deoxyribonucleic acid (dna)stannscollegehyd.com/department/wp-content/uploads/2018/08/dn… · 1)...
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DEOXYRIBONUCLEIC ACID (DNA) - Dr. Tasneem Jahan
Lecturer, Dept. of Zoology St. Ann’s College for Women Hyderabad
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DEOXYRIBONUCLEIC ACID (DNA):
The Watson and crick model of the double helical structure of DNA.
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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.
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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).
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Complementary,
antiparallel
nucleotide
chains in a DNA
molecule.
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• 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.
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• 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.
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Ribose and deoxyribose sugars.
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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.
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DEOXYRIBOSE SUGAR MOLECULE LINKED WITH ONE
PHOSPHATE GROUP AT 5TH
POSITION AND
ANOTHER PHOSPHATE GROUP AT 3RD
POSITION.
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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.
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CYTOSINE THYMINE
URACIL
ADENINE GUANINE
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FRAGMENT OF DNA MOLECULE
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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.
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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.
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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.
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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.
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Molecular structure of a double stranded DNA
Phosphate deoxyribose
hydrogen-bonded base pair phosphate-deoxyribose Backbone
backbone
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NUCLEOTIDES OF DNA
THYMIDYLIC ACID CYTIDYLIC ACID
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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.
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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.
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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.
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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.
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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.