DNA structureDNA structure
ByByDr. NAGLAA FATHY Dr. NAGLAA FATHY
Ass. Prof. of Biochemistry & Ass. Prof. of Biochemistry & Molecular Biology Molecular Biology
Faculty of Medicine Faculty of Medicine Benha University Benha University
E-mail : E-mail : [email protected][email protected]
[email protected]@fmed.bu.edu.eg
IntroductionIntroduction
The Central Dogma The Central Dogma of Molecular Biologyof Molecular Biology
DNA
mRNA
Transcription
Cell
Polypeptide(protein)
TranslationRibosome
OH
OCH2
Sugar
H
HH
A NucleotideA NucleotideAdenosine Mono Phosphate (AMP)Adenosine Mono Phosphate (AMP)
OH
NH2
N
N N
N
BaseP
O
OH
HO O
Phosphate
2’3’
4’
5’
1’Nucleotide
Nucleoside
H+
-
Pyrimidines
NH2
O
N
N NH
N
Guanine
N
N
Adenine
N
N
NH2
N O
NH2
N O
NH2
NCytosine
Purines
Uracil(RNA)CH3
N ON
O
NH
N ON
O
NH
Thymine(DNA)
NO
H
NO
N
NH C
ytosine
H
O
NN
N
N
N
H
H
Guanine -+
+
+
-
-
Base PairingBase PairingGuanine And CytosineGuanine And Cytosine
CH 3
N
O
N
ON
H+
- ThymineN
NN
N
HN H
-
+Adenine
Base PairingBase PairingAdenine And ThymineAdenine And Thymine
Base PairingBase PairingAdenine And CytosineAdenine And Cytosine
NO
H
NO
N
NH C
ytosine-
+
-
N
NN
N
HN
H
-
+
Adenine
Base PairingBase PairingGuanine And ThymineGuanine And Thymine
CH
3
NO
N
O
NH+
- Thymine
H
O
NN
N
N
N
H
H
Guanine
+
+
-
SU
GA
R-P
HO
SP
HA
TE
BA
CK
BO
NE
H
P
O
HO
O
O
CH2
HOH
P
O
O
HO
O
O
CH2
H
P
O
OH
HO
O
O
CH2
NH2
N
N
N
N
O
O
NH2N
NH
N
N
N O
NH2
N
B A
S E
S
DDNNAA
OH
P
O
HO
O
O
CH2
HO
O
H 2N
NHN N
N H
H
P HO
O
O
CH2
OO
N
O
H 2N
NH
H2O
H OH
P
O
HO
O
O
CH2
CH 3
O
O
HNN
H2O
5’Phosphate group
3’Hydroxyl group
5’Phosphategroup
3’Hydroxyl group
The Watson - Crick The Watson - Crick Model Of DNAModel Of DNA
3.4 nm1 nm
0.34 nm
Majorgroove
Minorgroove
A T
T AG C
C G
C GG C
T A
A T
G CT A
A TC G
--
-
-
---
--
--
--
-
--
--
-
---
--
--
--
-
-
Forms of the Double HelixForms of the Double Helix
0.26 nm
2.8 nmMinorgroove
Majorgroove
C GA T
T AG C
C G
G CT A
A T
G CT A
A TC G
A T
G C
1.2 nm
A DNA
1 nm
Majorgroove
Minorgroove
A T
T AG C
C G
C G
G CT A
A T
G CT A
A TC G
0.34 nm
3.9 nm
B DNA
+34.7o Rotation/Bp11 Bp/turn
-30.0o Rotation/Bp12 Bp/turn
+34.6o Rotation/Bp10.4 Bp/turn
C GG C
G CC G
C G
G CG C
G CC G
G CC G
0.57 nm
6.8 nm
0.9 nm
Z DNA
C-DNA:C-DNA:– Exists only under high dehydration conditionsExists only under high dehydration conditions– 9.3 bp/turn, 0.19 nm diameter and tilted bases9.3 bp/turn, 0.19 nm diameter and tilted bases
D-DNA:D-DNA:– Occurs in helices lacking guanineOccurs in helices lacking guanine– 8 bp/turn8 bp/turn
E-DNA:E-DNA:– Like D-DNA lack guanineLike D-DNA lack guanine– 7.5 bp/turn7.5 bp/turn
P-DNA:P-DNA:– Artificially stretched DNA with phosphate groups found Artificially stretched DNA with phosphate groups found
inside the long thin molecule and bases closer to the inside the long thin molecule and bases closer to the outside surface of the helixoutside surface of the helix
– 2.62 bp/turn2.62 bp/turn
Even More Forms Of DNAEven More Forms Of DNA
B-DNA appears to be the B-DNA appears to be the most common form most common form in in vivovivo. However, under . However, under some circumstances, some circumstances, alternative forms of DNA alternative forms of DNA may play a biologically may play a biologically significant role.significant role.
Denaturation and RenaturationDenaturation and Renaturation Heating double stranded DNA can overcome the Heating double stranded DNA can overcome the
hydrogen bonds holding it together and cause the hydrogen bonds holding it together and cause the strands to separate resulting in denaturation of the strands to separate resulting in denaturation of the DNADNA
When cooled relatively weak hydrogen bonds When cooled relatively weak hydrogen bonds between bases can reform and the DNA renaturesbetween bases can reform and the DNA renatures
TACTCGACATGCTAGCACATGAGCTGTACGATCGTG
Double stranded DNA
TACTCGACATGCTAGCACATGAGCTGTACGATCGTG
Double stranded DNA
Renaturation
TACTCGACATGCTAGCAC
ATGAGCTGTACGATCGTG
Denatured DNA
Denaturat
ion
Single stranded DNA
Denaturation and RenaturationDenaturation and Renaturation DNA with a high guanine and cytosine content has DNA with a high guanine and cytosine content has
relatively more hydrogen bonds between strandsrelatively more hydrogen bonds between strands This is because for every GC base pair 3 hydrogen This is because for every GC base pair 3 hydrogen
bonds are made while for AT base pairs only 2 bonds bonds are made while for AT base pairs only 2 bonds are made are made
Thus higher GC content is reflected in higher melting or Thus higher GC content is reflected in higher melting or denaturation temperaturedenaturation temperature
Intermediate melting temperature
Low melting temperature High melting temperature67 % GC content -
TGCTCGACGTGCTCGACGAGCTGCACGAGC
33 % GC content -
TACTAGACATTCTAGATGATCTGTAAGATC
TACTCGACAGGCTAGATGAGCTGTCCGATC
50 % GC content -
Comparison of melting temperatures can be used to determine the GC content of an organisms genomeTo do this it is necessary to be able to detect whether DNA is melted or notAbsorbance at 260 nm of DNA in solution provides a means of determining how much is single strandedSingle stranded DNA absorbs 260 nm ultraviolet light more strongly than double stranded DNA does although both absorb at this wavelengthThus, increasing absorbance at 260 nm during heating indicates increasing concentration of single stranded DNA
Determination of GC ContentDetermination of GC Content