Lecture 2
By Dr. M A Fouad
‫جامعة أم القرى‬
‫كلية العلوم الصحية بالليث‬
‫قسم الصحة العامة‬
• In 1953, Watson and Crick,
discovered the structure of DNA
• DNA Is a Structure That Encodes
Biological Information.
• DNA is abbreviation of DeoxyriboNucleic Acid
• Nucleic acid is composed of a long polymer of individual
molecules called nucleotides .
• Each nucleotide is composed of:
• Nitrogenous base,
• a sugar molecule, and
• a phosphate molecule.
Nucleotide
Nitrogenous base
Ribose
Phosphate
• The four nucleotides found
in DNA differ only in their
nitrogenous bases.
• There are two types of
nucleotides, purines and
pyrimidines.
• Purines have two rings in
their structure, while
pyrimidines have just one.
• The purines include:
• Adenine
• Guanine
• The pyrimidines include:
• Cytosine
• Thymine (only in DNA)
• Uracil (only in RNA).
• DNA  Deoxyribose.
• RNA  Ribose.
• Nucleotides are linked by
phosphodiester bonds
between the 3' and 5‘
carbons of adjacent
sugars.
• The alternating
phosphate and sugar
residues make the
backbone of the DNA
strand.
• The DNA molecule is composed
of two chains of nucleotides
arranged in a double helix.
• The two chains being held
together by hydrogen
bonds between the
nitrogenous bases which
point in towards the center
of the helix.
• The DNA chain end terminated
by the 5' carbon atom of the
sugar molecule is referred to as
the 5' end (5 prime), and the
end terminated by the 3‘
carbon atom is called the 3' end
(3 prime).
• In a double helix the direction
of the nucleotides in one
strand is opposite to their
direction in the other strand
• In the DNA duplex the 5' end
of one strand is opposite the
3' end of the other.
• The arrangement of the bases in the DNA is not
random:
• G in one chain always pairs with C in the other chain.
•A always pairs with T
• i.e. this base pairing forming complementary strands.
• This base pairing means that the two DNA strands are
complementary strands.
There are two different types of nucleic acid:
• Ribonucleic acid (RNA): contains the sugar
ribose (five carbon sugar)
• Deoxyribonucleic acid (DNA):, contain the five
carbon sugar deoxyribose.
• RNA contains the sugar ribose; DNA contains deoxyribose.
• RNA contains the base uracil; DNA contains thymine instead.
• RNA is usually single stranded; DNA is usually double stranded.
• RNA is short: one gene long at most; DNA is long, containing many
genes.
• In Deoxyribose, the
hydroxyl group at the 2'
position of the ribose
sugar is replaced by a
hydrogen (i.e. an oxygen
molecule is lost, hence
'deoxy').
• DNA and RNA both contain
the purine bases adenine
and guanine and the
pyrimidine cytosine
• Thymine occurs only in DNA
and uracil is found only in
RNA.
DNA
Sugar
Bases
Number of
Strand
Strand length
RNA
Deoxy ribose
Adenine,
Guanine,
Cytosine, and
Thymine
Two
Ribose
Adenine,
Guanine,
Cytosine, and
Uracil
One
Long
Short
Quizzes
Quiz
1. What are the three parts of a nucleotide?
• ------------------------• -------------------------• --------------------------
2. Which parts make up the backbone of a DNA strand?
• ------------------• --------------------
3. If six bases on one strand of a DNA
double helix are AGTCGG, what are the six
bases on the complementary section of the
other strand of DNA?
4. In DNA, thymine always pairs with
A.
B.
C.
D.
E.
adenine
cytosine
guanine
thymine
uracil
5.
A.
B.
C.
D.
E.
In DNA guanine always pairs with
adenine
cytosine
guanine
thymine
uracil
6.
Complete the following table
DNA
Sugar
Bases
Number of
Strand
Strand length
RNA
………………….
Adenine,
………………,
…………….., and
……………………
………………………..
Ribose
Adenine,
………………,
…………….., and
……………………
………………………
………………………..
………………………..
7. If one side of a DNA molecule contains the
following sequence of nucleotides, AGTCCG, the
complementary sequence on the other side would
be:
A.
B.
C.
D.
E.
GCCTGA
AGTCCG
TCAGGC
CTGAAT
None of the above
8. The two complementary strands of the
DNA double helix are held together by:
A.
B.
C.
D.
Covalent bonds
Hydrogen bonds
Ionic bonds
Phosphodiester bonds
9. In each strand of DNA, the nucleotides are
held together by:
A.
B.
C.
D.
Hydrogen bonds
Ionic bonds
Phosphodiester bonds
Peptide bond
10. An example of a purine is
A.
B.
C.
D.
uracil.
cytosine.
guanine.
thymine.
11. The sugar in RNA is
A.
B.
C.
D.
Ribose
deoxyribose
Glucose
Fructose
12. The sugar in DNA is
A.
B.
C.
D.
Ribose
deoxyribose
Glucose
Fructose
13. Which of the following are pyrimidines?
A.
B.
C.
D.
guanine and adenine
cytosine and thymine
thymine and guanine
adenine and thymine
14. Which of the following are purines?
A.
B.
C.
D.
cytosine and guanine
guanine and adenine
adenine and thymine
uracil and cytosine