Fig. 9-CO, p.215
Nucleic Acids: How Structure Conveys
Information
1. What Is the Structure of DNA?
2. What Are the Levels of Structure in
Nucleic Acids?
3. What Is the Covalent Structure of
Polynucleotides?
4. Knowing the types & general features of BDNA.
5. How Does the Denaturation of DNA
Take Place?
DNA structure
DNA is the largest macromolecule in the cell. In
eukaryotic cells, 99% of the cell DNA is present
in linear form folded on itself several times to
occupy small space within the chromosomes of
the nucleus .
Each chromosome contains single DNA
molecule. Small amounts of DNA (about 1%) are
circular shape present inside the mitochondria.
The DNA can be described as a polymer of
nucleotides (Polynucleotide).That is a long
chain of repeating nucleotide units connected
together strongly (by covalent bonds).
Therefore, nucleotide is the unit of DNA
structure that has complex structure made of
3 different components:
1 .Nitrogen base
2. Pentose sugar
3. Phosphate groups.
Without phosphate groups, the combination of
nitrogen base and sugar is called a nucleoside.
The nitrogen bases are heterocyclic
(combination of carbon and nitrogen atoms)
present in the cell with 5 different types.
Two purines : adenine and guanine
(with 9 atoms of 4 nitrogens and 5 carbons
arranged in two rings) and
Three pyrimidine: Cytosine, Thymine and
Uracil having 6 atoms ring including 2 nitrogens
and 4 carbons
In the nucleotide structure, the pentose sugar
connects to nitrogen base at carbon one from one
direction and to phosphate group at carbon 5 of
the other side
• There are 4 different types of
nitrogen bases present in each
nucleic acid. Adenine, Guanine and
cytosine are present in both DNA
and RNA .However only RNA
contains Uracil and only DNA
contains Thymine nitrogen bases.
Also RNA nucleotides have ribose
while DNA has deoxyribose as
pentose sugars.
Fig. 9-1, p.216
• The difference is - 2'-OH in ribo and 2'-H in
deoxyribose
The difference is - 2'-OH in ribose and 2'-H in deoxyribose
• In single polynucleotide chain of
DNA(and similarly for RNA structure)
the chain is made from covalent linkage
of sugar phosphate backbone .In this
arrangement the nitrogen bases are
exposed freely to the inside of the
backbone structure.
• In DNA structure two
polynucleotides are twisted around
each others in double helical
arrangement so that bases of
opposite polyncleotides are
specifically connected by weak
hydrogen bonds.
Opposite direction of DNA
polynucleotide chains
• The two strands have their 3’ and 5’ terminals
at opposite ends ( antiparallel )
• 5’ terminal: at one end of each DNA strand is
a phosphate group linked to carbon atom 5 of
deoxyribose
• 3’ terminal: at one end of each DNA is a
hydroxyl group attached to carbon atom 3 of
deoxyribose.
Fig. 9-5, p.219
Has polarity
Has a Hydroplilic side
Has a Hydrophobic side
Fig. 9-6, p.219
James Watson and Francis
Crick, 1959
won the 1962 Nobel Prize in Medicine for their
discovery of the structure of DNA. This was one of the
most significant scientific discoveries of the 20th
century
ROSALIND FRANKLIN
1920-58
 Gave an idea that the structure
of DNA is a helical structure.
 Physical Chemistry & X-ray
crystallography expert
 Died of ovarian cancer
The 3-dimensional double helix structure of DNA, correctly
elucidated by James Watson and Francis Crick.
Complementary bases are held together as a pair by
hydrogen bonds
Fig. 9-7, p.221
Fig. 9-8a, p.222
Fig. 9-8b, p.222
Less Common Nucleobases
Fig. 9-2a, p.216
Hypoxanthine + Sugar = Inosine
Fig. 9-2c, p.216
5
Fig. 9-2b, p.216
Fig. 9-3, p.217
Commonly Occurring Nucleotides
Fig. 9-4a, p.218
Fig. 9-4b, p.218
commonly occurring nucleotides
Fig. 9-4c, p.218
Fig. 9-4d, p.218
Three different conformations of the DNA double helix.
(A) A-DNA is a short, wide, right-handed helix. (B) B-DNA,
the structure proposed by Watson and Crick, is the most
common conformation in most living cells. (C) Z-DNA, is a
left-handed helix unlike A- and B-DNA, is a left-handed
helix.
Fig. 9-10, p.224
Fig. 9-11, p.225
Fig. 9-12, p.225
Fig. 9-14a, p.227
Fig. 9-14b, p.227
Fig. 9-15, p.228
Chromatin is comprised of histones and DNA: 147
base pairs of DNA wraps around the 8 core histones
to form the basic chromatin unit, the nucleosome
The function of chromatin is to efficiently package
DNA into a small volume to fit into the nucleus of a
cell and protect the DNA structure and sequence
Fig. 9-16, p.229
Fig. 9-17, p.229
Fig. 9-18, p.231
END