1DNA structure

Prepared By ZAHIDA UMAR

Dept of Environmental Sciences

GC, University, Lahore

What DNA is?

DNA structure

2

3

What DNA is?

DNA structure

1. A polymer of deoxyribonucleotides i.e. DNA

2. Double-stranded

3. A “double helical” structure

• common axis for both helices

• antiparallel relationship between 2 DNA strands

4. Structural purposes, or are involved in regulating the use

of this genetic information.

5. Individual deoxynucleoside triphosphates are coupled by

phosphodiester bonds

• Esterification

• terminal ends : 5’ and 3’

4

Discovering the structure of DNA

but 1st described by James Watson and Francis Crick in 1953.

DNA Discovery

DNA structure was discovered in 1869 by chemist Friedrich Miescher.

5

DNA structure

DNA Organization

This image illustrates how DNA is organized in human body.

6

Chemical structure of DNA

DNA Topology

•DNA forms a double helix in which the nucleotide bases are attached to deoxyribose units linked through phosphate groups. The bases in the center of the DNA helix always occur in complementary matched pairs, with cytosine linking to guanine and thymine linking to adenine through hydrogen bonding.

Sugar Phosphate Backbone

Esterification

7

•This backbone is constructed from alternating ribose sugar and phosphate molecules which are highly polar. Because the backbone is polar, it is hydrophilic which means that it likes to be immersed in water. The asymmetric ends of DNA strands are referred to as the 5′ (five prime) and 3′ (three prime) ends, with the 5' end being that with a terminal phosphate group and the 3' end that with a terminal hydroxyl group.

Nitrogenous Bases

DNA Bases

8

•The interior portion of a DNA molecule is composed of a series of 4 nitrogenous bases: Adenine (A),Guanine (G), Thymine (T), Cytosine (C). These bases are classified into two types; Purines , Pyrimidines

9

“Chargoff’s rule” A = T & C = G

Nitrogenous Bases

DNA Bases

10

DNA Grooves

DNA structure

•There are two grooves twisting around the surface of the double helix:

1.Major groove 2. Minor groove

•The major groove is 22 Å wide and the other, the minor groove, is 12 Å wide. The narrowness of the minor groove means that the edges of the bases are more accessible in the major groove.

11

Sense & AntisenseDNA strands

DNA STRANDS

•If DNA sequence is the same as that of a messenger RNA copy that is translated into protein is called SENSE & sequence on the opposite strand is called ANTISENSE

12

DNA Supercoiling

Almost all DNA molecules in cells can be

considered as circular, and are on average

negatively supercoiled.

Counter helical turn

DNA Supercoiling

13

Alternative Structure of DNA

DNA structure

A-Dna B-Dna Z-Dna

14

DNA structure

•Bases are the anti conformation

•Right handed helix•planes of bases are tilted 20 ° relative the helix axis. •2.3 A rise between base pairs

•Sugars are in the 3’ endo conformation.

O

OH (OH)

O

BASEO

O

H (OH)

OBASE

2' endo (3' exo) B-DNA

1'

3' endo (A-DNA)

3'

1'3'

2'

5'

5'

2'

5.9 A

•11 bases per helix turn •Helical pitch = 25.3 A

A – DNA

15

DNA structure

•Bases are the anti conformation

B – DNA

•Right handed helix•planes of bases are nearly perpendicular to the helix axis. •3.4 A rise between base pairs

•Sugars are in the 2’ endo conformation.

•10.4 bases per helix turn •Helical pitch = 34 A

O

OH (OH)

O

BASEO

O

H (OH)

OBASE

2' endo (3' exo) B-DNA

1'

3' endo (A-DNA)

3'

1'3'

2'

5'

5'

2'

7.0A

16

DNA structure

•Narrow and deep minor groove

Z – DNA

•Left handed helix

•planes of bases are tilted 9° relative the helix axis. •3.8A rise between base pairs

•Flat major grooves

•12bases per helix turn

•Helical pitch = 45.6 A

17

DNA structure

Z – DNA 5’-GCGCGCGCGCGCG3’-CGCGCGCGCGCGC

C: sugar is 2’-endo, base is antiG: sugar is 3’-endo, base is syn

Sugar and base conformations in Z-DNA alternate

18

Alternative DNA

19

Prepared By ZAHIDA UMAR

B.Sc Hons, Dept of Environmental

Sciences