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DNA: Structure & Organization

Dr.P.Mohanalakshmi

Professor

Dept. of Biochemistry

SMMCH&RI

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Learning Objectives

At the end of the session the student should know:
Structure of DNA
Watson and Crick model of DNA
Types of DNA
Properties of DNA
Organization of DNA

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NUCLEOTIDE

Base + Sugar + Phosphate

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ADENINE NUCLEOTIDE

BASE (Adenine)

SUGAR (Deoxy Ribose)

PHOSPHATE

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Primary Structure of DNA

Poly deoxy ribonucleotide
Nitrogenous Bases in DNA
Ribose sugar in DNA
Phosphodiester bond/linkage
Base sequence in DNA is very important
Polarity of DNA

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PRIMARY STRUCTURE OF DNA

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Nitrogenous bases in DNA

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PENTOSE SUGAR IN RNA & DNA

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GCCAATT -------- Protein A
GCCAATG -------- Protein B
GCCAATA -------- Protein C

DNA BASE SEQUENCE IS UTMOST IMPORTANT

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POLARITY OF DNA

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WATSON & CRICK MODEL OF DNA

James Watson & Francis Crick

Right handed double helix
Antiparallel nature
The Base-pairing rule
Hydrogen bonding
Other features

- Grooves

- Spiral – pitch (3.4 nm)

- Bases – distance (0.34 nm)

- Width of the helix

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5’

3’

5’

3’

ANTI-PARALLEL NATURE

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Sugar & Phosphate

Bases

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Chargaff’s -Patterns of Base Pairing

A T

G C

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CHARGAFF’S RULE

A+G

~ 1

T+C

No. of Purines = No. of Pyrimidines

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A T

G C

HYDROGEN BONDING

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MAJOR GROOVE

MINOR GROOVE

SPIRAL-PITCH

BASE DISTANCE

WIDTH OF HELIX 2.0 nm

KEY FEATURES OF DNA

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Forms/Types of DNA

Type A
Type B
Type C
Type D
Type E
Type Z

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Forms/Types of DNA

Properties	A-DNA	B-DNA	Z-DNA
Shape	Broadest	Intermediate	Narrow
2. Type of helix	Right handed	Right handed	Left handed
3. Width of the helix	2.55 nm	2.37 nm	1.84 nm
4. Major groove	Narrow	Wide	Flat
5. Minor groove	Very broad	Narrow	Very narrow

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Other properties of DNA

Denaturation

Annealing

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Difference between DNA & RNA

Properties	DNA	RNA
1. Pyrimidine Base	T C	C U
2. Pentose sugar	Deoxy Ribose	Ribose
4. Chargaff’s rule	Obey	Not obey
5. Stability	Strong	weak
6. Hydrolysis by alkali	Stable	Unstable

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Organization of Eukaryotic DNA

Double stranded DNA

Nucleosome

Chromatin

Chromosome

Histones

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CLASSES OF HISTONES

Types:

H1, H2A, H2B, H3, H4

Two molecule of each H2A, H2B, H3, H4 associate with one another to form a structure core called histone octomer for individual nucleosome

Eukaryotic DNA interacts with an equal weight of basic protein histones

Which contain a large amount of basic amino acid arginine and lysine

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N-TERMINAL MODIFICATION OF HISTONES BY

Acetylation
Phosphorylation
Ribosylation
Methylation

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NUCLEOSOME

DNA

HISTONES

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DNA double helix

2 nm

Beads on a string form of chromatin 10nm

Chromatin fiber of packed nucleosomes (solenoid )

30 nm

Nucleosome core particals

DNA wrapped around the Nucleosome core to the next form a chromatin fiber

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Noncondensed loops of chromosome attached to nuclear lamina

300 nm

Condensed loops of chromosome 700 nm

Metaphase chromosome

700 nm (1400)

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MCQ

In a DNA double helix the bases are held together by hydrogen bonds. These hydrogen bonds are _________

A .Ionic bonds

B .Covalent bonds

C . Non-covalent bonds

D .Van der Waals forces

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2. The type of sugar in DNA are

A .Hexose

B .Pentose

C .Tetrose

D . Triose

3.Which of the following processes would not lead to disruption of the nucleosomal structure?

A .Carboxylation

B .Acetylation

C .Methylation

D .None Of The Above

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4.Choose the incorrect statement about histones

A .H2A and H2B form dimers

B .H3 and H4 form a tetramer

C .H2A, H2B, H3, and H4 form the core of a nucleosome

D . histones are tightly bound to chromatin

5.Chromatin is composed of

A. Proteins Only

B. Nucleic Acids Only

C. Nucleic Acids And Protein

D .None Of These