DNA structute

1. Nucleic acid (DNA structure)

2. DNA
•DNA forms the genetic material in most of
the organisms.
•Deoxyribonucleic acid

3. Nucleic acid
 Friedrich Miescher in 1869 isolated what he
called nuclein from the nuclei of pus cells
 Nuclein was shown to have acidic properties,
hence it became called nucleic acid
 Two types of nucleic acid are found
 Deoxyribonucleic acid (DNA)
 Ribonucleic acid (RNA)

4. Nucleic acid structure
 Nucleic acids are polynucleotides
 Their building blocks are nucleotides

5. Nucleotide structure
PHOSPATE SUGAR
Ribose or
Deoxyribose
NUCLEOTIDE
NITROGENOUS BASE
PURINES PYRIMIDINES
Adenine (A)
Guanine(G)
Cytocine (C)
Thymine (T)
Uracil (U)

6. Sugars (Pentose sugar)

7. Nitrogenous Bases

8. Nitrogenous Bases

9. Nucleotide
O
O=P-O
O
Phosphate
Group
N
Nitrogenous base
(A, G, C, or T)
CH2
O
C1
C4
C3 C2
5

10. •The bases are covalently attached to the 1’ position of a pentose sugar
ring, to form a nucleoside
Glycosidic bond
R
Ribose or 2’-deoxyribose
Nucleosides =ribose/deoxyribose + bases
1

11. Adenosine, guanosine, cytidine, thymidine, uridine

12. Nucleotides = nucleoside + phosphate

13. BASES NUCLEOSIDES NUCLEOTIDES
Adenine (A) Adenosine Adenosine 5’-triphosphate (ATP)
Deoxyadenosine Deoxyadenosine 5’-triphosphate
(dATP)
Guanine (G) Guanosine Guanosine 5’-triphosphate (GTP)
Deoxyguanosine Deoxy-guanosine 5’-triphosphate
(dGTP)
Cytosine (C) Cytidine Cytidine 5’-triphosphate (CTP)
Deoxycytidine Deoxy-cytidine 5’-triphosphate
(dCTP)
Uracil (U) Uridine Uridine 5’-triphosphate (UTP)
Thymine (T) Thymidine/
Deoxythymidie
Thymidine/deoxythymidie
5’-triphosphate (dTTP)

14. 14
One Strand of nucleotide
 One strand of DNA is a
polymer of nucleotides.
 One strand of DNA has
many millions of
nucleotides.
nucleotide

15. Polymer of Nucleotide

16. The structure of a DNA chain can
be concisely represented
• The base chain is written in the 5’ →3’
direction

17. How DNA looks

18. James Watson Francis Crick
Cambridge University
Rosalind Franklin Maurice Wilkins
King’s College

19. X-Ray Diffraction and the
Structure of DNA
Watson was shown this picture by Wilkins in early 1953.
From the picture it was possible to calculate:
1) the distance between bases (3.4A°)
2) the length of the period (34A°)
3) the rise of the helix (36 degrees)

20. Chargaff’s Rule
(Erwin Chargaff)
 Molar ratio of purines is equal tomolar
ratio of pyrimidines.

21. Base pairing
A:T G:C
1
23
4
8
9
7 6
5
4
3 2
1

22. How Watson and Crick Solved the DNA Structure
They already knew from Franklin and Wilkins’ work that DNA was in
the form of a Double Helix
They used Chargaff’s Rule to figure out how the 4 Bases match up in pairs

23. How Watson and Crick Solved the DNA Structure
They discovered that:
• The Phosphate Backbone was on the outside
• Which protected the Bases on the inside
• DNA acts as a Template or a Copying Mechanism for reproduction

24. Physical Structure (cont’d)
 Chains are anti-parallel (i.e in opposite directions)
 Diameter and periodicity are consistent
 2.0 nm
 10 bases/ turn
 3.4 nm/ turn
 Width consistent because of pyrimidine/purine
pairing

25. 1. DNA contains two helical strands wound around a single axis.
2. Hydrophilic backbone containing deoxy ribose sugar and the phosphate
group remain to the outside of the helix facing the aqueous phase.
3. The hydrohphobic purines and pyrimidines remain inside the double
helix.
4. The purines of one strand pairs with the pyrimidines of the other strand
by hydrogen bonding. There are 3 hydrogen bonds between guanine
and cytosine and 2 hydrogen bonds between adenine and thymine.
5. The pairings of two strands create major groove and minor grooves on
the surface of the DNA double helix.
6. The nucleotides in one strand are joined to each other by phosphor
diester bond between 5’-phosphate of one nucleotide with the 3’-
hydroxyl group of the next nucleotide. All the phosphodiester bonds are
in the same orientation in a chain.
7. The two strands are antiparallel to each other i.e, the direction of 5’-3’
phosphodiester bond in the two strands are opposite.
8. Distance between two adjacent nucleotide pair in the double helix is 3.4
A°.
9. There are ten base pairs in each turn of the DNA double helix which
accounts for 34 A° length.

26. Conformational variation in
double-helical structure
• B-DNA
• A-DNA
• Z-DNA

27. • B-form: the duplex structure proposed by Watson and Crick is referred as
the B-form DNA.
•It is the standard structure for DNA molecules.
•A-form: at low humidity the DNA molecule will take the A-form:
•The A-form helix is wider and shorter, with a shorter more
compact helical structure, than the B-form helix.
• Z-form: the Z-form DNA is adopted by short oligonucleotides.
•It is a left-handed double helix in which backbone phosphates
zigzag (Alternate purines and pyrimidines).

30. Thank you