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 : naglaa_fathy722000@yahoo.comnaglaa_fathy722000@yahoo.com

nagla.alhusseini@fmed.bu.edu.egnagla.alhusseini@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