T and C in the other chain will be linked

5’-3’. This is analogous to 2 parallel streets,

each running one way, but carrying traffic in

opposite directions. • The mean diameter of the double helix is 2.00nm and

one full turn of the double helix has a pitch of 3.4nm. 10

base pairs exist within a single turn. • The double helix reveals two grooves-a major deep

groove and a minor or shallow groove winding along the

molecule parallel to the phosphodiester backbones. In

these grooves, specific proteins interact with DNA

molecules.

Ribose and deoxyribose sugars.

1) SUGAR:

The sugar present in the DNA is called deoxyribose.

It is a pentose sugar which contains five carbon

atoms (C5H10O4). It contains one O atom less than

the ribose sugar. At carbon no.2 of deoxyribose, is

present a H-C-H group. But in ribose sugar the

second carbon atom contains H-C-OH group.

2)PHOSPHORIC ACID (H3PO4):

Phosphoric acid links consecutive nucleotides by joining their pentose sugars with a phosphate

diester bond. This bond links carbon 5’ in one nucleoside with carbon 3’ in the next nucleoside.

DEOXYRIBOSE SUGAR MOLECULE LINKED WITH ONE

PHOSPHATE GROUP AT 5TH

POSITION AND

ANOTHER PHOSPHATE GROUP AT 3RD

POSITION.

These are N2 containing organic compounds. They are of two types, namely Purines and Pyrimidines.

Purines are two-ringed N2 compounds. They are

of two types, namely adenine and guanine. Their

structural formulae are represented below:

These are single ringed N2 compounds. They are of two types, namely Thymine and Cytosine.

CYTOSINE THYMINE

URACIL

ADENINE GUANINE

FRAGMENT OF DNA MOLECULE

The nucleotides of DNA are named according to the

type of nitrogen bases present. As there are four types of

nitrogen bases, DNA contains four types of nucleotides,

namely

1) AMP-Adenosine monophosphate (Adenylic acid) 2) GMP-Guanosine monophosphate (Guanylic acid) 3) TMP-Thymidine monophosphate (Thymidylic acid) 4) CMP-Cytidine monophosphate (Cytidylic acid).

In each nucleotide, the deoxyribose sugar is attached

to a phosphoric acid at one side and a nitrogen base at the

other side.

The phosphoric acid molecule is linked to the sugar at carbon atom number 3 or 5.

The nitrogen base molecule is joined to the sugar by a

glycosidic bond. This bond is formed between carbon

atom number 1 of deoxyribose and nitrogen atom 3

or 9 of nitrogen base.

Many nucleotides are linked together to form a

polynucleotide chain. Two nucleotides are joined by a

phosphodiester bond. It is formed between the carbon

atom number 3 of sugar of one nucleotide and the

phosphate component at another nucleotide.

At one end of the polynucleotide chain, the 3rd

carbon of the sugar is free and it is not linked to any

nucleotide. This end is called 3 prime (3’) end. At the

other end the 5th

carbon of the sugar is free and this end is called 5 prime (5’) end.

Each DNA molecule has two polynucleotide chains.

The nucleotides of adjacent chains are linked. The

linking is always between purines and pyrimidines. So

adenine of one chain is linked with thymine (A-T).

Similarly guanine of one chain is linked with cytosine

(G-C).

The amount of adenine is equalent to the amount

of thymine and the amount of guanine is equalent to

the amount of cytosine.

A DNA molecule looks like a ladder. The sugar and

phosphate form the back bones and base-pairs

form the horizontal rungs.

The two complementary chains are twisted around

each other to form a double helix. One turn of helix

measures about 34A. It contains 10 paired nucleotides.

Distance between two pairs is 3.4 A. The mean

diameter of the double helix is 2.00nm.

THE HYDROGEN BONDS BETWEEN ADENINE AND

THYMINE AND GUANINE AND CYTOSINE IN THE

DOUBLE HELICAL STRUCTURE OF DNA.

Molecular structure of a double stranded DNA

Phosphate deoxyribose

hydrogen-bonded base pair phosphate-deoxyribose Backbone

backbone

NUCLEOTIDES OF DNA

THYMIDYLIC ACID CYTIDYLIC ACID

DNA is often called ‘’the secret of life’’. Kendrew

(1967) very appropriately called it ‘’the thread of life’’.

The most important functions of DNA are as follow:

DNA has the property of self-replication. It is,

therefore a reproducing molecule. This unique

property of DNA is at the root of all reproduction

through its replication. DNA acts as the key to

heredity. In the replication of DNA, the two stands of

a double helix unwind and separate as a template for

the formation of a new complementary strand.

DNA REPLICATION ABOVE,THE 2-STRANDED DNA MOLECULE,WHICH

‘’UNWINDS’’INTO TWO SINGLE CHAINS. BELOW THE TWO NEW COMPLEMENTARY

STRANDS THAT HAVE BEEN SYNTHESIZED, TWO DNA DOUBLE CHAINS THUS RESULT.

The specific sequence of pyrimidine, purine base pair

in DNA represents coded information for the

manufacture of specific proteins. These coded

instructions first are transcribed into the matching

nitrogen-base sequences within mRNA, and the

instructions in such RNA subsequently are translated

into particular sequence of amino acid units within

the polypeptide chains and proteins.

The major steps in the utilization of the genetic information can be represented as:

DNA—replication—DNA—transcription—RNA— translation—Protein.

CODE TRANSCRIPTION FROM DNA TO mRNA, A, T, G, C, U NITROGEN-BASES: R, RIBOSE: P,

PHOSPHATE (1) THE DNA DOUBLE CHAIN UNWINDS. (2) THE NUCLEOTIDE DIPHOSPHATE RAW

MATERIALS ADP, GDP, CDP AND UDP BECOME H-BONDED TO APPROPRIATE NITROGEN BASES

ALONG THE DNA CHAIN.(3) FORMATION OF A LINKED RIBOSE-PHOSPHATE CHAIN, THEN

YIELDS FINISHED MRNA.

This scheme is called the ‘’Central Dogma’’ in modern

biology. By so controlling protein manufacture.

DNA ultimately controls the entire structural and

functional make up of every cell.

3) MUTATION:

Under certain conditions, the nitrogen base sequence of a

particular amino acid gets altered. Such, alteration then are

stable and persist into succeeding molecular generations of

DNA. When such changes occur, the structural and

functional traits of a cell also change correspondingly.

Through changes in its cells, a whole animal and its progeny

may thus become changed in the course of successive

generations; this is equivalent to evolution.