DNA Structure and Composition

1. ‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬
Molecular Biology
Course
Dr. Mohamed Abdel-Fattah
A. Professor of Immunology & Biotechnology
Teacher Home Page: http://uqu.edu.sa/staff/ar/4320553

2. Deoxy ribo-Nucleic Acid
(DNA)

3. Definition
Molecular Biology :
 Is the branch of biology that deals with the molecular basis of
biological activity
 Is the branch of biology that study the structure, function and
manipulation of nucleic acids and proteins.
 The Molecular biology field overlaps with other areas,
particularly genetics and biochemistry.
 DNA (Deoxy ribo-Nucleic Acid): Is the hereditary material

4. Components involved in Molecular Biology
DNA
RNA
Protein

6.  Deoxy ribo-nucleic acid (DNA) consists of a
chemically linked sequence of subunits.
 Each subunit contains:
1- A nitrogenous base
Provide the nitrogenous bases in the nucleic acids
2- Deoxyribose sugar
A five-carbon sugar in a ring form.
3- Phosphate group
PO4
Composition of DNA

7. 1- Nitrogenous Bases
 Nitrogenous bases are divided according to their
chemical structures into two types:
Purines: double ringed structure (Adenine and Guanine).
Pyrimidines: single ring structure (cytosine and thymine)

8. A- Pyrimidines:
Cytosine and Thymine are present in DNA while
Cytosine and Uracil are present in RNA
1- Nitrogenous Bases (cont.)
A heterocyclic ring of carbon and nitrogen atoms.

9. B- Purines:
Adenine and Guanine are present in both DNA
and RNA
1- Nitrogenous Bases (cont.)
Have fused five- and six-member rings of carbon
and nitrogen atoms.

10. 1- Nitrogenous Bases (cont.)
■ DNA: Four different types of nucleotides differ in
nitrogenous base:
■ A is for adenine;
■ G is for guanine;
■ C is for cytosine and
■ T is for thymine.
■RNA: thymine base replaced by uracil base (U).

11. Two types of pentose sugar are found in nucleic acids.
 In RNA Ribose sugar
 In DNA 2-deoxyribose sugar
The difference between Ribose & Deoxyribose sugar lies in the
absence/presence of the OH group at position 2 of the sugar
ring.
2- Deoxy Ribose Sugar

12. The nitrogenous base is linked to position (1) on the pentose
ring by a glycosidic bond from N1 of pyrimidine or N9 of
purine.
To avoid interference between the numbering systems of
the heterocyclic rings and sugar, positions on the pentose
are given a prime(‘).
1
1
Glycosidic
bond
Glycosidic
bond
1
9
2- Deoxy Ribose Sugar (Cont.)

13. O
CH2OH
OH OH
OCH2
O
ll
O-P-O
l
O
OH
O
ll
O-P-O
l
O
O
ll
O-P-O
l
O ---
-
Mono-
Phosphate
Nucleotide
Di-
Phosphate
Nucleotide
Tri-
Phosphate
Nucleotide
N.BASE
Pentose
Nucleoside Nucleotide
OH
1
5
3
(5)
(4)
(3) (2)
(1)
3- Phosphate Group

14.  Nucleoside:
A base linked to a sugar (Base+Sugar)
 Nucleotide:
A phosphate group is added to nucleoside
(Base+Sugar+Phosphate)
3- Phosphate Group (Cont.)

15.  Nucleotides provide the building blocks from
which nucleic acids are constructed.
 Nucleotides are linked together into polynucleotide
chain by backbone consisting of an alternating series
of sugar and phosphate residues.
 The 5‟ position of one pentose ring is connected to
the 3‟ position of the next pentose ring via a
phosphate group. Thus
 The sugar-phosphate backbone is said to consist of
5’-3’ phosphodiester linkages.
 The nitrogenous bases „stick out‟ from the backbone.
Polynucleotide Chain

16. A polynucleotide
chain consists
of a series of
5‟-3‟
sugar-phosphate
links that form
a backbone
From which
the bases
protrude.
Polynucleotide Chain (Cont.)

17.  The terminal nucleotide at one end of the chain has free
5’ group; the terminal nucleotide at the other end has free
3’ group.
 It is conventional to write nucleic acid sequences in the
5’-3’ direction, that is, from 5’ terminus at the left to 3’
terminus at the right.
Polynucleotide Chain (Cont.)

18.  X-ray diffraction data showed that DNA has the form
of a regular helix, making a complete turn every 34Å
(3.4nm), with a diameter of ~20Å (2nm).
 Since the distance between adjacent nucleotides is
3.4Å, there must be 10 nucleotides per turn.
 The density of DNA suggests that the helix must
Contain two polynucleotide chains.
 The proportion of G is equal to the proportion of C
in DNA, and the proportion of A is equal to the
proportion of T. (i.e C=G A=T)
DNA is a Double Helix

19.  DNA is negatively charged due to the phosphate ions
present in the ribose-phosphate backbone.
 It moves towards the positive pole during
electrophoresis.
 The definition cation/anion is confusing because:
1. The anion moves to the anode
2. As the anode is positive, thus
3. The anion is negative
DNA is anion

20. Base Pairing
 The arrangement of the bases in the DNA is not random
 The polynucleotide chains in the double helix associate
by Hydrogen bonding between the nitrogenous bases.
– G in one chain always pairs with C in the other chain with
Three Hydrogen Bonds
– A always pairs with T with Two hydrogen Bonds
i.e. this base pairing forming complementary strands.

22.  The process of copying a DNA helix is called
DNA Replication
 The Double-Stranded nature of the DNA allows each
Original strand to serve as template for the formation
Of complementary new strand
DNA chains separate, each chain is
used as a template to produce a new
chain, each new DNA helix contains
one “old” and one “new” chain
 DNA Replication is termed as
semiconservative because each
new double helix has one original
strand and one new strand
DNA Replication

23. 1. Helicase enzyme unwinds double stranded DNA
i. e. breaks weak hydrogen bonds between the paired bases
2. New complementary DNA nucleotides fits into DNA strands
By complementary base pairing process.
3.The positioned nucleotides are joined together by
DNA polymerase
4. Two produced DNA double helix molecules are identical to
each other and to the original DNA double helix molecule.
If any error happens during replication, mutation occurs which
may cause change in phenotype, genotype and diseases.
Steps of DNA replication

24. A mutation is a change in the genetic material of
an individual (permanent DNA replication error)
3’---TACAAAGAGACT---5’
5’---ATG TTTCTC TGA---3’
3’---TACAAA GAGACT---5’ DNA template
5’---ATG TTTCTC TGA---3’
3’--- TACAAA GAGACT---5’
3’---TACAAAGAGACT---5’
5’---ATG TTTCTC TGA---3’
3’---TACAAAGAGACT---5’ DNA template
5’---ATG TTTCTC TGA---3’
5’--- ATGTTCCTCTGA---3’ new DNA
3’---TACAAA GAGACT---5’
5’---ATG TTT CTC TGA---3’
Mutation